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Nutrition and Disease

Dietary Protein Digestion and Absorption Rates and the Subsequent Postprandial Muscle Protein Synthetic Response Do Not Differ between Young and Elderly Men^1,2

³ Department of Human Movement Sciences; ⁴ Department of Human Biology, Nutrition and Toxicology Research Institute Maastricht, Maastricht University, Maastricht 6200 MD, The Netherlands; ⁵ Human Nutrition Unit, Université Clermont Auvergne, Centre de Recherche en Nutrition Humaine BP 321, 63009 Clermont-Ferrand Cedex 1, France; ⁶ DSM Food Specialties, R&D, Biochemistry and Nutrition Department, Delft 2600 MA, The Netherlands; and ⁷ Top Institute Food and Nutrition, Wageningen 6700 AN, The Netherlands

Impaired digestion and/or absorption of dietary protein lowers postprandial plasma amino acid availability and, as such, could reduce the postprandial muscle protein synthetic response in the elderly. We aimed to compare in vivo dietary protein digestion and absorption and the subsequent postprandial muscle protein synthetic response between young and elderly men. Ten elderly (64 ± 1 y) and 10 young (23 ± 1 y) healthy males consumed a single bolus of 35 g specifically produced, intrinsically L-[1-¹³C]phenylalanine-labeled micellar casein (CAS) protein. Furthermore, primed continuous infusions with L-[ring-²H₅]phenylalanine, L-[1-¹³C]leucine, and L-[ring-²H₂]tyrosine were applied and blood and muscle tissue samples were collected to assess the appearance rate of dietary protein-derived phenylalanine in the circulation and the subsequent muscle protein fractional synthetic rate over a 6-h postprandial period. Protein ingestion resulted in a rapid increase in exogenous phenylalanine appearance in both the young and elderly men. Total exogenous phenylalanine appearance rates (expressed as area under the curve) were 39 ± 3 µmol·6 h·kg^–1 in the young men and 38 ± 2 µmol·6 h·kg^–1 in the elderly men (P = 0.73). In accordance, splanchnic amino acid extraction did not differ between young (72 ± 2%) and elderly (73 ± 1%) volunteers (P = 0.74). Muscle protein synthesis rates, calculated from the oral tracer, were 0.063 ± 0.006 and 0.054 ± 0.004%/h in the young and elderly men, respectively, and did not differ between groups (P = 0.27). We conclude that protein digestion and absorption kinetics and the subsequent muscle protein synthetic response following the ingestion of a large bolus of intact CAS are not substantially impaired in healthy, elderly men.

^* To whom correspondence should be addressed. E-mail: l.vanloon{at}HB.unimaas.nl.

Manuscript received 21 April 2009. Initial review completed 21 May 2009. Revision accepted 29 June 2009.

Published online 22 July 2009.