Chewing and its influence on swallowing, gastrointestinal and nutrition-related factors: a systematic review

Figures & data

Table 1. PECOs criteria for inclusion and exclusion of studies.

	Inclusion criteria	Exclusion criteria
Population (P)	Adults and older individuals Edentulous and/or dentulous Healthy and/or not healthy With or without different medical conditions	Children below 18 years
Exposition (E)	Chewing, mastication, physical breakdown of food Direct objective assessment of chewing function or chewing/eating intervention or intentional/deliberate change in chewing behavior	Self- or proxy-assessed (indirect) masticatory function Subjective assessments measured by questionnaires and interviews evaluating the status of dentition, prosthesis use, and eating behavior
Comparison (C)	Healthy cohort, younger/older participants. Control condition without chewing intervention of condition.	No restrictions/exclusions were applied based upon presence/absence of comparator groups.
Outcomes (O)	Bolus formation and properties, swallowing threshold, and dysphagia Gastric emptying, gastric hormones, pathological conditions of the gastrointestinal tract Nutritional status, body mass index, micronutrient concentrations, body composition, malnutrition	No direct relationship between chewing intervention and outcome No clear finding related to chewing and outcome Effect of gum chewing on postoperative ileus and recovery of the bowel function after cesarean section
Studies design (S)	Randomized control trials Prospective cohort Prospective observational study with/without comparison group Cross sectional study with or without comparison group English-language studies	All reviews (narrative or systematic), meta-analysis, opinion pieces. Study protocols, conference abstracts, letters to editors, commentaries, preprints. Case reports, invitro, and animal studies. Articles published in languages other than English Article not published in peer-reviewed journals

Figure 1. PRISMA flow diagram showing the screening and study selection.

Table 2. Summary of the included studies.

ID	Authors (year)	Study design	Study group (N, age, sex)	Comparison group (if applicable)	Intervention /exposure (Chewing)	Outcome measurement	Results	Study quality
Effect of chewing on swallowing
1	Saitoh, Shibata (Saitoh et al. 2007)	Cross-sectional	N = 15 Age= 30 ± 5.2 yrs. Sex: 6 women		Chew and swallow food with 4 different consistencies	Video fluorography of 4 stages of swallowing The duration of each stage was calculated.	Chewing reduced the effectiveness of the posterior tongue-palate seal, allowing oral contents to spill into the pharynx. Consuming foods with both solid and liquid phases increase the risk of aspiration in dysphagic individuals	High
2	Mioche, Bourdiol (Mioche, Bourdiol, and Monier 2003)	Cross-sectional	N = 25 Age = 25-30 yrs. Sex: 11 Women		Chewing two samples of meat (hard and tender) for 7 s and spitting them. EMG for masseter and temporalis muscle was recorded.	Meat samples were weighed before and after chewing to determine weight changes due to saliva incorporation and the release of meat juice	Higher EMG for the hard meat and more saliva in the spitted bolus than tender meat.	High
3	Berretin-Felix, Machado (Berretin-Felix et al. 2009)	Cohort	N = 15 Age = 60-76 yrs. Sex: 10 Women	Pre-tx (removable denture) and post-tx 3, 6, and 18 months after implant overdenture tx	Chewing ability (interview) and chewing/swallowing of water, paste, and hard consistencies. Chewing cycles and bolus formation evaluated	Subjective evaluation Swallowing: Lip seal, bolus propulsion duration, oral food residue, and amplitude of laryngeal elevation	Patients reported more masticatory and swallowing disturbances before tx as compared to after tx. Swallowing of liquids is not affected in the removable denture, but bolus propulsion of solid foods was affected before tx and improved after tx.	Moderate
4	Fukatsu, Nohara (Fukatsu et al. 2015)	Cross-sectional	N = 33 Age = 25.6 ± 3.5 yrs. Sex: 20 women		Eating 5 g of cooked rice (white and green; 3 trials) in the usual manner or chewed well. Chewing cycles were counted.	Videoendoscopy to evaluate swallowing. Food bolus was graded subjectively into 3 categories.	The grinding of the swallowed food bolus increases with the increase in chewing cycles.	High
5	Goto, Nakamich (Goto et al. 2015)	Cross-sectional	N = 15 Age = 20.2 ± 0.54 yrs. Sex = all women,		3 volumes (0.5, 1, and 2) of 3 foods: boiled rice, fish sausage, and peanuts. Spitted at 50%, 100% or 120% chewing	Physical properties of solidity, adhesiveness, and cohesiveness were measured in the bolus	The number of chews significantly increased with an increase in the mouthful volume for all of the foods. No change in bolus properties	Moderate
6	Kim and Han (Kim and Han 2005)	Cross-sectional	N = 10 (Stroke patients) Age 50-82 yrs. Sex: 5 Women	N = 10 Healthy controls Age = 51-75 yrs. Sex: 5 women	Chew different pudding, thick rice gruel, curd-type yogurt, and liquid barium. Spit before swallowing. Also, chewing and swallowing thick rice gruel.	Food bolus viscosity (spit food). Also, chewing time and the number of chews was counted for the Thich rice gruel.	No differences in food bolus viscosity. But stroke patients used more chewing cycles and chewing time than healthy individuals.	Moderate
7	Kohyama, Sawada (Kohyama et al. 2007)	Cross-sectional	N = 15, Age= 20-42 yrs. Sex: All women		The participants ate 2 cubes of standard rice cake (6 and 9 g), and a modified cylindrical rice cake for the older adults (9 g).	Swallowing was measured by EMG of suprahyoid muscle, laryngeal movement, and swallowing sound.	Food size and texture affected chewing behavior but not swallowing behavior. Swallowing was induced whenever the food bolus was ready for swallowing.	High
8	Matsuno, Nohara (Matsuno et al. 2017)	Cross-sectional	N = 30 Age: 73.7 ± 6.3 yrs. Sex: 14 women	N = 30 Age = 25.6 ± 3.5 yrs. Sex: 19 women	Participants ate 5 g of cooked rice (white and green). Chewing cycles were counted.	Videoendoscopy to evaluate swallowing. Food bolus was graded subjectively into 3 categories.	Old adults showed similar food mixing and grinding scores to young adults, but with longer chewing cycles and lower food bolus aggregation scores.	Moderate
9	Abe, Furuya (Abe, Furuya, and Suzuki 2011)	Cross-sectional	N = 10 27.1 ± 1.7 yrs. Sex: 5 women		Rice cake cubes (2 green layers and one white layer in between) eaten in two ways: usual or well-chewing and under 4 restricted chewing cycles (10. 15. 20 or 30 cycles).	Videoendoscopy to evaluate swallowing. Food bolus was evaluated with the bolus formation index (BFI) (ratio of nonwhite rice pixels to the total food bolus pixels)	The BFI score increased with the increase in chewing times and cycles. BFI scores also increased with the well-chewing than usual chewing.	Moderate
10	Yamashita, Sugita (Yamashita, Sugita, and Matsuo 2013)	Cross-sectional	N = 25 Age= 20-37 yrs. Sex: 10 women		Participants ate 7 g of gummy jelly (3 trials). Number of chewing cycles pre- and post-stage II food transport.	Videoendoscopy to determine the onset of stage II food transport (bolus aggregation on the oropharynx).	The number of chewing cycles for pre-stage II transport was significantly higher than that for post-stage II transport. Bolus aggregation in the pharynx is related to the number of chewing strokes	High
11	Son, Seong (Son et al. 2013)	Cross-sectional	N = 20 (edentulous) Age = 73.3 ± 11.4 yrs. Sex: 10 Women	Measurements before and after the removal of the removable denture	Chewing performance was measured with chewing soft candy (30 cycles; with dentures only) then the particles were dissolved in water and the glucose contents were analyzed and categorized into: 0%—25%, 25%—50% and ≥50%.	Videofluorography of swallowing a mixture of younger and water-soluble barium. Food bolus oral and pharyngeal transit times, food residue, and aspiration.	Wearing dentures delayed the oral transit time of food bolus. Aspiration and food residue did not differ with or without wearing the dentures.	Moderate
12	Yamamoto, Furuya (Yamamoto et al. 2013)	Cross-sectional	N = 15 (edentulous) Age = 78 ± 5.6 yrs. Sex: 11 women	Measurements before and after the removal of the removable denture	10 g of minced agar jelly (with barium) was ingested with and without dentures (3 times each).	Videofluorography of 4 swallowing stages: 1- oral cavity, 2- upper oropharynx, 3- valleculae, and 4- hypopharynx. Duration of each stage was calculated.	Swallowing onset without dentures was delayed, with poorer food bolus aggregation. With dentures, food transit time was shorter than without dentures.	Moderate
13	Furuya, Hara (Furuya et al. 2014)	Cross-sectional	N = 25 Age = 27.6 ± 2.9 yrs. Sex: 8 women	With or without volition.	Masticatory performance was evaluated by color-changeable chewing gum.	Solid agar jelly (with barium) was ingested with or without volition (3 times each). Videofluorography of 4 swallowing stages: 1- oral cavity, 2- upper oropharynx, 3- valleculae, and 4- hypopharynx. Duration of each stage was calculated.	With volition, the food chewing and the onset of swallowing were delayed than without volition. However, chewing with volition showed faster transport of food bolus transport with swallowing.	Moderate
14	Engelen, Fontijn-Tekamp (Engelen, Fontijn-Tekamp, and van der Bilt 2005)	Cross-sectional	N = 266 Age = 42 ± 12 Sex: 187 women		Comminution test Optocal plus 15 chewing strokes Besides, participants also chewed carrot, peanut, and cheese.	Swallowing threshold	The chewing performance showed small, but significant correlations with the swallowing thresholds of the relatively hard to chew products Hard and dry products require more chewing cycles and longer time in mouth until swallowing	Moderate
15	Fontijn-Tekamp, Slagter (Fontijn-Tekamp et al. 2004)	Cross-sectional	N = 87 Age = 41.6 ± 12 Sex: 62 Women		Median particle sizes (X50) for Optocal Plus after 15 chewing strokes and at the moment of swallowing Swallowing threshold tests were performed with peanuts, carrots, and cheese	Swallowing threshold	The number of chewing strokes for bolus preparation correlated significantly with the size of food. Food with suitable texture for swallowing was positively associated masticatory efficiency.	High
16	Wada, Kawate (Wada, Kawate, et al. 2017)	Cross-sectional	N = 100 Age ≥ 65 Sex: 67 Women		Color-changeable chewing gum 120 s of chewing	Food bolus texture at the swallowing threshold	With larger portion sizes, subjects needed significantly more chewing strokes and time until swallowing.	Moderate
17	Bayram, Ilgaz (Bayram et al. 2021)	Cross-sectional	N = 55 older participants with dysphagia	62 older participants without dysphagia	Mixing ability test Color-changeable chewing gum	Dysphagia risk evaluated by Eating Assessment Tool (EAT-10).	Poor oral health and decreased masticatory performance are directly associated with dysphagia and may lead to a decrease in ADL	Low
18	Maeda, Takei (Maeda et al. 2020)	Cross-sectional	N = 29 Age= 29.6 ± 6.8 Sex= 14 women		Chew the food in their usual manner and then spit it out at 50%, 100% and 150% of chewing duration, In the swallowing test, participants were asked to chew the food in the same manner and then to swallow at 50%, 100% and 150% of chewing duration, cued by the investigator	Bolus analysis done to measure rheological properties such as hardness, adhesiveness, and cohesiveness	Hardness, cohesiveness, and adhesiveness changed with increasing the number of chewing cycles Hardness and cohesiveness but not the adhesiveness at least changed with chewing duration.	High
19	Lucas and Luke (Lucas and Luke 1986)	Cross-sectional	N = 35 Age = 18-41 (median 21 years)	6 participants with partial dentures Age (41–74) mean 60	Chew and spit food varying numbers of chews and median particle size calculated	Number of chews needed to swallow the food sample	Within the limitations of the study the no. of chews before swallow was correlated with median particle size in dentate individuals	Moderate
20	Takeshima, Fujita (Takeshima, Fujita, and Maki 2019)	Cross-sectional	N = 120	Group 1: primary dentition N = 20, Age= 5.68 ± 0.68 Sex: 10 girls Group 2 early mixed dentition N = 20 Age = 7.75 ± 0.96 Sex = 10 girls Group 3: late mixed dentition stage N = 20 Age = 10.40 ± 1.09 10 females, Group 4: permanent dentition N = 20, age, 16.50 ± 2.25 years Sex: 10 women, Group 5 permanent dentition (n = 40), age= 25.05 ± 2.89 20 women	Comminution text Concentration of dissolved glucose obtained from a cylindrical shaped (diameter, 15 mm; height, 8 mm) gummy jelly consisting of 40% maltose	Participants were instructed to chew until feeling the desire to swallow Number of chewing cycles and chewing duration to the onset of swallowing was recorded	Swallowing threshold did not coincide with masticatory performance according to dental formula stage	High
21	Kochi, Takei (Kochi et al. 2021)	Cross sectional	N = 24 adults Age= 29 ± 6.1 Sex: 10 women		Chew 8 g of steamed rice and spit it out at 50%, 100% and 150% of their own chewing duration, defined as the time of chewing from onset of the first chewing cycle to onset of the first swallow	Bolus properties, i.e., hardness, cohesiveness, and adhesiveness as well as water content of the bolus were measured	Bolus hardness decreased, and the cohesiveness and water content of the bolus did not change up to 50% of chewing duration followed by a slight but significant increase Variation of chewing duration and swallowing initiation was not dependent on bolus properties during the chewing of steamed rice, but mainly depended on the surface lubrication of the boli There was a significant negative correlation between time and hardness, and positive correlations between time and cohesiveness, between time and adhesiveness and between time and water content	High
22	Medeiros, Figueredo (de Medeiros et al. 2021)	Cross-sectional	N = 179 Age: 78.9 ± 9.0 years Sex: 119 women		Mixing ability test with two-colored chewing gum	Swallowing thresholds were evaluated by asking them to chew a 3.7-g portion of unsalted roasted peanuts	The masticatory performance and swallowing threshold were not correlated.	High
23	Magara, Onuki (Magara et al. 2022)	Cross-sectional	N = 20 healthy subjects Age= 28.1 ± 3.2 years Sex= 10 women		Freely chewing gum for three min	Repeated swallowing 1 ml of water (at 36 ◦C) delivered to the posterior dorsal tongue using an infusion catheter	Masticatory movement may suppress swallowing related activity within the pharyngeal motor circuit	High
24	Onuki, Magara (Onuki et al. 2021)	Cross-sectional	209 patients median age, 72 years (69.0–77.0); 128 women)		Comminution test Masticatory performance evaluated by estimating the glucose concentration after chewing gummy jellies with Gluco sensor device	Dysphagia screening questionnaire	Deterioration of swallowing function was related to reduced occlusal force and decreased masticatory performance	High
Effect of chewing on physiologic/pathologic processes of the GIT
25	Hattori, Mito (Hattori, Mito, and Watanabe 2008)	Cross-sectional	N = 13 Age range = 20—21 yrs. Sex: All men	With or without splint (on separate days, 7 days in between)	Comminution test 3 g of peanut (20 cycles) Mandibular intraoral appliance covering the teeth anterior to the second premolars	Gastric emptying rate measured by 13C-octanoic acid breath test after a solid test meal	Chewing performance decreased with the splint. No change in gastric emptying with or without the splint.	High
26	Sierpinska, Golebiewska (Sierpinska et al. 2007)	Cross-sectional	N = 40 (Dyspeptic patients with a suspected masticatory deficiency), Age = 24—65 yrs. Sex: 18 women	N = 40 (Dyspeptic patients without masticatory deficiency), Age= 24-70 yrs. Sex: 17 women	Comminution test (Optosil tablets chewed for 20, 40, 60, and 80 chewing strokes).	Histopathologic changes of gastric mucosa and severity of Helicobacter pylori infection scored according to Sydney Classification of Chronic Gastritis.	Reduced masticatory efficiency was associated with more severe chronic inflammatory changes and Helicobacter pylori infection of the gastric mucosa.	Moderate
27	Arya, Agarwal (Arya et al. 2017)	Cross-sectional	N = 12 (Diagnosed with GERD) Age = 44.6 ± 11.6 Sex: 7 women		Chewing normal Vs increasing the chewing cycles 20 times or until the food becomes a puree.	48-hour pH monitoring DeMeester score	An increase in chewing cycles did not influence the DeMeester score significantly.	Moderate
28	Kimura, Nomura (Kimura et al. 2006)	Cross-sectional	N = 30 (Tubal feeding without mastication and swallowing) Age = 74 ± 4.3 Sex: 24 women	N = 30 Ingested food with mastication and swallowing) Age = 77.2 ± 6.1 Sex: 18 women	Ingestion of 250 ml of a liquid diet.	Before and 30 min after ingestion, the electrogastrography (for gastric motility) and abdominal ultrasonography (for gastric emptying) were measured.	Gastric motility increased for both the groups, but the ratio of the increase was lower in the tube-fed group than in controls. Gastric excretion was lower in tube-fed individuals than controls.	Moderate
29	Pera, Bucca (Pera et al. 2002)	Cross-sectional	N = 12 Age range= 8-35 yrs. Sex:3 women		Comminution test Evaluated by chewing ham for 50 cycles and passed through a sieve.	Solid meal consists of egg with 100 mg sodium octanoate, ham (21 g), crackers (25 g), and water (500 ml). Eaten with 50 cycles in one session, while in another session only the egg and crackers chewed for 25 cycles and swallowed the ham. Gastric emptying using the 13 C-octanoic acid breath test Lag phase and gastric half-emptying time were measured.	Masticatory performance was inversely related to both lag phase and gastric half-emptying time. Both lag phase and gastric half-emptying time were shorter during 50 chewing cycles than 25 chewing cycles.	Moderate
30	Pennings, Groen (Pennings et al. 2013)	Cross-sectional	N = 10, Age = 74 ± 2 yrs. Sex: all men		Participants consumed 135 g of L-[1-13C] phenylalanine–labeled beef both as beef steak and minced beef	Blood sample to evaluate postprandial protein kinetics and insulin and amino acid levels.	Minced beef is more rapidly digested than beef steak leading to increased amino acid availability and greater postprandial protein retention Does not result in greater postprandial muscle protein synthesis rates	Low
31	Koike (Koike et al. 2013)	Cross-sectional	N = 11 (With malocclusion) Age= 25.5 ± 4.8 yrs. Sex: all women	N = 11 (With normal occlusion) Age= 26 ± 5–1.0 yrs. Sex: all women	Masticatory ability was evaluated with questionnaires. Masticatory efficiency was measured with color-changeable gums.	Gastric emptying was assessed using a [13C]-labelled acetate breath test with a liquid meal Frequency scale for symptoms of gastroesophageal reflux disease (FSSG) Questionnaire on food intake	Masticatory function in patients with malocclusion was significantly lower than in subjects with normal occlusion No difference in the FSSG results between the two groups Malocclusion group showed lower CO2 excretion with a slower gastric emptying rate	Moderate
32	Carretero, Sanchez-Ayala (Carretero et al. 2011)	Cross-sectional	N = 38, (Non-ulcerative functional dyspepsia) Age= 71.8 ± 7.7 yrs. 16 women	N = 38, (Healthy) Age = 71.9 ± 7.0 15 women	Number of occluding teeth pairs and Comminution test (Optosil 20 chewing strokes)	--	Masticatory performance of subjects presenting with dyspepsia and 0–4 occlusal pairs was significantly lower when compared to the control group.	High
33	Dubey, Nundy (Dubey and Nundy 1984)	Cross-sectional	22 patients with duodenal ulcer Age = 36 ± 17	30 control subjects without any gastrointestinal disease	A masticatory and a non-masticatory (homogenized) meal having the same chemical composition.	Gastric acid secretion by intubation	No difference in the gastric acid response to a masticatory or a non-masticatory diet. However, a masticatory diet has a buffer capacity after ingestion which is significantly greater than that of a non-masticatory diet. Masticatory diets may play a part in protecting individuals from developing duodenal ulcer.	Moderate
34	Sumonsiri, Thongudomporn (Sumonsiri et al. 2019)	Cross-sectional	N = 40 healthy subjects Age= 23.9 ± 3.8 years Sex= 25 women		Food comminution test with chicken sausage cubes where participants chewed and expectorated the cubes after 30 cycles Median particle size was calculated by the multiple sieving method	Gastric emptying was measured by scintigraphy	No significant relationships between masticatory performance, and number of chewing cycles, and any parameter of gastric emptying scintigraphy using frankfurter sausage as a test food in healthy dentate subjects.	High
Effects of chewing on nutrition-related parameters
35	Flores-Orozco, Tiznado-Orozco (Flores-Orozco et al. 2016)	Cross-sectional	N = 100 Age = 21.9 Women 47		Comminution test (Optosil tablets chewed for five trials with 20 cycles/trial).	Anthropometric measurements and blood samples were collected.	No relation between masticatory performance and any nutritional status indicator	Moderate
36	Okada, Enoki (Okada et al. 2010)	Cross-sectional	N = 200 Age 76.6 ± 7.1 yrs. Sex: 122 women		Masticatory performance was measured with color-changeable gums.	Anthropometric measurements and blood samples were collected.	Serum albumin concentrations were well-correlated with chewing performance and anthropometric measurements	Moderate
37	Leischker, Kolb (Leischker, Kolb, and Felschen-Ludwig 2010)	Cross-sectional	N = 31 Age= ≥70 yrs. Sex: 13 women		Comminution test (Standardized slices of carrots chewed for 45 s)	MNA, anthropometric measurements, and more extensive blood samples were collected.	The nutritional status was higher in patients with a better chewing function, but these correlations were not significant.	Low
38	Kimura, Ogawa (Kimura et al. 2013)	Cross-sectional	N = 269 Age = 80.6 ± 4.7 yrs. Sex: 181 Women		Masticatory efficiency was measured with color-changeable gums. Chewing ability A questionnaire was used for self-assessment of chewing.	The 11-item Food Diversity Score Kyoto (FDSK-11).	Participants with low chewing performance had lower food variety and less frequent intake of beans, vegetables, seaweed, and nuts.	Low
39	Isabel, Moysés (Isabel et al. 2015)	Cross-sectional	N = 160, Age = 17.8-34.3 yrs. Sex: 80 Women		Comminution test (Optosil tablets chewed 20 cycles)	BMI	Chewing parameters did not differ between different BMI groups.	High
40	Liedberg, Stoltze (Liedberg et al. 2007)	Cross-sectional	N = 481, Age= 67-68 yrs. Sex: All men		Mixing ability test (Two colored chewing gum chewed 10 strokes, 3 times) Chew an almond until swallowing Chewing ability (Interviews for self-assessment of chewing)	Nutritionist used dietary history interview by Steen at al. BMI, 24-hour urinary nitrogen excretion.	No significant differences between those with adequate or inadequate nutrition were found regarding chewing performance, including self-assessed chewing. Inadequate dietary habits were associated with high BMI.	High
41	Wallace, Samietz (Wallace et al. 2018)	Randomized controlled clinical trial	N = 44 (RDPD) Age = 79.7 ± 5.41 yrs. Sex: 23 Women	N = 45, (SDA) Age= 79.6 ± 6.4 Sex: 20 women	Mixing ability test Two colored chewing gum chewed 20 cycles	MNA and blood samples at baseline and 12 months post-treatment.	Masticatory performance and MNA increased significantly in both groups with no significant differences between the groups. Nutritional status could not be predicted from the masticatory performance.	Moderate
42	Zhu and Hollis (Zhu and Hollis 2014b)	Randomized crossover design	N = 45 Age= 20.9-27.1 yrs. Sex: 25 Women	Normal = N = 16 Overweight N = 16. Obese N = 15	Pizza chewed 100%, 150%, or 200% of the normal number of chews	Weighing the plate before and after serving	Food intake was reduced significantly by increasing the number of chews. Meal duration was prolonged, eating rate reduced but subjective appetite the same.	Low
43	Zhu and Hollis (Zhu and Hollis 2015b)	Cross-sectional	N = 64 Age= 20.9-25.4 yrs. Sex: 39 Women		Consumed five portions of pizza rolls	BMI	Negative correlation between body mass index and the number of chewing cycles Chewing behavior is associated with body weight status in fully dentate healthy adults	High
44	Ranawana Viren, Henry (Ranawana, Henry, et al. 2010)	Randomized non-blind	N = 11 Age = 18-65 yrs. Sex: 8 Women		Consumed rice or spaghetti on six nonconsecutive days, each food tested on 3 random days	Fasting blood samples at −5, 0 minutes before consumption and at 15, 30, 45, 60, 90, and 120 minutes after.	Significant correlations between the degree of breakdown during mastication and the GR were observed for rice, but none for spaghetti.	Low
45	Cassady, Hollis (Cassady et al. 2009)	Randomized crossover trial	N = 13 Age = 19-43 yrs. Sex: 5 Women		Comminution test 5 g of raw almonds chewed 10, 25, or 40 times before swallowing	Particle sieving and stool samples were collected. Blood glucose, insulin, plasma lipids, ghrelin, PYY, and GLP-1 at 15,30, 45, 60, 90, 120, and 180 min after almond consumption.	40 chews compared to 10 and 25 chews, induced more satiety and higher PYY and GLP-1 levels but not ghrelin, glucose, or insulin levels	Low
46	Campos, Goncalves (Campos, Goncalves, and Rodrigues Garcia 2014)	Cross-sectional	N = 8 Age: 60.1 ± 6.6 yrs. Sex: 6 Women	RPD before and after implant support (2 months interval)	Comminution test measured with Optocal.	Swallowing threshold and subjective nutrient intake.	Smaller particle size and improved subjective nutrient intake with the implant, compared to no implant.	Moderate
47	Müller, Duvernay (Müller et al. 2013)	Randomized clinical trial	N = 16 IOD group: Age = 85.0 ± 6.19 yrs.	CD group: 18, 84.1 ± 5.55 yrs.	Maximum bite force Masticatory performance with a two-color mixing ability test	MNA and blood samples (hemoglobin, albumin, folic acid, vitamin B12, and C-reactive protein) at baseline, 3- and 12-months post-treatment.	IOD compared to CD, had higher bite force but no differences in masticatory performance or MNA or blood markers.	Moderate
48	Suzuki, Fukushima (Suzuki et al. 2005)	Cross-sectional	N = 10 Type2 diabetes: Age = 44.1 ± 4.4 yrs. Sex = 2 women	Control: 16, 7 F: 9 M, 35.6 ± 2.1 yrs.	Chewing and eating 130 g hamburger steak and 100 g rice in the “usual” manner and “thoroughly”	Plasma glucose and serum insulin concentrations were measured for 3 hours postprandially	Thorough mastication in subjects with type 2 diabetes elicited a higher postprandial plasma glucose concentration Postprandial plasma glucose concentration was lower during thorough mastication in controls	High
49	Kokkinos, le Roux (Kokkinos et al. 2010)	Cross-sectional	N = 17 Age= 29.7 ± 1.2 Sex: All men		Eating ice cream fast (5 min) vs slow (30 min)	Blood glucose, insulin, plasma lipids, ghrelin, PYY, and GLP-1	Eating slowly induced more satiety and higher PYY and GLP-1 levels but not ghrelin, glucose, or insulin levels	Moderate
50	Zhu, Hsu (Zhu, Hsu, and Hollis 2013)	Cross-sectional	N = 21 Age= 18—40 yrs. Sex: all men		Consumed equal-sized portions of pizza either in 15 or 40 chewing cycles.	Appetite was determined subjectively. Blood sample to evaluate satiety-related hormones, glucose, insulin, and GIP concentrations.	Subjective hunger and desire to eat were lower during the 40 cycles than 15 cycles. Glucose, insulin, and GIP concentrations were higher with 40 than 15 cycles.	High
51	Wöstmann, Michel (Wöstmann et al. 2008)	Cross-sectional	N = 47 Age = 72.6 ± 6.7 Sex: 28 women		Chew a standardized cube of carrot (45 seconds) and degree of breakdown evaluated	MNA and Albumin, pre-albumin and zinc	Despite an improvement in masticatory ability and efficiency, no change in nutritional status particularly albumin, zinc, and MNA values remained unchanged	Moderate
52	Goncalves, Campos (Goncalves, Campos, and Garcia 2015)	Cross-sectional	N = 12 Age = 62.6 ± 7.8 Sex: 8 women		Comminution test with Optocal until swallow	Nutritional intake measured by dietary dairies and BMI	Efficient mastication as a result of Implant-supported fixed partial prosthesis improves dietary intake	Low
53	Liedberg, Norlén (Liedberg et al. 2004)	Cohort	N = 44 FPDs Age = 67–68 Sex: All men	N = 40 RPDs Age = 67–68 Sex: All men	Chewing gum color mixing and bolus-shaping Almonds used to record the swallowing threshold	Body mass index and Participants interviewed for the choice of food	Chewing gum bolus differed between the two groups No changes in energy intake and nutrients or body mass index between the groups	High
54	Sanchez-Ayala, Campanha (Sanchez-Ayala, Campanha, and Garcia 2013)	Cross-sectional	N = 100 Age = 39.7 ± 16.6 Sex: 67 Women		Comminution test The participants chewed Optosil cubes for 20 chewing cycles	Patients were weighed on a scale arm bascule determine body fat BMI	Lower masticatory efficiency maybe at risk for increased body fat	High
55	Amaral, Souza (Amaral et al. 2019)	Cross-sectional	N = 12 Age = 68.6 ± 5.2 years 8 women		40 cycles of Optocal mastication performed	Computerized dietary tables were used to evaluate intake of energy	Improved masticatory performance after participants with conventional complete dentures were rehabilitated with single-implant overdentures. Intake of nutrients remains the same except sodium ingestion, which was decreased.	High
56	Aquilanti, Alia (Aquilanti et al. 2020)	Cross-sectional	N = 76 Age: 75.8 ± 5.6 Sex: 34 women		Two-color mixing test	Qualitative and a quantitative food interview to assess nutritional status	No association between reduced masticatory performance and a worsening of nutritional parameters or bioimpedance parameters such as Impedance, Resistance, Phase Angle and Reactance	High
57	de Medeiros, Pinheiro (Medeiros et al. 2020)	Cross-sectional	N = 344 Older adults Age= 77.7 ± 9.1 Sex: 206 Women		Masticatory performance using two-color chewing gum	Swallowing threshold evaluated while eating peanuts Mini-Nutritional Assessment—Short Form Body composition was assessed by a bioelectrical impedance analysis	Nutritional status and body composition of institutionalized older adults was not correlated with their poor masticatory performance and swallowing threshold. Worse masticatory performance and swallowing threshold negatively impacted the OHRQoL.	High
58	Flores-Orozco, Pérez-Rodríguez (Flores-Orozco et al. 2020)	Cross-sectional	N = 100 non-indigenous population Age= 21.9 Sex: 53 women	N = 42 indigenous population Age =40 years Sex: 31 women	Chewing 2 g of silicon for 20 cycles. Tablets of Optosil (5 mm thick, 20 mm in diameter) (Optosil P Plus ®)	Weight, height, and body circumferences were recorded	No relation between any aspect of masticatory function (masticatory performance, masticatory laterality, and chewing rate) and nutritional status indicator was detected in the indigenous group	High
59	Fujimoto, Suito (Fujimoto et al. 2020)	Cross-sectional	N = 153 Age= 75.3 ± 6.5 year 47 Women		Comminution text with gummy jelly and glucose concentration in the supernatant liquid collected by washing the chewed pieces was measured	Body height and weight were self-reported in participant interviews and were subsequently used to calculate the BMI for assessing the nutritional status	BMI not associated with objective masticatory efficiency*	High
60	Motokawa, Mikami (Motokawa et al. 2021)	Cross-sectional	N = 509 community-dwelling older 294 women		Mixing ability text Color-changeable chewing gum	BMI blood serum albumin values Food Frequency Questionnaire	Chewing performance was related to nutrient and food group intake among community-dwelling older adults After adjusting for BMI, the chewing ability was significantly related to undernutrition	High
61	Paphangkorakit, Kanpittaya (Paphangkorakit et al. 2019)	Cross-sectional	N = 30 Age range= 20–24 yr, 20 women		Participants were asked to eat a meal and chewing rate was calculated	BMI	Neither chewing rate nor the number of chews per mouthful was correlated with BMI.	Moderate
62	Saksono, Hijryana (Saksono et al. 2019)	Cross-sectional	N = 158 Age = 67 117 women	Two groups based on the Eichner Index A2–B3 (with at least one posterior support zone) (n = 114) and Eichner Index B4–C3 with no posterior support zone) (n = 44)	Color-changeable chewing gum	Mini-Nutritional Assessment (MNA) Food-frequency questionnaire (FFQ)	Despite significant differences between masticatory performance no significant differences in nutrition intake*	Moderate
63	Iwasaki, Motokawa (Iwasaki et al. 2022b)*	Cohort	N = 715 adults Age= ≥65 years Sex: 436women		Comminution test Chew gummy jelly for 30 strokes. The degree of comminution evaluated compared to a visual reference scored on a scale of 0 to 9.	MNA	7-component oral hypofunction (which includes masticatory performance in food commination test) phenotype contributed to the increase in the odds of the presence and severity of malnutrition	High
64	Madhu, Shirali (Madhu et al. 2016)	Cross-sectional	N = 100 Normoglycaemic 37.9 ± 14.2 Sex = 43 Women	N = 100 Dysglycaemic 56.74 ± 11.78 Sex = 44 Women	Chew test food normally and chew 40 times	fasting and post-prandial blood sugar values estimated	Postprandial blood glucose concentration upon thorough mastication was significantly lower, due to early-phase insulin secretion in Normoglycaemic group	Low
65	Goh, Chatonidi (Goh, Chatonidi, et al. 2021)	Cross-sectional	Slow chewers N = 24 Age = 26.0 ± 4.2 Sex = all men	Fast chewers N = 15 Age= 27.3 ± 4.6 Sex = all men	Participants classified into Slow chewers/eaters’ Fast chewers eaters based on chewing rate	Bolus and saliva properties Post-prandial glucose response	Eating rate, oral exposure time and saliva uptake were shown to be the main contributors to an increase in post-prandial glycemic responses in slow eaters.	High
66	Goh, Choy (Goh, Choy, et al. 2021)	Cross-sectional	Slow chewers N = 15 Age= 48.1 (8.8) Sex = women	Fast chewers N = 15 Age= 43.0 (9.5) Sex = 3 women	Participants with higher risk for type 2 diabetes classified into Slow chewers and Fast chewers based on chewing rate	Bolus and saliva properties Plasma glucose, insulin, and satiety measures	Increased chews per bite and a longer oral sensory exposure, led to increased bolus particle surface area and greater saliva uptake and were associated with higher PP glucose, PP insulin and satiety responses.	High
67	Ohta, Imamura (Ohta et al. 2022)*	Cross-sectional	Sixty patients Age= 82.5 ± 7.0 Sex: 38 women		Comminution test Masticatory performance evaluated by estimating the glucose concentration after chewing gummy jellies with Gluco sensor device	Nutritional risk was screened using the Mini-Nutritional Assessment Short Form (MNA-SF) Nutritional Risk Screening (NRS) scores Malnutrition was assessed according to the Global Leadership Initiative on Malnutrition (GLIM) criteria	Oral hypofunction that included masticatory function correlated with nutritional risk, evaluated by the MNA-SF and Nutritional Risk Screening score	High
68	Nishi, Ohta (Nishi et al. 2022)*	Cross-sectional	117 women: 73, mean age: 50 ± 7 years		Comminution test Masticatory performance evaluated by estimating the glucose concentration after chewing gummy jellies with Gluco sensor device	Multi-frequency bioelectrical impedance analysis (MF-BIA)	No association between masticatory performance and nutritional status.	High
69	Sawada, Takeuchi (Sawada et al. 2021)	Cross-sectional	203 Age = 74.0 (70.0, 79.0) 150 women		Comminution test Masticatory performance evaluated by estimating the glucose concentration after chewing gummy jellies with Gluco sensor device	Mini Nutritional Assessment	Masticatory function was not associated with national status	High
70	Karawekpanyawong, Nohno (Karawekpanyawong et al. 2022)	Cross-sectional	N = 84 Sex: 45 women		Comminution test Masticatory performance evaluated by estimating the glucose concentration after chewing gummy jellies with Gluco sensor device	Brief-Type Self-Administered Diet History Questionnaire, Body mass index (BMI) assessments	Masticatory performance <173 mg/ dL had lower intake of folic acid and vitamin A than those with masticatory performance ≥173 mg/dL Lower masticatory performance, lower SSF and fewer teeth were associated with a lower intake of several micronutrients, such as vitamin A, β-carotene, and folic acid	High
71	Hamada and Hayashi (Hamada and Hayashi 2021)	Cross-sectional	N = 11 Healthy participants Age= 23 ± 1 years		Participants chewed 20-mL test drink for 30 s at a frequency of once per second, and then swallowed it	Gas‑exchange variables, DIT, and substrate oxidation.	Oral stimuli (i.e., the duration of tasting food in the mouth and the duration of chewing) increase DIT	Moderate

GIT: gastrointestinal tract; Tx: treatment; MNA: mini nutritional assessment; BMI: body mass index; RDPD: removable partial dental prosthesis; SDA: shortened dental arch; GR: glycemic response; GERD: gastroesophageal reflux disease; GIP: glucose-dependent insulinotropic peptide; GLP-1: glucagon-like peptide; PYY: peptide YY; IOD: implant overdenture; CD: complete removable denture; IBS: irritable bowel syndrome; ADL: activities of daily living; EMG: electromyography; Co2: carbon dioxide; OHRQoL: Oral Health Related Quality of Life; DIT: diet induced thermogenesis.

* Partial finding.

Figure 3.  Bar graphs showing the overall A) number of studies (in percent) supporting and refuting the hypothesis that chewing is a mechanical or physiological contributor in the swallowing process, physiologic/pathologic processes of the gastrointestinal tract, and nutrition-related parameters. The different shades of the bars represent the methodological quality of the studies based on the Joanna Briggs Institute (JBI) Critical Appraisal Checklist. B) showing the number of studies (N) supporting and refuting the hypothesis.

Table 3. A. Question: Does chewing function affect or contribute to swallowing function

No. of studies	Certainty assessment
	Study design	Risk of bias	Inconsistency	Indirectness	Imprecision	Certainty	Bibliography
Impaired masticatory function is associated with swallowing function and dysphagia^#
3	Cross-sectional	Serious	Very serious	Not serious	Not serious	Very low	Magara et al. 2022; Bayram et al. 2021; Onuki et al. 2021
Chewing with dentures causes delayed oral transit time, swallowing onset and changes in bolus propulsion and aggregation^#
3	Cross-sectional (2) Cohort (1)	Not serious	Very serious	Serious	Serious	Very low	Son et al. 2013; Yamamoto et al. 2013; Berretin-Felix et al. 2009
Increased chewing cycles/duration/swallowing threshold causes changes in bolus properties, bolus aggregation and bolus forming index^#
5	Cross-sectional	Not serious	Serious	Not serious	Serious	Low	Kochi et al. 2021; Maeda et al. 2020; Fukatsu et al. 2015; Matsuno et al. 2017; Abe, Furuya, and Suzuki 2011

B. Question: Does chewing function affects or contributes to the physiologic/pathologic processes of gastrointestinal tract

No. of studies	Certainty assessment
	Study design	Risk of bias	Inconsistency	Indirectness	Imprecision	Certainty	Bibliography
Impaired masticatory performance is associated with physiologic/pathologic processes of gastrointestinal tract^*#
3	Cross-sectional	Not serious	Very serious	Serious	Very serious	Very low	Sierpinska et al. 2007; Koike et al. 2013; Carretero et al. 2011

C. Question: Does chewing function affects or contributes to nutrition-related factors

No. of studies	Certainty assessment
	Study design	Risk of bias	Inconsistency	Indirectness	Imprecision	Certainty	Bibliography
Masticatory function was correlated with nutrition-related factors^**#
5	Cross-sectional (4) Randomized control trials (1)	Serious	Very serious	Serious	Serious	Very low	Okada et al. 2010; Motokawa et al. 2021; Ohta et al. 2022; Ranawana, Henry, et al. 2010; Zhu and Hollis 2015b
Increased masticatory function reduces food intake and affects postprandial glucose concentration and gut hormones^***#
6	Cross-sectional (4) Randomized control trials (2)	Serious	Very serious	Very serious	Very serious	Very low	Suzuki et al. 2005; Cassady et al. 2009; Zhu and Hollis 2014a; Madhu et al. 2016; Hamada and Hayashi 2021; Zhu, Hsu, and Hollis 2013
Improved masticatory function results in improved diet and nutritional intake increased risk of fat and associated with malnutrition^#
6	Cross-sectional (5) Cohort (1)	Serious	Very serious	Serious	Serious	Very low	Iwasaki et al. 2022a; Sanchez-Ayala, Campanha, and Garcia 2013; Campos, Goncalves, and Rodrigues Garcia 2014; Goncalves, Campos, and Garcia 2015; Karawekpanyawong et al. 2022; Kimura et al. 2013
Chewing rate affects expression of gut hormones^***#
3	Cross-sectional	Not serious	Not serious	Not serious	Serious	Moderate	Goh, Chatonidi, et al. 2021; Goh, Choy, et al. 2021; Kokkinos et al. 2010
Masticatory function is not associated with BMI, nutritional status body composition or nutritional biomarkers^#
8	Cross-sectional (6) Randomized control trials (2)	Not serious	Serious	Not serious	Serious	Low	Isabel et al. 2015; Müller et al. 2013; Liedberg et al. 2007; Wallace et al. 2018; Aquilanti et al. 2020; Medeiros et al. 2020; Nishi et al. 2022; Sawada et al. 2021; Zhu and Hollis 2014b
Masticatory function is not correlated with BMI and nutritional status^#
5	Cross-sectional	Serious	Not serious	Not serious	Not serious	Moderate	Isabel et al. 2015; Flores-Orozco et al. 2020; Fujimoto et al. 2020; Paphangkorakit et al. 2019; Flores-Orozco et al. 2016
Improved masticatory function does not change the nutritional intake BMI or nutrition status^#
4	Cross-sectional (3) Cohort (1)	Not serious	Very serious	Very serious	Serious	Very low	Amaral et al. 2019; Saksono et al. 2019; Liedberg et al. 2007; Wöstmann et al. 2008

# Congregated outcomes with 3 or more studies have been analyzed.

* Chronic inflammatory changes in the gastrointestinal tract, helicobacter infection, dyspepsia, lowers CO2 excretion and gastric emptying.

** Serum albumin concentrations, glycemic responses, body mass index, nutrient intake, and nutritional risk.

*** Gut hormones: peptide YY, glucagon-like peptide, post-prandial glycemic responses, and post-prandial insulin.

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Data availability statement

Data can be obtained from the corresponding author on reasonable request.