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Abstract
Fatty acid (FA) synthesis in vitro was examined in human adipose tissue slices and in an optimally fortified cytoplasmic assay system that measured the overall enzyme capacity for incorporation of acetyl units into FA from acetate and citrate. Six obese women were examined in a steady weight, eating their usual food, and after over-feeding for 5 days with a high-carbohydrate low-fat diet. Palmityl(+)carnitine did not increase incorporations by the cytoplasmic assay system, indicating that this assay approaches optimal conditions for determination of the upper overall enzymatic limitation of FA synthesis. Contrary to earlier reports the present study demonstrates that, in vitro, the enzyme capacity for citrate incorporation is at least as large as the capacity for acetate incorporation into FA under optimal conditions for both precursors. Citrate incorporation demonstrates a higher adaptability than acetate incorporation during short-term hypercaloric high-carbohydrate feeding. The insulin-stimulated incorporation of glucose into FA by slices increased from 1 to 9 nmoles X (g TG)−1 X (hour)−1 (p < 0.10) while the capacity of the enzymes for incorporation of acetyl units into FA increased from 33 to 77 nmoles X (g TG−1 X (hour)−1 (p < 0.005). The supply of acetyl units was probably rate-limiting for fatty acid synthesis. An over-night fast was not sufficient to significantly depress adipose tissue metabolism, including FA synthesis. In agreement with available in vivo experiments, it is concluded that de novo FA synthesis is of little quantitative importance for fatty acid storage or consumption of glucose carbon in human adipose tissue. In plasma, insulin and triglycerides increased, cholesterol decreased, while blood glucose and glucose tolerance were unchanged.