Aerobic exercise blunts the amino acid-stimulated muscle protein synthesis in the immediate post-exercise period in humans with obesity

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Obesity is associated with altered skeletal muscle protein metabolism. While plasma amino acids (AA) stimulate protein synthesis in muscle, it remains unclear how aerobic exercise modifies this response in obesity. In this study, we evaluated basal and amino acid-stimulated skeletal muscle protein fractional synthesis rate (FSR) in humans with obesity, induced by a primed, constant AA infusion (15% Clinisol; 4 mg·kg FFM-1·min-1), immediately after a single bout of acute aerobic exercise. We studied two groups of subjects with obesity: Group 1: AA infusion alone for 240 mins (AA; N = 8; BMI: 34.9 ± 1 kg/m 2 ; body fat percent: 35.2 ± 2) and group 2: acute aerobic exercise followed by AA infusion for 240 mins (AA+EX; N=8; BMI: 36.1 ± 2 kg/m 2 ; body fat percent: 42.4 ± 2). Subjects in the AA+EX group performed 45 minutes of cycling on a stationary bike at 65% VO2 max, and their amino acid infusion started immediately after the exercise ended. A d10-leucine tracer was infused for the entire duration of the study to measure mixed-muscle protein FSR (MMP-FSR). Muscle biopsies from the vastus lateralis were taken at baseline, 45 mins after the start of the amino acid infusion and at the end of the infusion study to determine MMP-FSR. Baseline MMP-FSR values did not differ between groups (p > 0.05). Participants receiving the AA infusion alone exhibited a significant increase in MMP-FSR from baseline to post-infusion (0.060 ± 0.005 to 0.119 ± 0.011 %/hr; p < 0.0001), whereas the AA+EX group showed no significant change from baseline (0.067 ± 0.004 to 0.084 ± 0.009 %/hr; p > 0.05). Plasma insulin concentrations increased significantly in response to the amino acid infusion (p < 0.0001) but did not differ between groups (p > 0.05). Baseline amino acid concentrations were not different between groups (p > 0.05). Among the plasma amino acids (μmol/L) changed, baseline plasma leucine (AA:137 ± 8, AA+EX: 144 ± 10; p > 0.05) increased significantly in both groups during AA infusion but was higher in the AA (424 ± 15) than the AA+EX (226 ± 13) group (p< 0.0001). Similarly, baseline plasma valine (AA: 226 ± 7, AA+EX: 234 ± 19; p > 0.05) increased significantly in both groups during AA infusion but was higher in the AA (553 ± 28) than the AA+EX (456 ± 23) group (p< 0.002). In contrast baseline arginine (AA: 59 ± 5, AA+EX: 71±11; p > 0.05), although it increased significantly in both groups during AA infusion, was lower in the AA (197 ± 3) than the AA+EX (266 ± 11) group (p< 0.0001). Despite marked increases in plasma amino acid and insulin concentrations, the AA+EX group exhibited a blunted muscle protein synthesis response to amino acid stimulation compared with the AA group. In conclusion, aerobic exercise blunts the amino acid-stimulated increase in muscle anabolism in the immediate period after exercise in humans with obesity. This research was supported by the National Institutes of Health/ National Institute of Diabetes and Digestive and Kidney Diseases. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

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