Differences in amino acid digestibility between young and older adults: a randomized crossover study using the dual tracer method

BACKGROUND: Older adults may have reduced amino acid (AA) digestibility due to physiological changes in the digestive tract, but quantification of reduced AA digestibility in humans is lacking. OBJECTIVES: This randomized crossover study investigated differences in indispensable AA digestibility, primarily focusing on threonine and lysine, between young and older adults for milk, sorghum, and black beans using the dual tracer method. METHODS: Ten young (21.8 ± 1.7 y) and 10 older (72.8 ± 3.8 y) adults ingested 20 g ²H-labeled protein from either milk, sorghum, or black beans, mixed with 400 mg of a ¹³C-labeled AA mixture, in a plateau feeding protocol on 3 separate test days. Blood was sampled before and at regular intervals over an 8-h period after meal consumption. The ²H- and ¹³C- enrichments of AAs in plasma samples were determined at steady state (5.5-8.0 h). The ratio of the ²H/¹³C-ratio between blood plasma and test meals was calculated. For lysine and threonine, this ratio was compared statistically between age groups and protein sources using linear mixed model analysis. RESULTS: The isotope plasma-to-meal ratio for lysine and threonine was 19% lower in older adults compared with young adults for sorghum (P = 0.041), whereas it was not significantly different for milk (P = 0.277) and black beans (P = 0.849). CONCLUSIONS: Lysine and threonine digestion might be lower in older compared with young adults, but the effect differed among protein sources. Furthermore, research on how aging affects protein digestibility across protein sources is needed to optimize dietary protein recommendations for older adults. This study was registered at clinicaltrials.gov as NCT05534464.