Reduced lipogenesis in rats fed a high-protein, carbohydrate-free diet: participation of liver and four adipose depots.

1. Rates of in vivo fatty acid (FA) synthesis were assessed with 3H2O in carcass, liver, intestine, muscle and four adipose depots from rats fed a high-protein, carbohydrate-free diet (70% casein, 8% fat, w/w) or a balanced diet (66% carbohydrate, 17% casein, 8% fat) for 25-30 days. 2. Rats adapted to the high protein (HP) diet showed a marked reduction of total FA synthesis from all carbon sources, which was due to a decreased synthesis of triacylglycerols. Rates of phospholipid-fatty acid synthesis in both carcass and liver were not affected by the diet. Rates of triacylglycerol-fatty acid (TAG-FA) synthesis were markedly reduced in adipose tissue from four different sites: epididymal (79%), retroperitoneal (78%), subcutaneous (65%) and intermuscular (82%). 3. In rats fed the balanced control diet, TAG-FA synthesis in adipose tissue accounted for about 75% of synthesis in whole carcass, whereas it was reduced to 36% in rats under the HP regimen. 4. Although hepatic lipogenesis was also reduced in HP-fed rats, the contribution of the liver to total TAG-FA synthesis was approximately the same in HP (24%) and control (20%) rats, whereas the contribution of adipose tissue was only 26% in HP-fed animals compared to 57% in controls. 5. Force-feeding fed rats with components of their own diets resulted in a significant (100%) increase of liver TAG-FA synthesis in animals fed the control diet, but did not significantly affect liver lipogenesis in HP rats.