Amino acid metabolism and whole body protein synthesis and degradation in the post-absorptive and post-prandial state following 3 weeks habituation to a normal and high protein intake

Background: Higher protein intakes are recommended to older adults. Such enhanced intakes lead to metabolic adaptations in the handling of amino acids. Objective: Elucidating how the habituated level of protein intake affects the fasted state, as well as the stimulatory effect of a standardized protein-rich meal on whole body protein turnover and amino acid utilization in older males. Design: 12 males (65-70 years) were included in a double-blinded cross-over intervention study, consisting of a 20-day habituation period to RDA or high-protein intake (1.1 g/kg lean body mass (LBM)/day or >2.1 g/kg LBM/day), each followed by an experimental trial with a primed, constant infusion of D8-Phe and D2-Tyr. Arterial and hepatic venous, and venous blood samples were obtained in the overnight fasted state and repeatedly four hours after a standardized meal including intrinsically labeled D5-Phe-whey protein concentrate and 15N-Phe-calcium-caseinate proteins. Blood was analyzed for amino acid concentrations and Phe/Tyr tracer enrichments and whole body protein kinetics were calculated. Gut microbiome (GM) composition was determined before habituation start and before infusion.Results: High compared to the recommended level of protein intake resulted in a higher fasting protein synthesis and breakdown resulting in a lower net balance (P2.1 g protein/kg LBM/day) let to a significantly lower fasting net balance, not compensated for by a meal high in protein. Thus, we suggest caution, before advising a protein intake for elderly, which by far exceeds the current official recommendations.