HEPATIC AND WHOLE-BODY FAT SYNTHESIS IN HUMANS DURING CARBOHYDRATE OVERFEEDING

The magnitude of the capacity to convert carbohydrate to fat in the hu man body is still controversial, as is the extent to which it takes pl ace in the liver as opposed to the adipose tissue. We calculated whole -body net fat synthesis from indirect calorimetry and substrate balanc e data from five healthy men in the basal state and after 1 and 4 d on a hyperenergetic carbohydrate diet (approximate to 2.5 times energy e xpenditure). At the same time, the secretion of fatty acids synthesize d in the liver was measured to determine the extent to which fat synth esis occurs in the liver in a lipogenic state. The respiratory exchang e 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 n et fat oxidation in the basal state (955 +/- 139 mg.kg(-1).min(-1)), t here 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 approximate to 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 porti on of total fat synthesis. We conclude that the liver plays a quantita tively minor role when surplus carbohydrate energy is converted into f at in the human body. The main site for fat synthesis is likely to be the adipose tissue.