Effect of protein source and quantity on protein metabolism in elderly women

To study sequentially the effect of meal feeding and the effect of protein source and quantity on whole-body protein metabolism, 12 elderly women cons umed 3 diets differing in both the quantity and source of protein (diet A: 5.3% of energy intake provided by animal protein and 5.0% by vegetable prot ein; diet B: 14.5% of energy provided by animal protein and 5.1% by vegetab le protein; diet C: 5.0% of energy provided by animal protein and 15.1% by vegetable protein). The diets were consumed for 2 wk with a 2-wk interval b etween diets. At the end of each dietary period, nitrogen balance and prote in turnover were measured. Protein turnover was measured during 4 h of fast ing followed by 4 h of feeding. Comparisons were made between fasted and fe d periods (within one diet) and between the diets to study the effect of th e protein source and quantity. Mean nitrogen balance did not differ signifi cantly from zero during diets B and C and was not affected by the protein s ourer. Meal feeding resulted in increased protein flux and protein oxidatio n and decreased protein breakdown compared with the postabsorptive values; there was no effect of feeding on protein synthesis. With the high-vegetabl e-protein diet, protein breakdown in the absorptive state was not inhibited to the same extent as during the high-animal-protein diet, resulting in le ss net protein synthesis during the high-vegetable-protein diet than during the high-animal-protein diet.