Composition of gain of rats fed low or high protein diets and grown at controlled rates from 80 to 205 grams.

Composition of gain was measured in 54 rats fed purified diets that contained either 10 or 20% casein. Rats were weighed three times weekly and their intakes adjusted so that five or six rats within each diet gained maximally or at 2, 3, 4 or 5 g/d. Live weight for each rat was 80 g at the start and 205 g when the experiment ended. Ten additional rats were killed at about 80 g to estimate initial composition. The gain contained less (P less than 0.005) water and protein and more (P less than 0.005) lipid as rate of gain increased. Gain of gastrointestinal tract was increased (P less than 0.005) with increasing rate of gain. Rats that gained faster had shorter (P less than 0.05) tibias. More (P less than 0.005) water and protein and less (P less than 0.005) lipid were in the gain of rats fed the 20% casein diet than of those fed the 10% casein diet. Rats fed the 10% casein diet had longer bodies, and those fed the 20% casein diet had longer tibias. Water and protein gain declined and tibia length was less while liver gain increased with increasing rate of gain, but all rates of change for these measurements were higher for rats fed the 10% casein diet. Dissected epididymal fat pad gain was unaffected by casein level or rate of gain. These results demonstrate that chemical composition of the total gain can be altered when rate of growth is controlled and that the epididymal fat pad is not representative of chemical composition.