Our recent in vivo observations in healthy nonobese humans have demonstrated that prolonged elevation of plasma free fatty acids (FFAs) results in diminished glucose-stimulated insulin secretion (GSIS) when the F FA-mediated decrease in insulin sensitivity is taken into account. In the present study, we investigated whether obese individuals and patients with type 2 diabetes are more sensitive than healthy control subjects to the inhibitory effect of prolonged elevation of plasma FFAs on GSIS. In seven patients with type 2 diabetes and seven healthy nondiabetic obese individuals, we assessed GSIS with a programmed graded intravenous glucose infusion on two occasions, 6-8 weeks apart, with and without a prior 48-h infusion of heparin and Intralipid, which was designed to raise plasma FFA concentration approximately twofold over basal. The nondiabetic obese subjects had a significant 21% decrease in GSIS (P = 0.0008) with the heparin and Intralipid infusion, associated with a decrease in whole body insulin clearance. The impairment in GSIS was evident at low (<11 mmol/l) but not at higher plasma glucose concentrations. In contrast, the patients with t y p e 2 diabetes had a slight increase in GSIS (P = 0.027) and no change in insulin clearance, although there was marked interindividual variability in response. Plasma proinsulin concentrations measured in a subset of subjects were not altered in either group by the infusion of heparin and Intralipid. In summary, 1) obese nondiabetic individuals are susceptible to a desensitization of GSIS with heparin and Intralipid infusion, and 2) patients with type 2 diabetes do not demonstrate such susceptibility when FFAs are elevated approximately twofold above basal with heparin and Intralipid. Our results suggest that FFAs could play an important role in the development of -cell failure in obese individuals who are at risk for developing t y p e 2 diabetes. They do not, however, seem to further deteriorate the -cell function of patients who already have established type 2 diabetes and may even result in a slight increase in GSIS in this latter group. D i a b e t e s 4 9 :3 9 9-408, 2000 I t is well accepted that type 2 diabetes is characterized by defects in both insulin action and insulin secretion (1,2), with a specific defect in glucose-stimulated insulin secretion (GSIS) early in the evolution of this disease (3,4). Over the past decade, several investigators have focused their attention on the possible role of free fatty acids (FFAs), which are often elevated in states of insulin resistance, in selectively desensitizing the -cell to glucose (5-9). The concept of FFA-induced -cell desensitization has received support from prospective epidemiological studies, which show that elevated plasma FFA levels are a risk marker for the long-term development of glucose intolerance and progression toward type 2 diabetes both in Caucasians (10) and in Pima Indians (11).The acute stimulating effect of FFAs on GSIS has been well described both in vitro (12,13) and in vivo (14-16), and...