Anthocyanins’ effects on diabetes mellitus and islet transplantation

Abstract

The incidence of diabetes mellitus is dramatically increasing every year, causing a huge global burden. Moreover, existing anti-diabetic drugs inevitably bring adverse reactions, and the application of islet transplantation is often limited by the damage caused by oxidative stress after transplantation. Thus, new approaches are needed to combat the growing burden of diabetes mellitus. Anthocyanins are of great nutritional interest and have been documented that have beneficial effects on chronic diseases, including diabetes mellitus. Here, we describe the health effects of anthocyanins on diabetes mellitus and islet transplantation. Epidemiological studies demonstrated that moderate intake of anthocyanins leading to a reduction in risk of diabetes mellitus. Numerous experiments both animal and clinical studies also showed positive effects of anthocyanins on prevention and treatment of diabetes and diabetic complications. These effects of anthocyanins may be related to mechanisms of improving glucose and lipid metabolism and insulin resistance, antioxidant, and anti-inflammatory activities. In addition, damage and function of pancreatic islets after transplantation are also improved by anthocyanins. These findings suggest that daily intake of anthocyanins may not only improve nutritional metabolism in healthy individuals to prevent from diabetes, but also as a supplementary treatment of diabetes mellitus and islet transplantation. Thus, more evidence is needed to better understand the potential health benefits of anthocyanins.

Keywords:

• Anthocyanins
• diabetes mellitus
• islet transplantation
• pre-diabetes

Introduction

Diabetes mellitus (DM), a group of metabolic diseases, is characterized by hyperglycemia resulting from reduces in islet β-cell numbers or defects in islet β-cell function, and as hyperglycemia continues to progress and rapid swings, it can affect nearly every tissue of body and lead to severe acute and chronic diabetes complications (Chellappan et al. 2018). DM has reached warning line across the globe, and approximately 463.0 million individuals are affected by DM in 2019. It is estimated that the number of patients will reach 578 million in 2030 and 700 million in 2045 (Saeedi et al. 2019). DM are mainly divided into various types by different causes, type 1 diabetes mellitus (T1DM), type 2 diabetes mellitus (T2DM), genetic defects of the β-cell function and other specific types of diabetes (Petersmann et al. 2019).

T1DM, an autoimmune disorder, accounting for approximately 5% of diabetic patients, is mainly due to the increase of the expression of surface leucocyte-homing receptors in the early endothelial cells, which causes immune cells to enter the endothelial tissue, leading to the destruction of β-cells (Eizirik, Pasquali, and Cnop 2020). T2DM, a metabolic disease with a strong genetic susceptibility, accounting for about 90% of all diabetic patients, is mainly due to insulin resistance or relative insulin deficiency without islet β-cell destruction (Mayans 2015). As insulin sensitivity progressively deteriorates, pancreatic cells increase insulin secretion leading to hyperinsulinemia, and as this coordination mechanism is out of balance, ultimately leading to β-cell dysfunction. There are different treatment principles between T1DM and T2DM. As for T1DM, primary treatment modality is exogenous insulin replacement therapy, while insulin therapy is used only after other anti-diabetic drugs cannot control hyperglycemia in T2DM (Lu and Zhao 2020). Furthermore, both T1DM and T2DM can induce a series of complications, including acute and chronic complications. Acute complications, including diabetic ketoacidosis and hyperosmolar hyperglycemia nonketotic coma, usually result from inadequate treatment. Chronic complications can be divided into microangiopathy and macroangiopathy, the former mainly includes diabetic retinopathy (DR), diabetic nephropathy (DN), and diabetic neuropathy, the latter mainly occurs in the coronary and cerebrovascular arteries (Cole and Florez 2020). DM and related complications have generated huge global burdens and become a global health problem.

Pancreatic islet transplantation, one of the safest and least invasive transplant procedures, has become as a reliable treatment for insulin-deficient diabetes(Shapiro, Pokrywczynska, and Ricordi 2017, Rickels and Robertson 2019). The therapeutic effect of islet transplantation depends on the numbers of the functional islet β-cells surviving after transplantation(Rickels and Robertson 2019). However, islets are damaged or even die due to ischemia-reperfusion and oxidative stress during islet isolation and following transplantation. Because of lacking an intrinsic vascular system, islets are rather vulnerable to ischemia-reperfusion injury which generated reactive oxygen species (ROS) further leading to the loss of islet β-cells(Miki et al. 2018). In addition, in the late stage of transplantation, the islet graft underwent immune reject reaction and gradually lose function due to oxidative stress and subsequent immune response(Montanari et al. 2019). Therefore, how to maintain the long-term activity and function of transplanted islets is a critical issue for its successful clinical application.

COVID-19, an ongoing pandemic, sweeping the world. Many diabetic patients with COVID-19 are insensitive to existing hypoglycemic drugs, and are even impaired by the side effects of the drugs(Panchamoorthy and Vel 2021). Thus, a growing body of research is focusing on novel natural products possessing anti-DM effect, such as medicine plant(Salehi et al. 2019, Ashande et al. 2022) and fresh fruit containing phenolic(Mhya, Nuhu, and Mankilik 2021) and flavonoids compounds(Al-Ishaq et al. 2019). Anthocyanins, natural plant flavonoid pigment, providing many fruits and flowers with various colors, which often provide benefits over other phytochemicals, have gathered widespread attention in recent decades (Fang 2014, Kalt et al. 2020). Due to unique properties and various effects, anthocyanins are widely used in a variety of industries, including food processing industry, clean energy industry, and pharmaceutical industry(Buchweitz et al. 2013, Chien and Hsu 2013, Alappat and Alappat 2020). The health benefits of anthocyanins are mainly associated with antioxidant, anti-inflammatory, anti-cancer, and gut microbiota modulation(Tian et al. 2019). Moreover, anthocyanins also show glucose regulation, and neuroprotective effects(Mattioli et al. 2020). At present, these effects have been proven to be effective in treating or preventing chronic diseases in studies, such as cardiovascular disease (CVD)(Wang et al. 2014), neurodegenerative diseases(Winter and Bickford 2019), vision loss disease(Nomi, Iwasaki-Kurashige, and Matsumoto 2019), tumor(Lin et al. 2017), and DM(Guo et al. 2016). Many features of DM, such as hyperglycemia, insulin deficiency, and insulin resistance, can be ameliorated by increasing consumption of anthocyanins(Cao et al. 2019). It is an appealing direction exploring the health benefits of anthocyanins for DM and islet transplantation, hence research on the role of anthocyanins in prevention and treatment of DM (including associated complications), functional preservation of islet grafts in islet transplantation is reviewed in this narrative, focusing on animal and human studies, as well as possible mechanism. Over nearly 150 references cited in this article were published in the recent ten years.

Current Status of knowledge

Chemical structure and bioavailability of anthocyanins

In fresh fruits and vegetables, anthocyanins can be divided into six common types, such as cyanidin, peonidin, pelargonidin, malvidin, delphinidin, and petunidin, based on the different substituent groups on the flavylium B‐ring(Millar, Duclos, and Blesso 2017) (Figure 1). They are similar molecules, including a benzoic ring(A ring), a non-benzoic ring with an oxygen atom inside(C ring), and another benzoic ring with a carbon–carbon bound (C–C) named as flavylium ion(B ring)(Andersen et al. 2010). In addition to providing various colors, these compounds play significant roles in the process of plant growth and development, including reproduction and defense mechanisms(Alappat and Alappat 2020). The main way to obtain anthocyanins is to extract and separate from plant tissues, such as blueberries, bayberries, and strawberries(Silva et al. 2017). Previously, it was difficult to associate anthocyanins with health outcomes, mainly due to their most striking features that quickly absorbed and excreted, but low in absorption efficiency, which means that the concentration of anthocyanins in plasma rarely reaches therapeutic level (Manach et al. 2005). Meanwhile, the stability of anthocyanins is poor and susceptible to temperature, pH, and light (Yang et al. 2018). The bioavailability of anthocyanins has been shown to be totally underestimated resulted from quickly absorbed and transformed into biologically active metabolites. And metabolized conjugates of these full sets of anthocyanins have not yet been completely determined(Lila et al. 2016). Meanwhile, to enhance the biological activity of anthocyanins, many advanced approaches have been designed, such as using lipid-, polysaccharide- and protein-based complexes, nanocarriers and exosome vehicles(Shen et al. 2022).

Figure 1. The structure of the anthocyanin backbone and the most common anthocyanins.

Relation of anthocyanins’ health effects to its structure

Anthocyanins possess antioxidant and anti-cancer abilities closely related to its unique chemical structure. The benzene ring skeleton provides anthocyanins with special function ensuring the stability of the structure under the disappearance of electrons. The antioxidant efficacy of anthocyanins is greatly dependent on their chemical structure, such as the number and position of hydroxyl groups, the 2, 3 double bond in conjugation, the degree of glycosylation and 4-oxofunction in C ring(Reis et al. 2016). Due to the hydroxyl groups in the B-ring and the oxonium ion in the c-ring, anthocyanins may have two antioxidant mechanisms: hydrogen atom transfer (HAT) and single electron transfer (SET). In the HAT mechanism, antioxidants Convert free radicals into stable products by donating a hydrogen atom. For the SET mechanism, antioxidants maintain stability by providing free radical electrons(Liang and Kitts 2014). The anticancer effect of anthocyanins is closely related to the substituents on the B ring. With the different substituents on the B ring, the anticancer effects are also different(Lin et al. 2017). Especially the ortho-dihydroxyphenyl structure on the B-ring showed the strongest anticancer effect, preventing tumor metastasis and growth (Hou et al. 2004, Xu et al. 2010).

In addition, anthocyanins play anti-inflammatory effect through multiple mechanisms: inhibiting the activation of nuclear factor-kB (NF-kB) pathway, which is a central orchestrator of the inflammatory response(Duarte et al. 2018); inhibiting inducible nitric oxide synthase(iNOS) to reduce the production of nitric oxide that promotes inflammation(Ma et al. 2021); increasing antioxidant capacity to reduce the inflammatory reaction(Huang et al. 2018); inhibiting cyclooxygenase activity to decrease the release of Prostaglandin E2 leading to acute inflammation(He, Hu, et al. 2017). And recent evidence suggests that anthocyanins may play a health-promoting role by modulating gut microbiota(Li, Wang, et al. 2017). Although the stomach and small intestine can absorb anthocyanins, more than half of the anthocyanins eventually reach the colon, and then play a prebiotic active role under the interaction with colonic microorganisms(He and Giusti 2010, Faria et al. 2014). Gathering information from in vitro and human studies, proves that anthocyanins have an influence on intestinal bacterial growth, especially beneficial bacteria, such as Bifidobaterium spp.and Lactobacillus-Enterococcus(Hidalgo et al. 2012, Boto-Ordóñez et al. 2014, Sun et al. 2018, Zhu, Sun et al. 2018).

Anthocyanins and prevention of diabetes mellitus

Pre-diabetes is the term that defines some people who with impaired glucose tolerance but do not meet the diagnostic criteria for diabetes and eventually develop DM(Beulens et al. 2019, Khan et al. 2019). In a simulation model, effective control of blood glucose can delay the progression of DM in patients with prediabetes(DeJesus et al. 2017). In addition, obesity was an independent predictor for pre-diabetes progression to DM, and with increasing BMI, the risk of progression to DM progressively increased(Ligthart et al. 2016).

Animal studies

Several animal studies have also been evaluated preventive effect of anthocyanins on DM (S1 Table). In an animal study, after the rats with prediabetes and hyperlipidemia were administered black chokeberry fruit extract that contains high amounts of anthocyanins, antioxidant status was increased, and blood lipids and blood glucose was decreased(Jurgoński, Juśkiewicz, and Zduńczyk 2008). Insulin resistance in mice with high fat diet were improved after 12 weeks of diet supplementation with cyanidin 3-glucoside-rich purple corn color(Tsuda et al. 2003). C57BL/6 mice with obese and hyperglycemic fed a high fat diet containing blueberry anthocyanins for 13 weeks had better glucose tolerance and fasting blood glucose levels, and lower serum cholesterol than mice fed a high fat diet without anthocyanins. In addition, blueberry anthocyanins significantly reduced glucose production in rat hepatocytes to improve blood glucose levels but did not increase glucose uptake by L6 myotubes(Roopchand et al. 2013).

Table 1. the anti-T2DM effect of anthocyanins in clinical trial.

Subject(numbers)	Study design	intervention duration	Administered Form and dosage of anthocyanins	Main results	references
Obese adults(n = 36)	Randomized crossover study	1 week	Anthocyanin-Rich Mixed-Berry; 143.5 mg/d	↓serum insulin area under the curve	(Solverson et al. 2019)
Adults with insulin resistance (n = 21)	Randomized, single-blinded controlled study	6 hours	freeze-dried whole strawberry powder	↓Postmeal insulin concentrations ↓ Oxidized low-density lipoprotein	(Park et al. 2016)
Adults with prediabetes or newly diagnosed diabetes (n = 160)	Randomized controlled study	12 weeks	Anthocyanins 320 mg/d	↑ adipsin and apoA-1 ↓ visfatin, HbA1c, and apo B	(Yang et al. 2021)
Adults with prediabetes or early untreated diabetes (n = 138)	Randomized, double-blinded controlled study	12 weeks	Purified anthocyanins 320 mg/d	↓ HbA1c, LDL-c, and apo B ↑ apoA-1	(Yang et al. 2017)
Adults with prediabetes or newly diagnosed diabetes (n = 160)	Randomized controlled study	12 weeks	Anthocyanins 320 mg/d	↑adiponectin ↓fasting glucose (in newly diagnosed diabetes)	(Yang et al. 2020)
Adults with prediabetes (n = 36)	Randomized controlled study	3 weeks	Delphinol standardized maqui berry extract 0,60 ,120, 180 mg/d	↓ basal glycemia and insulinemia	(Alvarado, Leschot, et al. 2016)
Adults with impaired glucose intolerance (n = 10)	Randomized controlled study	12 hours	Delphinol standardized maqui berry extract; 200 mg/d	↓post prandial blood glucose and insulin	(Hidalgo et al. 2014)
Adults with obese andinsulin resistant (n = 32)	Randomized, double-blinded controlled study	6 weeks	blueberry bioactives; 45g/d	↑ insulin sensitivity	(Stull et al. 2010)
Adults with diagnose, moderate glucose intolerance (n = 31)	Randomized controlled study	3 mouths	maqui berry extract Delphinol 180 mg/d	↓ HbA1c and LDL ↑ HDL	(Alvarado, Schoenlau, et al. 2016)
Healthy volunteers (n = 14) T2DM (n = 12) T2DM-at-risk (n = 14)	Case-control study	4 weeks	Bilberries and Black currants; 320 mg/d	↓plasma IL-6, IL-8, TNF-α ↑IL-10 and adiponectin ↓fasting blood glucose ↓LDL-cholesterol	(Nikbakht et al. 2021)
T2DM (n = 20)	Randomized, double-blinded controlled study	4 weeks	Bilberry extract; 1.4 g/d	No significant differences across treatments	(Chan et al. 2021)
Obese male volunteers with T2DM (n = 8)	Randomized, double-blinded controlled study	2 weeks	Bilberry extract; 50g	↓AUC for both glucose and insulin	(Hoggard et al. 2013)
Adults with T2DM (n = 37)	Randomized, double-blind controlled study	2 months	Whortleberry fruit hydroalcoholic extract; 1.05 g/d	↓blood levels of fasting glucose, 2-h postprandial glucose, and HbA1c	(Kianbakht, Abasi, and Dabaghian 2013)
Adults with T2DM (n = 58)	Randomized, double-blinded controlled study	4 weeks	Purified anthocyanin; 320 mg/d	↓LDL cholesterol, triglycerides, apolipoprotein (apo) B-48, and apo C-III ↑HDL cholesterol ↑ Antioxidant capacity ↓fasting plasma glucose and homeostasis model assessment for insulin resistance index ↑adiponectin and β-hydroxybutyrate	(Li et al. 2015)
Female with T2DM (n = 36)	Randomized, double-blind controlled study	6 weeks	Freeze-dried strawberry; 100 g/d	↓C-reactive protein levels and lipid peroxidation ↓HbA1c and total antioxidant status	(Moazen et al. 2013)
Healthy volunteers(n = 50) T2D (n = 85)	Case-control study	3 hours	Pomegranate juice; 1.5 mL / kg	↓ fasting serum glucose and insulin resistance ↑β-cell function	(Banihani et al. 2014)
Obese volunteers with T2DM (n = 16)	Randomized crossover study	3 weeks	bilberry extract; 0.47g/d	↓ OGTT venous plasma glucose AUCi	(Alnajjar et al. 2020)

Animal models fed a high-fat diet often develop obesity, insulin resistance, impaired glucose tolerance, and eventually DM(Heydemann 2016). Anthocyanins can improve these manifestations and thus play a role in preventing DM. Tian and colleagues reportedcyanidin-3-glucoside(C3G) reduced body-weight gain and improved glucose tolerance in mice fed a high-fat diet(Tian et al. 2020). Purple sweet potato extract induces adipocyte browning by increasing expression of adipose browning-related genes in high-fat-diet-induced obese mice, which means converting stored adipocytes into heat-producing adipocytes to improve lipid deposition(Lee et al. 2021). C57BL/6 mice fed a high-fat (45%) diet plus anthocyanin-rich tart cherry extract for 8 weeks had lower the leptin and IL-6 levels and higher adiponectin concentration, compared to controls group only fed by high-fat diet(Nemes et al. 2019). Blueberries, a rich source of anthocyanins, have been shown in multiple studies to reduce body weight, improve lipid profiles, down-regulate the expression of inflammatory factors, and improve insulin sensitivity in high-fat diet-fed animal models(Seymour et al. 2011, Wu, Jiang, et al. 2016, Lee et al. 2018). Black current that has a variety of anthocyanins have been reported to significantly inhibit the elevation of cholesterol and triglycerides, and thereby reduce hepatic triglyceride levels by reducing the expression of genes related to fatty acid synthesis in ovariectomized rats(Park et al. 2015). In addition, black currant has been shown to improve Lipid metabolism by regulating the expression of genes related to the synthesis and degradation of lipids and cholesterols in other animal experiments(Song, Shen, Wang, et al. 2021). Other anthocyanin-rich black rice(Song, Shen, Wang, et al. 2021), blackelderberry(Farrell et al. 2015),fruit of acanthopanax senticosus(Saito et al. 2016),aronia melanocarpa(Kim et al. 2018, Lim et al. 2019), andraspberry(Wu et al. 2018) improved lipid metabolism in liver or serum and insulin resistance in diet-induced obese mice.

Clinical trial

In a cohort study, 60586 women with T2DM follow for 20 years demonstrated the importance of anthocyanin-rich foods, which can prevent T2DM risk(Laouali et al. 2020). A dose-response manner between anthocyanin intake and T2DM risk was demonstrated (HR: 0.86; 95% CI: 0.81, 0.91) in two meta-analysis studies, with increased intake of anthocyanins leading to T2DM risk reduction(Rienks et al. 2018, Xu et al. 2018). In a study that data from 3 prospective cohort studies in US proved that the risk of developing T2DM was significantly reduced after a high intake of anthocyanins (RR = 0.85; 95% CI: 0.80, 0.91). Furthermore, the increased consumption of anthocyanin -rich foods, such as blueberries, apples, and pears, also decrease T2DM risk(Wedick et al. 2012, Palacios, Kramer, and Maki 2019). In its subsequent study, it was further demonstrated that consuming fruit juice 3 times a week, which contains high levels of anthocyanins, reduces the risk of T2DM by 14%-33%(Muraki et al. 2013). Similar results were also demonstrated when was determined in a Polish cohort (RR: 0.68, 95% CI: 0.48–0.98)(Grosso et al. 2017).A dose–response meta-analyses suggests that a daily intake of 7.5 mg increment of dietary anthocyanins or 17 g of berries per day reduces T2DM risk by 5%(Guo et al. 2016). It is worth noting that not all observational studies have shown that anthocyanins are associated with the risk of T2DM(Jacques et al. 2013).

Many studies have demonstrated a decrease in the risk of T2DM with increased intake of anthocyanins(Wedick et al. 2012, Guo et al. 2016). Insulin resistance and insulin deficiency due to pancreatic beta-cell dysfunction are core pathophysiological properties of T2DM. In a randomized cross-over trial, obese subjects had lower serum insulin area under the curve following anthocyanin-rich mixed-berry treatment, implying the potential ability of anthocyanins to improve insulin resistance(Solverson et al. 2019). Then, after intake of beverage containing freeze-dried whole strawberry powder by 21 adults with insulin resistance, a reduction in was documented in insulin demand after meal in obese patients, meaning insulin sensitivity of subjects was improved(Park et al. 2016). In a randomized controlled trial serum adipsin increased and visfatin decreased in participants with prediabetes or newly diagnosed diabetes following 12 weeks of anthocyanin supplementation. In addition, HbA1c, apolipoprotein A-1 and apolipoprotein B were also significantly improved(Yang et al. 2021). Similar results were observed in participants with prediabetes or early untreated DM in a randomized controlled trial in China(Yang et al. 2017). However, in another randomized controlled trial it was found that the improvement in blood glucose and lipids was only in patients with newly diagnosed DM, not in patients with prediabetes(Yang et al. 2020). In 36 prediabetic subjects, fasting blood glucose and insulin levels was dose-dependently reduced after delphinol one hour before glucose intake(Alvarado, Leschot, et al. 2016). And Hidalgo et al found that delphinol can effectively improve post prandial blood glucose and insulin in volunteers with moderate glucose intolerance(Hidalgo et al. 2014). But in another three-month clinical trial focusing on the effects of delphinol on prediabetic individuals, after 3 months of delphinol intake, average levels of HbA1c were significantly improved, but not fasting insulin and glucose(Alvarado, Schoenlau, et al. 2016). Insulin sensitivity improved after individuals with insulin resistance were treated with freeze-dried whole blueberry powder for 6 weeks(Stull et al. 2010).

Anthocyanins can also play a role in preventing DM by improving lipid metabolism. In a double-blind, placebo-controlled study of obese adults, lipid metabolism improved after 8 weeks of ingestion of anthocyanin-rich black soybean testa extracts(Lee et al. 2016). In a pilot study, 11 obese women had lower total and LDL cholesterol after a 12-week diet of commercial red orange juice(Azzini et al. 2017). Likewise, volunteers also experienced a drop in LDL cholesterol after consuming red orange juice daily for 8 weeks(Silveira, Dourado, and Cesar 2015). In clinical research on black rice extract, postmenopausal women reduced fat accumulation after oral ingestion for 12 weeks(Jung et al. 2021). Intake of blackberries that has rich anthocyanins for 7 days by 27 overweight or obese men was associated with an increase in fat oxidation and an improvement in insulin sensitivity(Solverson et al. 2018).

The result of the clinical trials discussed above about the benefits of anthocyanins on T2DM is presented in Table 1.

Anthocyanins and treatment of diabetes mellitus

Type 2 diabetes mellitus

Animal trial

Table 2 demonstrates the beneficial effect anthocyanins on T2DM in animal studies. T2DM mice that consumed bilberry extract that contain large amounts of anthocyanins had remarkable reduced the blood glucose concentration and enhanced insulin sensitivity(Takikawa et al. 2010). Anthocyanins from maqui berry improved fasting blood glucose levels and glucose tolerance, decreased hepatic glucose production, and increased glucose uptake by L6 myotubes in hyperglycemic obese C57BL/6J mice(Rojo et al. 2012). In a study, T2DM mice were fed with blackcurrant extract that contain high levels of anthocyanins, blood glucose concentration decreased, and glucose tolerance greatly improved(Iizuka et al. 2018).Several reports also have found an improvement in blood sugar and glucose tolerance(Guo, Xia, et al. 2012, Kurimoto et al. 2013, Choi et al. 2016, Oliveira et al. 2017), but other reports have not found such protective effects of anthocyanins in animal models(Mykkänen et al. 2014, Oyama et al. 2016). T2DM mice were fed a diet containing 0%, 5%, or 10% buckwheat sprouts, and the results showed that as the concentration of buckwheat sprouts in the diet increased, blood glucose and lipids improved more obviously(Watanabe and Ayugase 2010). And anthocyanins from other plants have also showed properties of improving blood glucose and lipid(Liu et al. 2020). Chen et al also found that anthocyanins extracted from black soybean seed coat could improve hyperglycemia and hyperlipidemia of STZ-induced T2DM mice and reduce the damage of liver, kidney, and pancreas(Chen et al. 2018). In other animal study, treatment of T2DM mice with blueberry anthocyanin extract improved blood glucose levels and glucose tolerance, relieved symptoms of polydipsia and polyuria, and decreased TC, TG, and insulin levels(Herrera-Balandrano et al. 2021). Diabetes model KK-Ay mice that consumed aronia juice had reduced body weights, blood glucose levels, and the weights of white adipose tissues, compared with the control group(Yamane et al. 2016). In addition to improving blood glucose and lipids level, anthocyanins from purple sweet potato were proved to decrease oxidative stress and liver damage in STZ-induced T2DM rats(Jiang et al. 2020).

Table 2. Anti-T2DM of anthocyanins in animal trial.

Subject(numbers)	Study design	intervention duration	Administered Form and dosage of anthocyanins	Main results	Mechanism	references
T2DM mice (KK-A^y mice)	Randomized controlled study	5 weeks	Bilberry extract; 10g/kg	↓ blood glucose ↑ insulin sensitivity ↓ glucose production and lipid content in the liver	↑ activation of AMPK signaling pathway ↑the expression of PPARα and Acox	(Takikawa et al. 2010)
T2DM mice (C57BL/6J mice fed a high fat diet)	Randomized controlled study	6 hours	Delphinidin 3-sambubioside-5-glucoside of maqui berry; 50 − 500 mg/kg	↓ fasting blood glucose levels ↑ glucose tolerance	↑ the expression of CPT1A ↓ the expression of acetyl-CoA carboxylase	(Rojo et al. 2012)
T2DM mice (KK-A^y mice) (n = 18)	Controlled study	7 weeks	Blackcurrant extract; 11 g/kg	↓ blood glucose ↑ glucose tolerance	↑activation of AMPK signaling pathway ↑the expression of GLP-1 concentration ↑ PC1/3	(Iizuka et al. 2018)
Mice with metabolic syndrome (C57BL/6J mice fed a high palatable diet )	Controlled study	150 days	blueberry fruit extract; 200 mg/kg	↓ insulin resistance ↓ body weight gain ↓visceral fat, blood glucose, TC and TG	NA	(Oliveira et al. 2017)
C57BL/KsJ-db/db mice (n = 24)	Controlled study	6 weeks	Mulberry fruit extract; 0.2%, w/w	↓ blood glucose, HbA1c, and insulin levels ↓ plasma lipid concentration	↑activation of AMPK signaling pathway ↓ the expression of G6Pase and PCK in the liver	(Choi et al. 2016)
T2DM mice (C57BL/6J mice fed a high fat diet)	Controlled study	5 weeks	cyanidin 3-glucoside; 0.2% of diet	↓ blood glucose ↑ insulin sensitivity ↓TC and steatosis in liver ↓ inflammation levels	↓the cJNK/FoxO1 signaling pathway ↓ the expression of TNF-α, IL-6, and MCP-1	(Guo, Xia, et al. 2012)
T2DM mice (KK-A^y mice) (n = 16)	Controlled study	6 weeks	Black soybean seed coat extract;22 g/kg	↓ blood glucose ↑ insulin sensitivity	↑activation of AMPK signaling pathway	(Kurimoto et al. 2013)
T2DM mice (KK-A^y mice)(n = 16)	Controlled study	5 weeks	Cyanidin 3-glucoside; 0.2% of diet	↓ blood glucose ↑ insulin sensitivity	↑ the expression of GLUT4 ↓ the expression of RBP4	(Sasaki et al. 2007)
Mice fed a high-fat, high-calorie diet	Randomized controlled study	10 weeks	Juçara freeze-dried powder ; 5 g, 20 g/kg	↓ blood glucose ↓ catalase activity and IL-10 level in epididymal adipose tissue	NA	(Oyama et al. 2016)
T2DM mice(C57BL/KsJ-leprdb/leprdb)	Randomized controlled study	21 days	Buckwheat sprouts; 0%, 5%, or 10% of diet	↓ TC, TG, TBARS, and lipids ↓ HbA1c and arteriosclerotic index	↑ the expressions of HMG-CoAR and CYP7A1 (CYP7A1> HMG-CoAR)	(Watanabe and Ayugase 2010)
T2DM mice (STZ- induced diabetic mice)	Randomized controlled study	4 weeks	Padus racemose.	↓ blood glucose ↓ triglyceride, cholesterol, and HDL	NA	(Liu et al. 2020)
T2DM mice (STZ- induced diabetic mice)	Randomized controlled study	4 weeks	Black soybean seed coat extract; 100, 200, 400 mg/kg/day	↓ fasting blood glucose levels ↓ insulin levels and HOMA-IR ↑ HOMA-β ↑ glycogen content in liver and muscle ↓ NEFA, TG, and TC ↑ HDL-c level ↓liver, kidney and pancreas damage	↑ the enzyme activity of SOD, CAT, GPX	(Chen et al. 2018)
T2DM mice (C57BL/6J genetic background)	Randomized controlled study	5 weeks	blueberry anthocyanin extract; 100,400 mg/kg	↓ fasting blood glucose levels ↓ TC and TG levels ↓ insulin level ↑ liver antioxidant	↑ activation of AMPK signaling pathway ↑ the expressions of Glut2 ↑the enzyme activity of SOD ↓ the expressions of PGC1α, FoxO1, PEPCK, G6Pase	(Herrera-Balandrano et al. 2021)
T2DM mice (KK-A^y mice) and health mice (C57BL/6JmsSlc mice)	Randomized controlled study	4 weeks	aronia juice;	↓ blood glucose ↓ body weights ↓ weights of white adipose tissues	↓ DPP IV activity ↓ α-glucosidase activity in the upper portion of the small intestine to decrease GIP level	(Yamane et al. 2016)
T2DM mice (STZ- induced diabetic mice)	Randomized controlled study	12 weeks	Purple Sweet Potato; 200, 500 mg/kg	↓ blood glucose ↑ glucose tolerance ↓ LDL-c, TG, and TC ↑ HDL-c level ↓ liver damage ↑ antioxidant capacity ↓ level of MDA	↑ protein level of INSR ↑ the enzyme activity of SOD, CAT, GPX ↑ the expressions of PK, PFK, GLUT2 ↓ G6Pase and PEPCK ↑ activation of AMPK signaling pathway	(Jiang et al. 2020)
T2DM mice (C57BL/6J genetic background)	Randomized controlled study	8 weeks	Mulberry anthocyanin extract; 50, 125 mg/kg	↓ fasting blood glucose levels ↓ the level of insulin, TC, TG, and leptin ↑ the level of adiponectin	↑activating AKT and downstream targets in liver, skeletal muscle and adipose tissues ↑ mRNA expression of PCG-1α and FoxO1 to inhibit the activities of PEPCK and G6Pase ↑ activating AKT and GSK3β to upregulate GYS2	(Yan, Dai, and Zheng 2016)
T2DM mice (C57BL/6J genetic background)	Randomized controlled study	6 weeks	Cyanidin-3-O-glucoside; 150 mg/kg	↓ fasting blood glucose levels ↓ accumulation of liver lipids ↑ glycogen synthesis	↑ GLUT-1 via the Wnt/β-catenin-WISP1 signaling pathway	(Ye et al. 2022)
T2DM mice (n = 16)	Randomized controlled study	30 days	Black soybean seed coat extract; 30 mg/kg	↓the body and white adipose tissue weight ↓the size of adipocytes	↑ gene expression of PPARγ and C/EBPα ↑ adiponectin secretion	(Matsukawa et al. 2015)
T2DM mice	Randomized controlled study	8 weeks	Cyanidin-3-O-β-glucoside; 2 g/kg	↑endothelium-dependent relaxation of the aorta	↑ adiponectin expression and secretion by promoting FoxO1expression	(Liu et al. 2014)
T2DM mice (C57BL/6J genetic background) (n = 18)	Randomized controlled study	6 weeks	Cyanidin-3-O-glucoside; 150 mg/kg	↓ fasting blood glucose levels and body weight gain ↓ pancreas damage ↓ the level of ROS	↑ activating mitophagy via the PINK1-PARKIN signaling pathway ↑the enzyme activities of SOD and CAT.	(Ye et al. 2021)

AMPK: AMP-activated protein kinase; CPT1A: carnitine palmitoyl-transferase-1A; Acox: Acyl-CoA Oxidase; GLP-1: Glucagon-like peptide-1; PC1/3: prohormone convertase 1/3; G6Pase: glucose 6-phosphatase; PCK: Phosphoenolpyruvate carboxykinase; cJNK/FoxO1: c-Jun N-terminal kinase/forkhead box O1; Glut: glucose transporter; RBP4: retinol binding protein 4; HMG-CoAR: the 3-hydroxy-3-methylglutaryl-CoA reductase; CYP7A1: cholesterol 7α-hydroxylase; SOD: Superoxide Dismutase;CAT: catalase; GPX: Glutathione peroxidase; PGC-1α: Peroxisome proliferator-activated receptor gamma coactivator 1-alpha; PEPCK: Phosphoenolpyruvate carboxykinase; INSR: Insulin Receptor; PFK: Phosphofructokinase; GSK3β: Glycogen Synthase Kinase 3β; GYS2: Glycogen Synthase 2; C/EBPα: CCAAT enhancer-binding protein; PINK1: PTEN-induced kinase 1.

Mulberry anthocyanin extract not only alleviated insulin resistance and lowered blood sugar in HepG2 cells in vitro studies, but also improved parameters of blood sugar and blood lipids and increased adiponectin levels in db/db mice in vitro studies(Yan, Dai, and Zheng 2016). Similar results were also seen in db/db mice after 6 weeks of C3G intervention(Ye et al. 2022). Black soybean seed coat extract and cyanidin-3-glucoside (C3G) were shown to induce adipocytes to differentiate into smaller, more insulin-sensitive adipocytes in vivo and in vitro studies(Matsukawa et al. 2015). Moreover, after diabetic db/db mice received dietary C3G supplementation for 5 weeks, the mice had reduced hepatic triglyceride content and steatosis, as well as decreased serum concentrations of inflammatory cytokines(Guo, Xia, et al. 2012). In addition, C3G can improve endothelial function and alleviate oxidative stress in pancreas islets.(Liu et al. 2014, Ye et al. 2021).

In addition, anthocyanins also have synergistic effects with conventional T2DM drugs. After combined treatment with fenofibrate and anthocyanins (from Black soybeans), the serum postprandial triglyceride level and LDL cholesterol concentration were significantly lowered in T2DM patients complicated with postprandial hyperlipidemia, compared to that with Fenofibrate alone(Kusunoki et al. 2015). In an animal experiment, malvidin, an anthocyanins compound, was combined with metformin to treat STZ-induced diabetic rats, and the results showed that the combination therapy had more alleviated insulin resistance, improved lipid metabolism, and reduced fasting blood glucose, serum insulin compared with single therapy(Zou et al. 2021). Similarly, metformin and anthocyanin exhibit significant synergistic effects in lowering blood glucose and improving insulin resistance both in vitro and in vivo(Tian et al. 2022).

Clinical trial

In a clinical trial, T2DM individuals after 4 weeks of ingestion of 320 mg of anthocyanins per day significantly reduced fasting blood glucose and LDL-cholesterol and some proinflammatory molecules such as IL-6, IL-18, and tumor necrosisi factor (TNF-α), increased some anti-inflammatory molecules such as IL-10 and adiponectin(Nikbakht et al. 2021). However, in another randomized, placebo-controlled study, short-term (4 weeks) supplementation with anthocyanins from bilberry had no significant effect on patients with T2DM. It may need longer treatment period to have beneficial effects based on trends in glycemic control(Chan et al. 2021). Moreover, bilberry extract has effects on glucose metabolism in T2DM, postprandial glycemia and insulin of T2DM subjects decreased after a single oral dose of bilberry extract that contains 36% (w/w) anthocyanins(Hoggard et al. 2013). Anthocyanins from Vaccinium Arctostaphylos extract lowered the blood levels of fasting glucose and 2-h postprandial glucose(Kianbakht, Abasi, and Dabaghian 2013). When some biochemical indicators in patients with T2DM after given 160 mg of anthocyanins for 24 weeks were examined, purified anthocyanin improving dyslipidemia, enhancing antioxidant capacity, and preventing insulin resistance(Li et al. 2015). In subject with T2DM, blood glucose and antioxidant capacity were improved, and lipid peroxidation and inflammatory responses were alleviated by freeze-dried strawberry intake for six weeks(Moazen et al. 2013). In a pomegranate juice study examining subjects with T2DM, after a 12-hour fast, subjects were given 1.5 mL/kg body weight pomegranate juice and then serum glucose levels were measured in 1 h and 3 h after the consumption of the juice. Results showed that subjects’ fasting blood glucose decreased, beta-cell function and insulin resistance improved(Banihani et al. 2014). In another clinical study, individuals with T2DM were given 0.47 g bilberry extract for three weeks. The postprandial glycemia decreased while insulin levels did not change(Alnajjar et al. 2020).

Mechanism of anthocyanins prevention and treatment of type 2 diabetes

With progression of insulin resistance, lipid metabolism disorders intensify, and blood glucose increases that promote inflammation and oxidative stress ultimately leading the progression of T2DM and the appearance of various complications. These above features of DM are often already present in prediabetes. Anthocyanins improve these features through various mechanisms to prevent prediabetes from progressing to DM. These mechanisms also involve the treatment of T2DM. Thus, we discussed the mechanism of anthocyanins in the prevention and treatment of T2DMtogether (Figure 2).

Figure 2. The mechanism of health benefits of anthocyanins on diabetes mellitus and pancreatic islets.

NF-κB: nuclear factor kappa-B; cJNK: c-Jun N-terminal kinase; AMPK: AMP-activated protein kinase; GLP-1: Glucagon-like peptide-1; PC1/3: prohormone convertase 1/3; FoxO1: forkhead box O1; PGC-1α: Peroxisome proliferator-activated receptor gamma coactivator 1-alpha; GSK3β: Glycogen Synthase Kinase 3β; GYS2: Glycogen Synthase 2; G6Pase: glucose 6-phosphatase; PEPCK: Phosphoenolpyruvate carboxykinase; ATGL: Adipose triglyceride lipase; CPT-1: Carnitine palmitoyl transferase I; PPAR: peroxisome proliferator-activated receptor; SREBP-1: sterol regulatory element binding protein; CYP7A1: Cytochrome P450 Family 7 Subfamily A Member 1; MMP: Matrix metalloproteinase; TIMP: Tissue matrix metalloproteinase; ICAM-1: leukocyte adhesion factors; VCAM-1: vascular cell adhesion molecule-1; Monocyte chemoattractant protein-1; Ngn3: neurogenin3; IRS: insulin receptor substrate; PIK3ca: phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha; PDK1: Phosphoinositide-dependent kinase-1;

1. improving insulin resistance:

   PI3K/AKT pathway, the insulin signal transduction pathway, accelerates glycogen synthesis and restrain the gluconeogenesis. Proinflammatory cytokines can suppress PI3K/AKT pathway by activation of the inhibitor of kappa B kinase (IKK)/nuclear Factor kappa B(NF-κB) pathway. Anthocyanins can reduce the activation of proinflammatory factors through their anti-inflammatory effects, but also directly activate the P13K/AKT pathway to improve insulin resistance(Yan, Dai, and Zheng 2016, Daveri et al. 2018). Furthermore, phosphorylation of insulin receptor substrate 1 (IRS-1) serine residues also inhibits insulin signaling causing insulin resistance. The activation of Jun NH2-terminal kinase (JNK) and NF-κB is inhibited by anthocyanins to reduce the phosphorylation of IRS-1 serine residues (Daveri et al. 2018, Lee et al. 2018). In addition, anthocyanin has been shown to induce adiponectin, which has some insulin-sensitizing properties thus this may be one of the mechanisms for improving insulin resistance(Liu et al. 2014, Li et al. 2015, Matsukawa et al. 2015, Yan, Dai, and Zheng 2016, Nemes et al. 2019, Yang et al. 2020).

2. improve blood glucose levels:

   Currently, anthocyanins improve the hyperglycemic state of T2DM through various mechanisms to decrease the risks of developing T2DM. First, anthocyanins activate AMP-activated protein kinase (AMPK) which is a key molecule in the regulation of bioenergy metabolism in white adipose tissue, skeletal muscle, and the liver(Takikawa et al. 2010, Kurimoto et al. 2013, Choi et al. 2016, Iizuka et al. 2018, Jiang et al. 2020). In white adipose tissue and skeletal muscle, activation of AMPK induces glucose transporters 4(GLUT4) to enhance glucose uptake and utilization(Sasaki et al. 2007, Iizuka et al. 2018). In the liver, activation of AMPK reduces glucose production. Second, anthocyanins as participants in glucagon-like peptide-1(GLP-1)/GLP-1 receptor (GLP-1R) related signaling pathway(Tsuda 2017, Daveri et al. 2018). GLP-1, a significant molecular target for the treatment of T2DM, produced in the gut binds to the GLP-1R on pancreatic β cells, then activates the cAMP/PKA pathway to promote release of insulin and growth of β cells (Jones et al. 2018).And anthocyanins increase the secretion of GLP-1 by upregulation of prohormone convertase 1/3 (PC1/3)and proglucagon that are involved in the production of GLP-1(Iizuka et al. 2018, Tian et al. 2020). Third, anthocyanins lower blood glucose by regulating glycogen metabolism. The activities of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase(G6Pase) inhibited by anthocyanins through upregulating mRNA expression levels of peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1α (PGC-1α) and forkhead box protein O1 (FoxO1), thereby reducing glycogen breakdown(Alvarado, Schoenlau, et al. 2016, Jiang et al. 2020). Furthermore, phosphorylation of AKT and glycogen synthase kinase-3β (GSK3β) are activated by anthocyanins, leading to the upregulation of glycogen synthase 2 (GYS2)(Yan, Dai, and Zheng 2016, Herrera-Balandrano et al. 2021). And anthocyanins inhibitα-amylase activity that is present in the lumen of the small intestine to improve postprandial hyperglycemia(Yamane et al. 2016, Chen et al. 2018, Alnajjar et al. 2020).Through the glucose transporters 2 (GLUT2) and sodium/glucoseco transporter - 1 (SGLT1), glucose can be absorbed from the small intestine into the blood vessels, thus causing postprandial hyperglycemia(Williamson 2013). Some studies have confirmed that anthocyanins can inhibit glucose transporters (SGLT1 and GLUT2) to reduce the absorption of glucose in the intestine, improving postprandial blood glucose(Hidalgo et al. 2014, Jiang et al. 2020, Herrera-Balandrano et al. 2021). In addition to GLUT-4 and GLUT-2, study have shown that anthocyanins may up-regulate the expression of GLUT-1 by activating the Wnt/β-catenin-WISP1 signaling pathway, thereby reducing blood glucose levels(Ye et al. 2022).

3. improving lipid metabolism:

   Several studies have reported multiple mechanisms of anthocyanins in improving lipid metabolism. First, many enzymes involved in fatty acid and triacylglycerol synthesis and metabolism, such as Acyl-CoA synthase1(ACS1), fatty acid synthase (FAS), and glycerol-3-phosphate acyltransferase (GPAT), can be inhibited by anthocyanins(Wu, Jiang, et al. 2016, Song, Shen, Wang, et al. 2021, Song, Shen, Wang, et al. 2021). And sterol regulatory element binding protein (SREBP-1) is regulated by insulin and involved in lipase expression. Anthocyanins reduced the expression of SREBP-1 to decrease the fatty acid synthesis(Park et al. 2015). Secondly, anthocyanins activated the gene expression of cholesterol metabolism-related enzymes such as cholesterol 7α-hydroxylase (CYP7A1) and 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoAR). Promoting cholesterol breakdown over cholesterol synthesis, thereby improving the blood lipid parameters of DM(Watanabe and Ayugase 2010). Some studies also showed anthocyanins play anti-obesity effects by enhancing antioxidant capacity and improving inflammation levels(Farrell et al. 2015, Wu et al. 2018, Nemes et al. 2019). Third, anthocyanins may improve adipocyte metabolism by regulating transcription factor FoxO1-mediated transcription of adipose triglyceride lipase (ATGL) (Guo, Guo, Guo, et al. 2012). Fourth, anthocyanins were shown in other studies to reduce hepatic triglyceride accumulation and improve lipid metabolism by activating AMPK signaling pathways that are key regulators of hepatic lipid metabolism(Hwang et al. 2011, Saito et al. 2016). Fifth, anthocyanins also inhibited the expressions of adipogenic genes such as PPARγ and enhanced the expression of carnitine palmitoyl transferase (CPT-1) and PPARα, which are associated with fat oxidation(Seymour et al. 2011, Farrell et al. 2015, Noratto et al. 2018, Lim et al. 2019, Park et al. 2019, Kim et al. 2020, Lee et al. 2021).

4. improving oxidative stress:

   Cell damage-induced by oxidative stress plays an important role in progression of T2DM, so the antioxidant capacity of anthocyanins is also one of the reasons for the anti-T2DM effect. Anthocyanins scavenge excess free radicals and reduce damage caused by oxidative stress by activating endogenous antioxidant defense systems(Li et al. 2015). Studies have consistently shown that anthocyanins improve the activity of enzymatic antioxidants in the body, such as superoxide dismutase (SOD) and catalase (CAT), enhance their ability to protect cells from oxidative damage by catalyzing the conversion of free radicals into hydrogen peroxide (Chen et al. 2018, Nemes et al. 2019, Ye et al. 2021). In addition, anthocyanins also enhance Glutathione peroxidase (GPx) and glutathione reductase (GR) activity, and accelerate the synthesis of glutathione (GSH), ultimately improving plasma antioxidant activity in T2DM populations(Chen et al. 2018, Qin et al. 2018). Oxidative stress has been shown to activate P38 MAPK signaling pathway, which play a crucial role in oxidative stress response. And the activation of P38 MAPK can be inhibited by anthocyanins(Liu et al. 2020). Andeven research shown that anthocyanins indirectly exert antioxidant effects by regulating the production of the superoxide anion and nitric oxide through their metabolites(Daveri et al. 2018).

5. improving inflammation:

   Long-term Inflammatory response may contribute to insulin resistance eventually leading to T2DM, and promote the progression of DM complications(Lontchi-Yimagou et al. 2013). Some inflammatory cytokines TNF-α, IL-6, IL-1 play a key role in the inflammatory response, while anthocyanins can inhibit the expression of these inflammatory cytokines, thus playing an anti-inflammatory role(Qin and Anderson 2012, Farrell et al. 2015, Hsu et al. 2016, Wu, Jiang, et al. 2016, Qin et al. 2018, Nemes et al. 2019). Monocyte chemoattractant protein 1 (MCP-1) is a chemokine that regulates the migration and infiltration of leukocytes and is involved in the pathogenesis of DM(Mussa et al. 2021). Many studies have proved that anthocyanins can reduce the expression of MCP-1(Farrell et al. 2015, Nemes et al. 2019). And anthocyanins also inhibit expression of these inflammatory cytokines gene by suppressing IKK/NF-κB pathway(Sarikaphuti et al. 2013, Daveri et al. 2018, Huang et al. 2018, Wu et al. 2018). At the same time, the secretion of adiponectin increased by anthocyanins, thereby reducing TNF-α and IFN-γ expression, stimulating the production of anti-inflammatory IL-10, and converting the phenotype of macrophages from proinflammatory type to the anti-inflammatory type(Oyama et al. 2016, Nemes et al. 2019). In addition, leptin produced by adipocytes can regulate immune response, including activation of immune cells and induction of pro-inflammatory mediators(Maya-Monteiro and Bozza 2008). And anthocyanins have been shown in multiple studies to inhibit leptin production(Sarikaphuti et al. 2013, Yan, Dai, and Zheng 2016, Nemes et al. 2019, Nikbakht et al. 2021).

Anthocyanins and complication of diabetes mellitus

DM and its macrovascular and microvascular complications cause enormous psychological and physical distress to patients and a huge burden on social care systems. Several studies have demonstrated positive effects of anthocyanins on DM complications (Table 3).

Table 3. The treatment of anthocyanins on diabetic complications in animal trial.

Subject	Organ	intervention duration	AdministeredForm and dosage of anthocyanins	Main results	Mechanism	references
STZ-induced diabetic rats	Heart	4weeks	Anthocyanin.; 250 mg/kg/day	↓ cardiac inflammation ↓ cardiac hypertrophy ↓ cardiac fibrosis ↑cardiaccontractile function	↓ TLR4/NFκB expression ↓TGF-β1 and FGF2 expression ↓ MMP-9 ↑TIMP-1	(Chen et al. 2016)
STZ-induced diabetic mice	Heart	12weeks	Anthocyanin; 250 mg/kg/day	↓ cardiac inflammation ↓cardiac fibrosis	↓ miR-214-mediated IL-17 expression ↓IL-1β, TNF-α, and IL-6 ↓collagen I and III	(Yue et al. 2020)
STZ-induced diabetic mice	Heart	4weeks	Anthocyanin; 250 mg/kg/day	↑ cardiomyocyte survival ↑ cardiac function	↓ cytochrome c and caspase ↑ IGFIR/PI3K/Akt survival pathway ↑ anti-apoptotic proteins Bcl-2	(Huang et al. 2017)
STZ-induced diabetic mice	Heart	12 weeks	Protocatechuic acid; 50,100mg/kg/day	↓ MDA levels ↓ cardiac contractile dysfunction ↑ heart rate variability	↑ anti-apoptotic proteins Bcl-2 ↓ cardiac mitochondrial dysfunction	(Semaming et al. 2014)
Db/db mice	Kidney	8 weeks	Purple corn extract; 10 mg/kg/day	↓ mesangial expansion-elicited renal injury ↓ the infiltration and accumulation of macrophages ↓ glomerulosclerosis.	↓ IL-8-Tyk2-STAT signaling ↓ ICAM-1 and CD11b expression	(Kang et al. 2012)
Db/db mice	Kidney	8 weeks	Purple corn extract; 10 mg/kg/day	↓ glomerular fibrosis ↓ renal filtration dysfunction ↓ glomerulosclerosis ↑ nephrin and podocin proteins	↓ IL-8-Tyk2-STAT signaling ↓ collagen IV expression ↓ renal TGF-β and CTGF expression ↑ MMP and ↓ TIMP-2	(Li, Kang et al. 2012)
STZ-induced diabetic mice	Kidney	8 weeks	M.cauliflora extracts; 0.1%, 0.5%,1% of diet	↓ renal injury ↓ the ratio of urine albumin/urine creatinine ↓extracellular matrix accumulation of glomeruli	↓Ras/PI3K/Akt signaling pathway	(Wu, Hung, et al. 2016)
STZ-induced diabetic mice	Kidney	8 weeks	M. cauliflora extracts; 0.1%, 0.5%,1% of diet	↓oxidant and inflammatory damage in the kidney ↓ renal fibrosis	↑ GSH, CAT, SOD, GST, GPx levels ↓ ICAM-1, VCAM-1, MCP-1, CSF-1, TNF-α, IL-1β and IL-6 expression ↓ JAK/STAT signaling pathway ↓ PKC/NF-kB signaling pathway	(Hsu et al. 2016)
STZ-induced diabetic mice	Kidney	8 weeks	Cyanidin-3-glucoside; 10, 20 mg/kg/day	↓ renal dysfuntion ↓ kidney index ↓ renal oxidative stress and inflammation ↓ renal fibrosis ↓ renal pathological change ↓ glomerulosclerosis	↓ TGF-β1/Smad2/3 signaling pathway ↓TNF-α, MCP-1, NF-kB, IL-1β and IL-6 expression ↑ GSH, CAT, SOD levels	(Zheng et al. 2020)
Db/db mice	Kidney	12 weeks	Cyanidin-3-glucoside; 20 mg/kg/day	↓ renal injury ↓ renal fibrosis ↓ renal inflammation	↑ GSH level ↓TNF-α, MCP-1 and IL-1α expression ↓ mRNA expression of collagen IV, fibronectin, TGFβ1, MMP9 and α-SMA	(Qin et al. 2018)
Db/db mice	Kidney	12 weeks	Grape seed procyanidin; 30 mg/kg/day	↓ renal cell apoptosis	↓ caspase-3, ↑ BCL-2, ↓ Bax ↓ BAX/BCL-2 ↓ p38 MAPK and ERK1/2 activity	(Wei et al. 2018)
Zucker diabetic fatty rat	Kidney	24 weeks	red orange and lemon extract; 90 mg/kg/day	↓ renal injury	↓ apoptotic protein BAX/BCL-2 ratio ↓ NADPH Oxidase 4(NOX4) expression	(Damiano et al. 2020)
Db/db mice	Kidney	14 weeks	Blueberry and sea buckthorn extract; 6 g/kg/day	↓ glomerular water permeability ↓ albuminuria ↓ glomerular fibrosis ↓ glomerular ultra-structural changes ↓ renal endothelial insult	↑anti-angiogenic VEGF-A expression ↓total VEGF-A expression ↓ NF-kB, Akt, ERK1/2 and COX-2 expression	(Stevens et al. 2019)
Db/db mice	Kidney	12 weeks	Seoritae extract; 10 mg/kg/day	↓ albuminuria ↓intra-renal lipid concentrations ↓glomerular matrix expansion and inflammation	↑ AMPK signaling pathway ↑ PPAR α and PPARγ level ↓SREBP-1 expression ↑ BCL-2, ↓BAX, ↓BAX/BCL-2	(Koh et al. 2015)
Zucker diabetic fatty rat	Kidney	24 weeks	red orange and lemon extract; 90 mg/kg/day	↓ renal oxidative stress	↑ GSH, CAT, SOD levels	(Damiano et al. 2019)
STZ-induced diabetic mice	Retina	12 weeks	Blueberry anthocyanins; 20, 40, 80 mg/kg/day	↓ oxidative stress and inflammation in the retina	↑ Nrf2/HO-1 signaling pathway ↑ GSH and GPx level	(Song, Huang, and Yu 2016)
STZ-induced diabetic mice	Retina	6 weeks	V. myrtillus extract; 100 mg/kg/day	↓ blood–retinal barrier breakdown ↑ blood–retinal barrier function	↓ VEGF protein and mRNA expression ↑ Tight junction proteins expression	(Kim, Kim et al. 2015)
STZ-induced diabetic mice	Retina	8 weeks	Cyanidin-3-o-glucosid; 20 mg/kg/day	↓ inflammation and microglial activation	↑ tight junction proteins ↓ TNF-α, IL-1β, and IL-6 expression	(Zhao et al. 2021)

TLR4: Toll-like receptors 4; NF-κB: nuclear factor kappa-B; TGF-β: transforming growth factor β1; FGF2: Fibroblast Growth Factor 2; MMP: Matrix metalloproteinase; TIMP: Tissue matrix metalloproteinase; IGFIR: insulin-like growth factor I receptor; ICAM-1: leukocyte adhesion factors; TGF-β: Transforming Growth Factor β; CTGF: Connective tissue growth factor; VCAM-1: vascular cell adhesion molecule-1; MCP-1: Monocyte chemoattractant protein-1; CSF-1: macrophage-stimulating factor; NOX4: NADPH Oxidase 4; AMPK: AMP-activated protein kinase;SREBP-1: sterol regulatory element binding protein; PPAR: peroxisome proliferator-activated receptor; Nrf2: NF-E2–related factor 2.

AOPPs: Advanced Oxidation Protein Products; OMPs: Ozone Mapping and Profiler Suite; AGEs: advanced glycation end products; MDA: Malondialdehyde.

Heart complications

Cardiovascular complications are the leading cause of death and disability in people with DM(Lin et al. 2014). The main risk factor for cardiovascular disease in T2DM is Dyslipidemia, which is characterized by low HDL-cholesterol concentrations, and high LDL-cholesterol concentrations and TGs concentrations(Li et al. 2015). By causing chronic inflammation of the heart, hypertrophy, apoptosis and fibrosis, DM eventually leads to myocardial remodeling, deteriorated cardiac function, and even most debilitating prognosis––heart failure(Chen et al. 2016).

Platelets play an important role in the development of atherosclerosis and consequently CVD in DM, and anthocyanins have potential antiplatelet ability that subsequently can prevent atherosclerosis and CVD(Gaiz et al. 2018). Bilberries and blueberries, anthocyanin-rich fruits, have the potential to curb cardio-metabolic perturbations(Crespo and Visioli 2017). In animal research on anthocyanins from purple rice, the extent of myocardial injury and cardiac function loss in mice with DM caused by STZ-induced decreased after 4 weeks of diet anthocyanins, and myocardial hypertrophy and fibrosis were significantly reduced(Chen et al. 2016). STZ-induced rats that consumed anthocyanins had enhanced cardiac function by improving inflammation and fibrosis in the heart tissue (Yue et al. 2020). In another study, STZ-induced rats enhanced cardiomyocyte survival and restores cardiac function after receiving 4 weeks of anthocyanin treatment(Huang et al. 2017). In addition, protocatechuic acid, a main metabolite of anthocyanin, was confirmed to have potential protective effects on the hearts of STZ-induced T1DM rats, mainly including improving cardiac function, preventing myocardial mitochondrial dysfunction, and increasing B-cell lymphoma-2 expression (Semaming et al. 2014).

Mechanism

High glucose induced inflammation and myocardial fibrosis are two major pathogenic factors of diabetic cardiac complications.Interleukin-17(IL-17), as a pro-inflammatory cytokine, is known to play a critical role in cardiac fibrosis(Valente et al. 2012). Anthocyanins can reduce the expression of IL-17 by regulating miR-214-3p, which is a short non-coding RNAs that controls the process of inflammation, thus protecting cardiac function(Yue et al. 2020). Metalloproteinase (MMPs) can promote the transformation of fibroblasts into myofibroblasts leading to myocardial remodeling, and this effect can be inhibited by their tissue inhibitors (TIMPs) (Hansson et al. 2009). Anthocyanins can regulate homeostatic equilibrium between MMP and TIMP-1 and inhibit transforming growth factor β(TGF-β) and fibroblast Growth Factor 2 (FGF2) that are associated with cardiac fibrosis(Chen et al. 2016). Chen, Y. F et al also found that anthocyanins reduced toll-like receptors-4(TLR4)/NF-κB activation and improved myocardial inflammation and function. Hyperglycemia-induced cardiomyocyte necrosis and apoptosis is also one of the pathogeneses of diabetic cardiac complications. IGFI-R/PI3K/AKT survival signaling, and Bcl-2 family associated anti-apoptotic mechanisms are enhanced by anthocyanins, while caspase family and Bad and Bak proteins related to apoptosis mechanisms are suppressed(Huang et al. 2017).

Diabetic nephropathy

DN is one of the main complications of DM, and it is also the main cause of end-stage renal disease in the world(Qi et al. 2017). Hyperglycemia and inflammation pathways play a significant role in the progression of DN, which is characterized by accumulation of extracellular matrix proteins and irreversible decline in renal function(Wada and Makino 2013). Purple corn extract inhibited glomerular monocyte activation, macrophage infiltration, mesangial hyperplasia, and collagen fiber accumulation and ameliorated severe albuminuria in db/db mice(Kang et al. 2012, Li, Kang et al. 2012). In a study where STZ-induced C57BL/6J mice were fed a high fat diet and varied concentrations of myrciaria cauliflora extracts rich in anthocyanins, after treatment mice had improved renal injury and decreased accumulation of saccharide and formation of collagen IV (Hsu et al. 2016, Wu, Hung, et al. 2016). Similar result was observed in rats with DN(Zheng et al. 2020). After db/db mice were treated with C3G, renal function and histology were improved, and renal hypertrophy was reduced. Moreover, C3G exhibits downregulation of glucose level, hypolipidemic effect, antioxidant effect, and anti-inflammatory effect in db/db mice, which may play a protective role against DN (Qin et al. 2018, Wei et al. 2018). Kidney damage and the oxidative DNA damage in Zucker diabetic fatty rat were alleviated after supplementation with red orange and lemon extract through gavage for 24 weeks(Damiano et al. 2020). DIAVIT, a blueberry and sea buckthorn extract, inhibited the development of albuminuria and the enhancement of glomerular water permeability and prevented fibrotic glomeruli, endothelial insult, and glomerular ultra-structural changes(Stevens et al. 2019). Anthocyanins also improved intra-renal lipid concentrations and reduced renal apoptosis and oxidative stress in db/db mice with improvement of glomerular inflammation(Koh et al. 2015). However, in another animal experiment, kidney inflammation did not significantly improve(Damiano et al. 2019).

Protocatechuic acid, an anthocyanin metabolite, was shown in an in vitro study to ameliorates extracellular matrix accumulation to prevent DN(Ma, Chen et al. 2018). In many vitro studies, purple corn extract was shown that suppressed renal mesangial fibrosis and inflammation induced by high glucose and prevented glomerular angiogenesis (Li, Kang et al. 2012, Li, Lim, et al. 2012, Kang et al. 2013).

Mechanism

Renal function can be improved by anthocyanins via deceasing renal inflammation and enhancing the ability of antioxidative(Koh et al. 2015, Hsu et al. 2016, Qin et al. 2018, Zheng et al. 2020). NF-κB plays a pivotal role in the process of cellular inflammation and immune response that triggers renal fibrosis(Manucha 2014). After oxidative stress was ameliorated by anthocyanins, a decrease in the expression of the PKC enzymes/NF-κB that mediated DN(Hsu et al. 2016). The Ras pathway is extensively activated in renal cells by high glucose, promoting cell proliferation and epithelial-mesenchymal transition leading to renal fibrosis(Hirose et al. 2010). Anthocyanins can reduce Ras-related protein expression and inhibit Ras/PI3K/Akt signaling pathway to ameliorate renal mesangial fibrosis(Wu, Hung, et al. 2016). The IL-8/Tyk2/STAT signaling pathway plays a major role in the progression of DN by promoting renal mesangial fibrosis and inflammatory cell infiltration, while this pathway can be blocked by anthocyanins(Kang et al. 2012, Li, Kang et al. 2012). In addition, anthocyanins can also inhibit the TGF-β1/Smad2/3 pathway that is involved in ECM synthesis and accumulation to improve renal fibrosis(Zheng et al. 2020).Apoptosis of kidney cells is also one of the main pathogeneses of DN(Sanchez-Niño et al. 2010). And this cell apoptosis can be attenuated by anthocyanins by inhibiting the expression of apoptosis-related proteins, such as caspase-3, modulating the Bax/Bcl-2 ratio, and reducing phosphorylation of p38 MAPK and ERK1/2(Wei et al. 2018, Damiano et al. 2020).

Diabetic retinopathy

DR, a serious complication of DM, is considered the leading cause of vision loss and blindness worldwide(Singh et al. 2019). DR can be divided into non-proliferative retinopathy (NPDR) and proliferative DR (PDR) based on different phases. The characterization of DR is dysfunction of retinal blood vessels and breakdown of retinal blood barrier, which can be caused by oxidative stress and chronic inflammation induced by altered metabolites and hormones in DM(Zhang, Liu, et al. 2011, Preedy 2014).

In a blueberry study observing STZ-induced rat, after 12 weeks of taking either 20, 40, or 80 mg/kg, blood–retinal barrier breakage improved in rat retinas of all groups taking blueberry anthocyanins(Song, Huang, and Yu 2016). At that time STZ-induced rat received gastric gavage of bilberries extract had lower the fluorescein leakage in the fluorescein-dextran angiography and the level of retinal vascular endothelial growth factor (VEGF) that a remark of DR than control group without bilberries extract(Kim, Kim et al. 2015). Similar results were observed in another study that retinal vascular leakage was attenuated after STZ-induced diabetic mice were administered with C3G. In addition, C3G can inhibit retinal glial cell proliferation and angiogenesis(Zhao et al. 2021).

Mechanism

Anthocyanins may have protective effects on retina by their antioxidant and anti-inflammatory properties. For example, anthocyanins may regulate Nrf2/heme oxygenase-1 (HO-1) signaling pathway to improve antioxidant capacity and inflammation levels in retinal cells(Song, Huang, and Yu 2016). Furthermore, anthocyanins inhibit the mRNA and protein expression of VEGF that is related to blood–retinal barrier breakage to prevent DR(Kim, Kim et al. 2015). Microglia, which are monocytes resident in the retina, have been shown to play an important role in diabetic retinopathy(Altmann and Schmidt 2018). And anthocyanins regulate activation of microglia to delay the process of DR. In addition, anthocyanins improve retinol vascular permeability by increasing the expression of tight junction proteins, such as occludin-1, claudin-1, and ZO-1(Kim, Kim et al. 2015, Zhao et al. 2021).

Type 1 diabetes mellitus

Animal trial

In this section, we analyzed the benefits of anthocyanins for T1DM (S2 Table). Overnight-fasted mice induced by single streptozotocin (STZ), which inhibits insulin secretion, can be used as an animal model of T1DM to explore the mechanism of action. In a study where STZ-induced diabetic mice were fed by normal diet or normal diet plus 10 mg/kg aronia berry (contains a large amount of natural antioxidant) extract or normal diet plus 100 mg/kg aronia berry extract, finally mice receiving normal plus 100 mg/kg aronia berry extract had better lipid and blood glucose metabolic markers and better insulin levels(Jeon et al. 2018). Anthocyanins from other four plants have also been shown to reduce blood glucose and blood lipids in STZ-Induced animal research(Strugała, Dzydzan et al. 2019, Vilhena et al. 2020). Meanwhile, the fasting glucose of delphinol-treated STZ-induced rats was significantly improved(Hidalgo et al. 2014). The extracts of red and yellow fruits of cornelian cherries, containing a significant number of anthocyanins, were used to feed rats with STZ-induced T1DM. After 14 days of feeding, rats dosed with the extracts had lower blood glucose concentrations and amounts of glycated hemoglobin, greater erythrocyte resistance to acid hemolysis, and higher reduced glutathione and mean red blood cell levels than control groups(Dzydzan et al. 2019). In another studies, rats with STZ-induced diabetes were given red cabbage extract, and after 4-week rats also received similar antidiabetic effects, and islet morphology and beta cell numbers also were restored (Buko et al. 2018). Yue et al proved that anthocyanins not only reduced blood glucose in T1DM mice, but also delayed progress in cardiovascular complications of DM(Yue et al. 2020). Nizamutdinova et al observed that anthocyanins from black soybean seed coats improved blood glucose and lipids and insulin resistance, reduced islet cell apoptosis, enhanced antioxidant capacity, and even improved cardiac function in STZ-induced rats(Nizamutdinova et al. 2009).

Mechanism

Autoimmunity, a major pathogenesis of T1DM, can not only directly lead to islet cell damage and destruction, but also release inflammatory cytokines to cause islet inflammation, which gradually leads to the progression of T1DM(Bending, Zaccone, and Cooke 2012). Furthermore, oxidative stress induced by hyperglycemia could cause islet β-cell apoptosis and exacerbate the inflammatory response via promoting the expression of inflammatory cytokines and chemokines(Novoselova et al. 2021). Cyclooxygenase-2 (COX-2) is the key to triggering inflammation, while inducible iNOS synthesizes large amounts of NO that lead to aggressive oxidative stress. Anthocyanins block mRNA expression level of COX-2 and iNOS thereby improving inflammation and oxidative stress(Jeon et al. 2018). In addition, improving the expression of some antioxidant enzymes, such as CAT, SOD, and GSH, is also anti-T1DM mechanism of anthocyanins(Soon and Tan 2002, Buko et al. 2018, Strugała, Dzydzan et al. 2019). The P38 MAPK pathway that is involved in apoptosis can be activated by oxidative stress. Some proteins including Bcl-2 family, Cyt-c and caspase-3 are negatively regulated by upstream protein MAPK, thus lead to apoptosis of cells(Fu et al. 2006). Anthocyanins can inhibit the activation of P38 MAPK pathways and regulate Bax/Bcl-2 ratios and caspase-3 activity to improve islet cell apoptosis and oxidative stress to maintain islet function(Nizamutdinova et al. 2009, Lee, Kim et al. 2015). In addition, anthocyanins can reduce the production of ROS by controlling blood glucose(Dzydzan et al. 2019).

Anthocyanins and islet function and transplantation

Pancreatic islets, which are composed of a group of endocrine cells, perform many functions by releasing hormones. The most important function is regulated glucose metabolism, with hyperglycemia-stimulating insulin release by β-cells and decrease in glycemia-triggering glucagon secretion by α-cells(Thorens 2014). Part of anthocyanins have been shown to be capable of promoting insulin secretion at 4 and 10 mM glucose concentrations in pancreatic β-cells(Jayaprakasam et al. 2005). Similar results were reported on isolated mice pancreatic islets in the presence of 5.5 mM and 16.5 mM glucose concentration(Doshi et al. 2015). Anthocyanins from purple corn have also been shown to promote insulin secretion in iNS-1E cells(Luna-Vital and Gonzalez de Mejia 2018). In an in vitro study, cyanidin has proven to stimulate islet β-cells to release insulin through by Ca2+ influx via voltage-dependent Ca2+ channels and showed dose response relationship. At the same time, cyanidin increased the expression of genes, such as GLUT2, Kir_6.2 and Cav_1.2, which are associated with insulin release(Suantawee et al. 2017). In addition to voltage-dependent Ca 2+ channels, anthocyanin also promoted insulin release through activating the PLC/IP3 pathway(Kongthitilerd et al. 2022). Blueberry-leaf extract reduced compensatory increase in the pancreatic islet size and insulin content induced by hyperglycemia. At that time the expression of β-cell proliferation-related genes increased and the expression of β-cell apoptosis-related genes inhibited after high fat diet-fed mice were given blueberry-leaf extract (Li et al. 2020). Anthocyanins prevented pancreatic cell apoptosis in STZ-induced diabetic rats by regulating apoptosis-related proteins(Nizamutdinova et al. 2009). In STZ-induced diabetic rats, anthocyanins can also protect the islet morphology and degeneration, and activate insulin receptor phosphorylation to restore the number of β-cells(Jayaprakasam et al. 2006, Nizamutdinova et al. 2009, Nasri et al. 2011, Sarikaphuti et al. 2013, Buko et al. 2018). In addition, after supplementation with anthocyanins in diabetic animal models, the antioxidant capacity in pancreas islets was improved, thus reducing islet cell damage, and improving islet function(Ji et al. 2015, Ye et al. 2021).

Previously, our group demonstrated that anthocyanins from Chinese bayberry have a positive effect on improving the antioxidant capacity of islet cells after islet transplantation(Zhang, Kang, et al. 2011, Zhang et al. 2013, Cai et al. 2015, Li, Yang, et al. 2017, Croden et al. 2021). In an in vitro study, pretreatment of INS-1 cells with anthocyanins reduced cell death caused by H₂O₂ compared to the control group. In subsequent animal experiments, anthocyanins were shown to effectively reduce graft apoptosis in the β-cell transplantation STZ-induced mice by up-regulating HO-1 expression through via activation of PI3K/Akt and ERK1/2(Zhang, Kang, et al. 2011). Oxidative stress-mediated autophagic cell death in INS-1 cells and grafts was reduced after pretreated with anthocyanins(Zhang et al. 2013). C3G, a major anthocyanin component of Chinese bayberry, was used to treat islets from B6 mice before transplantation. After transplantation, islets treated with C3G showed stronger viability and function than untreated islets(Cai et al. 2015). In an animal experiment, transplantation of C3G-treated porcine islets into STZ-induced mice further demonstrated our previous study that anthocyanins reduced damage caused by oxidative stress and enhanced islet function after islet transplantation(Li, Yang, et al. 2017). In vitro, C3G was shown to effectively reduce the expression of inflammatory markers IL-1β and OD-like receptor thermal protein domain associated protein 3 (NLRP3) and increase the expression of LC3 autophagic marker and HO-1 in human islet after transplantation, and thus may play a protective role in islet cell(Croden et al. 2021). The mechanism by which anthocyanins improve islet transplantation and islet function was shown in Figure 3.

Figure 3. The mechanism of anthocyanins on islets function and transplantation in the cellular level.

Future perspective

A growing body of scientific evidence carried out on animals demonstrates that anthocyanins play a role in prevention and treatment of DM. And different mechanisms have been raised to explain anti-diabetic effect of anthocyanins, but most of the mechanism lacks further clinical studies to prove that, even if there is, the results are not entirely meaningful. In addition, anthocyanin intake has been considered to have a dose-response relationship with risk of DM in many systematic review studies. However, the intake quantity of anthocyanins treatment in these studies varied greatly, with some subjects received 320 mg/d or greater, and other under 100 mg/d, and these results may not be applicable to other ethnic groups. Thus, more large-scale human studies are necessary to conclusive evidence of the optimal daily intake of anthocyanin that minimizes the risk of DM.

All the time, the bioavailability of anthocyanins is considered to be extremely low, which contradicts the health benefits of anthocyanins. However, the biological metabolic rate of anthocyanins is severely underestimated, a recovery of 12.4% in a human bioavailability labeled by the cyanidin-3-glucoside(Czank et al. 2013). This underestimation is largely due to the facts that anthocyanins are transformed to various molecular in presystemic metabolism before they reach the plasma and the knowledge about the mechanisms of anthocyanin absorption, catabolism and metabolism is very limited(Lila et al. 2016). Currently, although different studies have been proposed to better understand of the anthocyanin bioavailability, the metabolized conjugates of these full sets of anthocyanins have not yet been determined. Thus, instruments with greater sensitivity and resolution needs to be designed to precise detection of anthocyanins and more in vivo and in vitro studies are needed to further explore the mechanism about anthocyanin catabolism in body.

Controlled release is an advanced approach to improve the biological activity of anthocyanins, controlling the slow release of anthocyanins at the target site leading concentration of anthocyanins reaches therapeutic level within a period(He, Ge, et al. 2017). Lipid-based, polysaccharide-based and protein-based complexes, nanoencapsulation and exosome have been designed to not only promote anthocyanin absorption and release at targeted sites, but also increase anthocyanin efficacy(Shen et al. 2022). However, few studies have focused on the controlled release of anthocyanins in the pancreas, which may enhance the ability of anthocyanins for the prevention and treatment of DM. And a growing body of research has demonstrated that direct delivery of antidiabetic drugs to the islet environment via nanocarriers and exosomes is a promising treatment option for T1DM and T2DM(Zeynaloo et al. 2022). Thus, it is very meaningful to carry out research on the controlled release of anthocyanins in the pancreas and these new approaches all require further studies to assess safety and efficacy.

The stability of anthocyanins is also the main reason for limiting the application of these compounds. The chemical structure of anthocyanins has a great influence on its stability. Unsaturated double bonds and oxidized groups will reduce its stability, while the methylation, glycosylation and acylation of glycosyl ligands will stabilize it. In recent years, great progress has been made in methods to improve the stability of anthocyanins, such as glycosyl acylation, whey protein-based, nanoemulsion and nanoliposome-based, etc.(Zhao et al. 2017, Chen and Stephen Inbaraj 2019, Ren, Jiménez-Flores, and Giusti 2021). But there are still many problems that need more research to further explore, say, how to solve undesirable exogenous materials be introduced in nanocarrier systems. Furthermore, the more precise and specific approaches need to be developed and large-scale medical application needs more clinical data.

With increasing drug dose, most of anti-diabetic agents caused various adverse reactions, while anthocyanins are of great nutritional interest with no adverse effects were reported. Combining anthocyanins with anti-diabetic agents not only effectively reduce the occurrence of adverse reactions, but also enhance the efficacy. Animal research indicate that synergistic effects of combined anthocyanins and metformin treatment on the blood glucose and insulin resistance(Zou et al. 2021, Tian et al. 2022). In addition, combined administration of the anthocyanins with fenofibrate enhanced ability of fenofibrate to improve lipid metabolism in patient with T2DM(Kusunoki et al. 2015). However, studies on combination therapy of antidiabetic drugs with anthocyanins are still limited. Further research in this direction may lead to anthocyanins as a supplementary treatment of anti-diabetic drugs for clinical application soon.

Few studies reported the benefits of anthocyanins in T1DM, as well as studies on anthocyanins for islet transplantation. Islet transplantation, as an effective treatment of T1DM, is often limited by the effects of oxidative stress produced in isolation and transplantation procedures. At present, anthocyanins were shown that may play a protective in islet after transplantation in vitro and animal studies by regulate the expression of HO-1, inflammatory markers IL-1β and NLRP3, and LC3 autophagic marker. However, these mechanisms need to be demonstrated in further in vivo studies, and more research needs to explore the protective mechanism of anthocyanins on islet transplantation.

Conclusion

The anthocyanins found in some natural plants produce positive effects that have been reported in medicinal field as potential supplementary treatment for DM. Here, this review has comprehensively assessed the healthy effect of anthocyanins on DM as well as its complications. Numerous studies confirm that anthocyanins have beneficial effects on glucose metabolism, lipid metabolism, insulin resistance, antioxidant, and anti-inflammatory through various mechanisms, thus plays a role in the prevention and treatment of DM. In addition, anthocyanins can also reduce organ damage caused by high glucose and lipid deposition, improve organ function, and prevent progression of the complication. At the same time, improvement in islet function and reduction in islet injury after transplantation associate with anthocyanin is demonstrated by animal studies. It can be anticipated that with the development of study in improving the stability of anthocyanins, more research on the effect of anthocyanins on DM will be carried out and the optimal intake of anthocyanin will be determined.

Abbreviation
DM	=	Diabetes mellitus
DR	=	diabetic retinopathy
DN	=	diabetic nephropathy
C3G	=	cyanidin-3-glucoside
NF-κB	=	nuclear Factor kappa B
AMPK	=	AMP-activated protein kinase
PPAR	=	peroxisome proliferator-activated receptor
ROS	=	reactive oxygen species
TLR4	=	Toll-like receptors 4
TNF-α	=	tumor necrosis factor
IL-6	=	Interleukin 6
GLP-1	=	Glucagon-like peptide-1
FAS	=	fatty acid synthase
TLR4	=	Toll-like receptors 4
MMP	=	Matrix metalloproteinase
TIMP	=	Tissue matrix metalloproteinase
IGFIR	=	insulin-like growth factor I receptor

Disclosure statement

No potential conflict of interest was reported by the authors. Figure 3 were created by Figdraw (www.figdraw.com).

Additional information

Funding

This work was supported by grants from the National Natural Science Foundation of China: No. 81570698 (Dr. Bo Zhang); the key project of Natural Science Foundation of Zhejiang Province (LZ20H160002); key research and development project of Zhejiang province (2021C03048).