Effect of dietary protein and energy on protein synthesis during lactation in rats.

A series of experiments was carried out in which effects of dietary protein and energy on protein synthesis during lactation were evaluated. Purified diets were fed to female rats from conception until d 15 of lactation. Protein supplied by wheat gluten, gluten + lysine + threonine or casein + methionine was fed at 20 or 25% in the diet. Protein synthesis in mammary gland and liver was measured with a large dose of L-[4-3H]phenylalanine. As lactation proceeded, protein absolute synthesis rates (ASRs) in casein-fed dams increased from 1056 to 2246 mg/d in mammary gland but remained unchanged in liver. In contrast, ASRs in wheat gluten-fed dams remained unchanged in both tissues. Feeding a protein-free diet or starving dams for 3 d reduced ASR in mammary gland 59 and 72%, respectively, when measured on d 15 of lactation. The corresponding reductions in liver ASR were 41 and 46%. A 20% restriction of dietary energy at a constant protein intake reduced protein synthesis in mammary glands 34, 55 and 28% in dams fed gluten, gluten + lysine + threonine or casein, respectively. In summary, protein synthesis during lactation was reduced less in liver than in mammary gland in response to dietary restriction of protein or energy.