Relationships among dietary protein, feed intake and changes in body and tissue composition of lactating rats.

To quantify effects of protein and energy intake on changes in tissue composition in lactating rats, 36 rats were fed three diets (L, M and H, containing 12,24, and 36% protein, respectively) at four levels of intake (100, 70, 50 and 30% of ad libitum) from d 7 to 14 of lactation. Three rats per treatment were killed on d 14; carcass, viscera, liver and mammary glands were dissected and analyzed for water, fat, protein and ash. Twelve rats killed on d 7 provided initial composition estimates. All rats lost weight loss was affected slightly by diet but drastically by intake, ranging from 22.4 g in ad libitum-fed rats to 79.9 g in the 30% group. Fat losses, mostly from carcass, were insensitive to diet but were increased to maximum at the 70% intake level, and further restriction did not result in significantly greater fat losses. Protein losses in all tissues increased with degree of feed restriction. Protein losses in viscera, liver and mammary decreased with higher dietary protein. Possible homeorrhetic adaptations responsible for these patterns of nutrient partition are discussed in terms of changes in rates of fat and protein turnover and hormonal effects upon these rates.