Metabolic isotopes have been used in human metabolism research for several decades ( 21) to “trace” numerous metabolites, including glucose, lipids, and amino acids. The application of appropriate methods for measuring protein turnover during periods of high physiological stress will improve our understanding of the adaptive, or maladaptive, responses to military training, and help inform the development of strategies to preserve lean mass for physical performance and injury protection.
Energy ( 5, 10) and protein ( 5, 11, 12) balance are difficult to maintain during military training, likely due to these physiological stressors consequently, military training and field exercises are often accompanied by a loss of lean body mass ( 4, 11, 13–15), suppressed circulating anabolic hormones ( 13, 16–18), and impaired physical performance ( 5, 13–16, 19, 20). Military field exercises, training courses, and operations are characterized by prolonged periods of intense physical activity ( 4–6), activities that engage multiple large muscle groups, restricted feeding ( 4, 8, 9), and restricted sleep ( 4).
Conversely, a chronic, negative muscle protein balance results in the loss of skeletal muscle mass, and compromised health and physical performance ( 2, 3). Exercise training and protein feeding are anabolic stimuli for skeletal muscle and result in a net positive muscle protein balance ( 1). Protein turnover is the continual, metabolic process regulating tissue mass and function, and reflects the balance between rates of protein synthesis and breakdown. Application of this method may improve our understanding of protein kinetics during conditions of high physiological stress in free-living environments such as military training. The end-product method using 15N-glycine is a practical method for measuring whole-body protein turnover in the field over short (24 h) time frames and has been used effectively in recent military field research. However, the end-product method can also be used to measure acute (over 9–24 h) whole-body protein turnover noninvasively by ingesting 15N-glycine, or equivalent isotope tracers, and collecting urine samples. Both methods require invasive procedures such as the infusion of amino acid tracers and muscle biopsies to assess the uptake of the tracer into tissue. The precursor method is the gold-standard for measuring acute (over several hours) skeletal muscle protein turnover, whereas the end-product method measures chronic (over several weeks) skeletal muscle protein turnover and provides the opportunity to monitor free-living activities. The 2 primary approaches for measuring protein turnover are based on precursor- and end-product methods.
We undertook a narrative review of published literature with the aim of reviewing the suitability of, and advancements in, stable isotope methods for measuring protein turnover in field research. Evaluating protein turnover in free-living environments, such as military training, can help inform protein requirements. skeletal muscle only) level using stable isotope methods. aggregated across all body proteins) or tissue (e.g. Protein turnover reflects the continual synthesis and breakdown of body proteins, and can be measured at a whole-body (i.e.
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