Adipose tissue from twelve normal-weight and ten obese subjects on weight-maintaining diets and nine obese subjects on hypocaloric diets was removed at surgery and incubated in vitro. Basal glucose oxidation correlated significantly (r = 0.68, p < 0.005) with fat-cell diameter in subjects on weight-maintaining diets. This relationship was significantly altered (p < 0.02) in subjects on calorie-restricted diets. In tissue from subjects on weight-maintaining diets, physiologic concentrations of insulin (25 μU./ml.) significantly increased glucose incorporation into carbon dioxide (p < 0.005) and glycogen (p < 0.001). Maximum insulin-stimulated glucose oxidation (increase over basal) was significantly enhanced (p < 0.05) in tissue from obese subjects, whereas insulin-mediated glucose incorporation into glycogen was similar in controls and obese subjects on weight-maintaining diets. Insulin-stimulated glucose oxidation was imparied in tissue from subjects on hypocaloric diets although fat-cell diameter was similar to those of obese subjects on weight-maintaining diets.
The effect of insulin on glucose incorporation into glycogen in isolated adipocytes was also studied. There was no correlation between insulin-stimulated glycogen synthesis and cell diameter. When cells from the same individual were separated into small and large adipocytes by differential flotation, the insulin effect was similar whether expressed as absolute or per cent increase over basal.
These results indicate that in vitro glucose oxidation by adipose tissue, in both the absence and the presence of insulin, is largely determined by dietary factors. This may also be true for insulin-stimulated glycogen synthesis. No evidence is provided for the concept that the enlarged human fat cell of obesity is insensitive to insulin in vitro.