Proteins are degraded and amino acids are metabolized in different quantities during endurance exercise. However, a clear consensus on protein oxidation during exercise has yet to be established. The main objective was to calculate estimates of protein oxidation during endurance exercise using available data. Additionally, we aimed to investigate the effects of exercise intensity, duration, and volume on protein oxidation. We systematically searched for research studies published in English in the online databases PubMed and Google Scholar in March 2023. The inclusion criteria were: (1) measurement of protein metabolism with nitrogen excretion, leucine oxidation, or indicator amino acid utilization method; (2) inclusion of an endurance exercise condition and a control condition without exercise; (3) inclusion of a description of the endurance exercise protocol (duration, intensity); and (4) inclusion of healthy participants over the age of 18. Endurance exercises were defined as exercise periods of at least 60 min' duration of running, cycling, or cross-country skiing. We included 30 articles (n = 286 participants). Protein oxidation increased by 1.02 ± 0.06 mg·kg-1·min-1 (95% CI [0.91, 1.14]) during endurance exercise, from the level of 0.81 ± 0.38 mg·kg-1·min-1 measured without exercise. Protein contributed 3.28% ± 0.15% (95% CI [2.97, 3.58]) of the total energy expenditure during exercise. Protein oxidation normalized by exercise duration significantly increased with exercise intensity. This review is the first to aggregate data on protein oxidation during endurance exercise, measured using different methods. Endurance exercise consistently increased protein oxidation, with protein metabolism more than doubling during exercise compared to rest. Protein oxidation increased with exercise intensity, but neither exercise duration nor volume augmented the rate of protein oxidation.
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