Mechanisms explaining the decrease in circulatory cholesterol levels after weight loss remain ill defined. The objective was to examine effects of weight loss as achieved through energy restriction upon human in vivo cholesterol biosynthesis. Six subjects (64-77 y, body mass index, 30.3 +/- 3.8 kg/m(2)) were recruited into a two-phase prospective clinical trial. In the first phase, subjects complied with American Heart Association (AHA) Step I diets for 3 mo with no change in their usual energy intake. After this weight-stable phase, subjects consumed an AHA Step I diet with a targeted reduction in energy intake of approximately 1000 kJ/d for 6 mo to achieve negative energy balance leading to weight loss. The incorporation rate of deuterium from body water into erythrocyte membrane free cholesterol over 24 h was utilized as an index of cholesterogenesis at the end of both phases. Subjects' mean weights decreased (P < 0.05) from 89.3 +/- 12.5 kg to 83.2 +/- 11.5 kg (6.8 +/- 2.6% of initial body weight) across phases. Circulating concentrations of total and LDL-cholesterol, and triglycerides also decreased (P < 0. 05) across phases. HDL-cholesterol concentrations were unchanged (P > 0.05). Cholesterol fractional synthetic rate (FSR) after phase 2 (3.04 +/- 1.90%/d) was lower (P < 0.05) than that after phase 1 (8. 42 +/- 3.90%/d). Absolute synthesis rate (ASR) after phase 2 [0.59 +/- 0.38 g/(kg. d)] also was lower (P < 0.05) than that after phase 1 [1.66 +/- 0.84 g/(kg. d)]. These data suggest that, in obese men, energy restriction resulting in even modest weight loss suppresses endogenous cholesterol synthesis, which contributes to a decline in circulating lipid concentrations.