To determine whether increased lipogenesis contributes to human obesity, we measured (postabsorptive state), in lean and obese subjects, lipid synthesis (deuterated water method) and the mRNA concentration (RT-competitive PCR) in subcutaneous adipose tissue of fatty acid synthase (FAS) and sterol regulatory element-binding protein (SREBP)-1c. Before energy restriction, obese subjects had an increased contribution of hepatic lipogenesis to the circulating triglyceride pool (14.5 +/- 1.3 vs. 7.5 +/- 1.9%, P < 0.01) without enhancement of cholesterol synthesis. This increased hepatic lipogenesis represented an excess of 2-5 g/day of triglycerides, which would represent 0.7-1.8 kg on a yearly basis. The lipogenic capacity of adipose tissue appeared, on the contrary, decreased with lower FAS mRNA levels (P < 0.01) and a trend for decreased SREBP-1c mRNA (P = 0.06). Energy restriction in obese patients decreased plasma insulin (P < 0.05) and leptin (P < 0.05) and normalized hepatic lipogenesis. FAS mRNA levels were unchanged, whereas SREBP-1c increased. In conclusion, subjects with established obesity have an increased hepatic lipogenesis that could contribute to their excessive fat mass but no evidence for an increased lipogenic capacity of adipose tissue.