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 consumed 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 protein; diet B: 14.5% of energy provided by animal protein and 5.1% by vegetable 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 between diets. At the end of each dietary period, nitrogen balance and protein turnover were measured. Protein turnover was measured during 4 h of fasting followed by 4 h of feeding. Comparisons were made between fasted and fed periods (within one diet) and between the diets to study the effect of the protein source and quantity. Mean nitrogen balance did not differ significantly from zero during diets B and C and was not affected by the protein source. Meal feeding resulted in increased protein flux and protein oxidation and decreased protein breakdown compared with the postabsorptive values; there was no effect of feeding on protein synthesis. With the high-vegetable-protein diet, protein breakdown in the absorptive state was not inhibited to the same extent as during the high-animal-protein diet, resulting in less net protein synthesis during the high-vegetable-protein diet than during the high-animal-protein diet.