The magnitude of the capacity to convert carbohydrate to fat in the human body is still controversial, as is the extent to which it takes place in the liver as opposed to the adipose tissue. We calculated whole-body net fat synthesis from indirect calorimetry and substrate balance data from five healthy men in the basal state and after 1 and 4 d on a hyperenergetic carbohydrate diet (approximately 2.5 times energy expenditure). At the same time, the secretion of fatty acids synthesized in the liver was measured to determine the extent to which fat synthesis occurs in the liver in a lipogenic state. The respiratory exchange ratio (RER) was 0.81 +/- 0.01 in the basal state and 0.99 +/- 0.025 and 1.15 +/- 0.022 on days 1 and 4, respectively. Although there was net fat oxidation in the basal state (955 +/- 139 mg.kg-1.min-1), there was net fat synthesis at the whole-body level both during early (day 1; 481 +/- 205 mg.kg-1.min-1) and late (day 4; 2243 +/- 253 mg.kg-1.min-1) carbohydrate overfeeding. Although hepatic secretion of fat synthesized de novo increased approximately 35-fold during the study (basal state, 1.0 +/- 0.3; day 1, 13.8 +/- 6.8; and day 4, 43.3 +/- 16.3 mg.kg-1.min-1) this could only account for a small portion of total fat synthesis. We conclude that the liver plays a quantitatively minor role when surplus carbohydrate energy is converted into fat in the human body. The main site for fat synthesis is likely to be the adipose tissue.