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

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. 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. Ten young (21.8±1.7 years) and ten older (72.8±3.8 years) adults ingested 20 grams 2 H-labelled protein from either milk, sorghum or black beans, mixed with 400 mg of a 13 C-labelled AA mixture, in a plateau feeding protocol on three separate test days. Blood was sampled before and at regular intervals over an eight-hour period following meal consumption. The 2 H- and 13 C- enrichments of AAs in plasma samples were determined at steady state (5.5-8.0 hours). The ratio of the 2 H/ 13 C-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. The isotope plasma-to-meal ratio for lysine and threonine was 19% lower in older adults compared to young adults for sorghum (P=0.041), whereas it was not significantly different for milk (P=0.277) and black beans (P=0.849). Lysine and threonine digestion might be lower in older versus young adults but the effect differed among protein sources. Further research on how ageing affects protein digestibility across protein sources is needed to optimize dietary protein recommendations for older adults. Clinicaltrials.gov: NCT05534464