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Background & aims: Aging and experimental bed rest are associated with muscle atrophy and resistance to post-prandial stimulation of protein synthesis or anabolic resistance (AR). We have used in young and older adult volunteers, during short-term bed rest, a quick and non-invasive method, based on a single oral bolus of the stable isotope L[ring-H-2(5)]phenylalanine (D(5)Phe), to determine post-prandial AR, defined as ratio between irreversible hydroxylation and incorporation into body protein of ingested phenylalanine.Methods: We compared in older (0, 59 +/- 1 y) and young (Y, 23 +/- 1 y) healthy male volunteers the effects of two-week bed rest on post-prandial protein kinetics, assessed during absorption of a standard ready to-use oral nutritional supplement, through stable-labeled isotope amino acid D(5)Phe, diluted in water, given as single oral load. The metabolic fate of D(5)Phe is either utilization for protein synthesis or irreversible hydroxylation to L[ring-H-2(4)]tyrosine (D(4)Tyr). AR was defined as ratio between the areas under the curves of D(4)Tyr-to-D(5)Phe plasma concentrations over 6 h meal absorption. To determine the relationships between AR and muscle changes following bed rest, quadriceps muscle volume (QMV) was determined by magnetic resonance imaging (MRI).Results: At baseline, in pooled Y and O subjects, values of AR were inversely correlated with QMV (R = -0.75; p < 0.03). Following 2-weeks of inactivity, there were significant bed rest effects on AR (p < 0.01) and QMV (p < 0.03), as well as significant bed rest x group interaction for AR (p < 0.03; +9.2% in Y; +21.9% in 0) and QMV (p < 0.05; -5.7% in Y; -%7.3 in O). In pooled subjects, the percentage delta changes in AR and QMV, induced by bed rest, were inversely correlated (R = -0.57; p < 0.05).Conclusion: Bed rest-induced AR is much greater in the older than in younger adults. We have developed a new, simple, non-invasive method for the assessment of AR. The results indicate that this metabolic abnormality is a key mechanism for sarcopenia of aging and inactivity. (C) 2016 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.

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