Postprandial amino acid profiles in older and younger adults following high and normal protein ingestion

Disruptions in protein metabolism can impact health outcomes in older age, with amino acid metabolism playing a central role. Aging is associated with impaired muscle protein synthesis, insulin resistance, and inflammation, all of which may alter postprandial amino acid responses. This study analyzed amino acid profiles in older (n = 40, female n = 32) and younger (n = 30, female n = 21) adults after consuming test meals with standard protein [NP; 16 energy percent (E%) protein, 68 E% carbohydrates] or high protein [HP; 77 E% protein, 17 E% carbohydrates; 450 kcal each]. Postprandial plasma amino acids were quantified at 0, 30, 60, 120, and 240 min using liquid chromatography coupled with tandem mass spectrometry, with plasma amino acids responses evaluated as the incremental area under the curve (iAUC). Fasting amino acid concentrations were similar between age groups, but significant postprandial changes (p < 0.05) were observed after both meals. Older adults exhibited altered branched-chain amino acids, proline, and tyrosine dynamics (all p < 0.05) after HP ingestion. Postprandial amino acid dynamics were comparable age between groups after NP ingestion, except for higher glutamine in older adults, but markedly higher postprandial amino acid concentrations occurred in older adults after HP. Regression analysis revealed insulin resistance, insulin response, and interleukin-6 as key determinants of amino acid iAUCs after NP, while skeletal muscle mass index, habitual protein intake and liver enzymes were significant factors after HP. These findings suggest that amino acid dynamics in older adults are shaped by habitual protein intake, muscle mass, insulin resistance, and inflammation rather than age alone. A more pronounced postprandial amino acid response to a high-protein meal in older adults may reflect reduced amino acid uptake due to lower muscle mass.