Dietary Protein Requirement of Men >65 Years Old Determined by the Indicator Amino Acid Oxidation Technique Is Higher than the Current Estimated Average Requirement

Background: The current estimated average requirement (EAR) and RDA for protein of 0.66 and 0.8 g . kg(-1) . d(-1), respectively, for adults, including older men, are based on nitrogen balance data analyzed by monolinear regression. Recent studies in young men and older women that used the indicator amino acid oxidation (IAAO) technique suggest that those values may be too low. This observation is supported by 2-phase linear crossover analysis of the nitrogen balance data. Objective: The main objective of this study was to determine the protein requirement for older men by using the IAAO technique. Methods: Six men aged > 65 y were studied; each individual was tested 7 times with protein intakes ranging from 0.2 to 2.0 g . kg(-1) . d(-1) in random order for a total of 42 studies. The diets provided energy at 1.5 times the resting energy expenditure and were isocaloric. Protein was consumed hourly for 8 h as an amino acid mixture with the composition of egg protein with L-[1-C-13] phenylalanine as the indicator amino acid. The group mean protein requirement was determined by applying a mixed-effects change-point regression analysis to (FCO2)-C-13 (label tracer oxidation in breath (CO2)-C-13), which identified a breakpoint in (FCO2)-C-13 in response to graded intakes of protein. Results: The estimated protein requirement and RDA for older men were 0.94 and 1.24 g . kg(-1) . d(-1), respectively, which are not different from values we published using the same method in young men and older women. Conclusions: The current intake recommendations for older adults for dietary protein of 0.66 g . kg(1) . d(1) for the EAR and 0.8 g . kg(-1) . d(-1) for the RDA appear to be underestimated by similar to 30%. Future longer-term studies should be conducted to validate these results.