Protein requirement of elderly women: Nitrogen balance responses to three levels of protein intake

Background. For elderly women, insufficient data exist to assess the accura cy of the assumed mean protein requirement of 0.6 g of protein . kg(-1) . d ay(-1), and the adequacy of the current Recommended Dietary Allowance (RDA) of 0.8 g of protein . kg(-1) . day(-1). The aims of this study were to ass ess the mean protein requirement and suggested safe and adequate protein in take (protein allowance) of elderly women using a shorter-term nitrogen bal ance protocol. Methods. During three separate 18-day trials, I I elderly women (age range, 70-81 years) were randomly fed eucaloric diets designed to provide either 0.50, 0.75, or 1.00 g of protein . kg(-1) . day(-1). Nitrogen balance was d etermined at Weeks 2 and 3 (Days 7-10 and 14-17, respectively) of each tria l using data from total nitrogen analyses of duplicate food composites, 24- hour urine collections, and stool collections. The mean protein requirement was calculated using linear regression of individual women's data from all three trials and inverse prediction. Results. At protein intakes of 0.53 +/- 0.02, 0.76 +/- 0.02, or 1.06 +/- 0. 05 g of protein . kg(-1) . day(-1), net nitrogen balances during Week 2 wer e -14.5 +/- 3.1, 3.8 +/- 2.5 and 23.4 +/- 3.3 mg of nitrogen . kg(-1) . day (-1), respectively, for these body weight- and body composition-stable wome n. At Week 3, the net nitrogen balances were -0.1 +/- 2.7, 8.5 +/- 3.6 and 42.0 +/- 3.0 mg of nitrogen . kg(-1) . day(-1). From Week 2 to Week 3, shif ts to more positive nitrogen balances occurred due to decreases in urinary nitrogen excretion. The mean protein requirement at Week 2 was calculated t o be 0.70 +/- 0.09 g of protein . kg(-1) . day(-1) (coefficient of variatio n [CV] = 13%) and at Week 3 was calculated to be 0.56 +/- 0.09 g of protein . kg(-1) day(-1) (CV 17%). From these data, an adequate protein allowance was estimated to be greater than the RDA at Week 2 (0.90 g of protein . kg( -1) . day [d](-1)), and not different than the RDA at Week 3 (0.76 g of pro tein . kg(-1) . d(-1)). Conclusions. The decrease over time in urinary nitrogen excretion from Week 2 to Week 3 suggests that these elderly women did not achieve a metabolic steady state during this shorter-term nitrogen balance study. Collectively, these data suggest that the total protein needs of elderly women are at or above the current RDA for protein. However, the results of this study indi cate that shorter-term nitrogen balance protocols are insufficient to firml y establish the RDA for protein of elderly women, and further research is r equired using alternative criteria measures.