Dual-tracer approach to study amino acid digestibility in old and young adults

For healthy ageing and prevention of sarcopenia, the amount and quality of dietary protein intake is important. The quality is determined by amino acid (AA) composition and digestibility of the protein source. Older adults are suspected to have reduced AA digestibility, due to a series of physiological changes in the ageing GI tract as recently reviewed for an older adult INFOGEST in vitro digestion model. However, there is limited in vivo evidence. This study aimed to determine the difference in AA digestibility of milk, sorghum and black beans between older (65-80 years) and younger (20-35 years) adults using the dual-tracer method.
The study was executed in 10 young and 10 older adults, with - up till now - data of 7 young and 7 old participants reported. Participants ingested 20g intrinsically 2H-labelled protein (from milk, sorghum or black beans) with 400mg 13C-labelled amino acid mixture in a plateau feeding protocol on three separate test days. The ratio between 2H and 13C enrichment of the indispensable AA in blood plasma was analysed and numerically compared between the two age groups, as a proxy of digestibility.
With data from 14 participants and combined for the three protein sources, the 2H/13C ratio was numerically lower in older compared to young adults for some measured indispensable AA. Plasma ratio of leucine was 0.24±0.21 (mean ± SD) in young and 0.22±0.24 in older adults (9% lower in older adults). Numerical reduction of 14% for histidine and 6% for methionine were observed in older compared to young adults. This reduction aligns with the hypothesized lower digestibility in older adults, but proper evaluation using the principles of the dual-tracer approach should be executed.
In conclusion, numerical differences in plasma 2H/13C ratio between young and older adults were observed in our preliminary dataset of 14 participants. With an indication of lower values in the older adults for some indispensable AA, that should be confirmed by fully analysing our data according to the principles of the dual-tracer approach. These data indicate at least that the dual-tracer approach can be applied to investigate differences in AA digestibility between young and older adults. Quantifying these differences in digestibility upon ageing provides essential information for product formulation and dietary advice to an older population.