Utilization and requirement of dietary protein taking into account the dermal and miscellaneous nitrogen losses in Japanese women.

Utilization and requirement of mixed protein in the conventional Japanese diet and the obligatory integumental and miscellaneous nitrogen losses were measured in female subjects. Twelve female students aged 18 to 24 years were given conventional low-protein diets at N intake levels of 50, 70, and 90 mg/kg/day for 10 days. Constituents of the diets corresponded to those of average intake of the Japanese in recent years (1982). N balances were estimated and regression equations between N intake (X) and N balance (Y) were calculated by the multiple level-constant variation method. The equation was Y = 0.426X - 40.0 (n = 36, r = 0.615, p less than 0.01) and the intersection of the regression line with zero nitrogen balance was 93.9 +/- 14.3 mg N/kg/day. The mean digestibility was 92.2 +/- 4.7%. In another experimental group, nitrogen losses due to hair, nails, and menstruation in ten Japanese women were 0.624 +/- 0.172, 0.020 +/- 0.005, and 1.76 +/- 0.68 mg/kg/day, respectively. In a third experimental group, mean of the integumental N losses was 2.8 mg/kg/day in both the subjects given a low-protein diet (19 women) and an ordinary-protein diet (4 women). It increased to 12.7 mg N/kg/day when subjects exercised in summer (4 subjects). The protein requirement in the conventional Japanese diet estimated as the sum of the mean requirement of dietary protein and obligatory dermal and miscellaneous nitrogen losses in Japanese women was 99.1 mg N/kg/day or 0.62 g protein/kg/day. The net protein utilization (NPU) of the proteins at the N intake level for N equilibrium was estimated as 48.