7/23/2023 0 Comments Protein turnoverIt is well known that the ingestion of protein or amino acids results in increased plasma amino acid levels (hyperaminoacidemia) that stimulate muscle protein synthesis (MPS) and that resistance exercise increases this feeding-induced stimulation of MPS even more (Breen & Phillips, 2012). With aging, changes in sex steroids in women (likely post-menopause) may result in an elevated rate of protein turnover, but with little response to feeding or contraction.Sex-based differences in muscle protein turnover are not observed when comparing young men and women.With resistance exercise training, MPS is elevated in both sexes and at all ages, but this does not change the feeding-induced increase in MPS.Older men have robust MPS responses to resistance exercise whereas older women are somewhat impaired in their ability to stimulate MPS with exercise.Despite marked differences in the levels of testosterone, young men and women show very similar responses to resistance exercise. Younger women exhibit no difference in MPS across the menstrual cycle and very little difference in amino acid kinetics.Older women in particular have a diminished MPS response to protein feeding. Younger men and women exhibit similar MPS responses to feeding, whereas older men and women show comparatively diminished MPS response, or become “resistant” to feeding, compared to their young counterparts.However, older women appear to have an elevated rate of MPS. Basal levels of muscle protein synthesis (MPS) are similar in young men and women as well as older men.Muscle proteins turn over in response to both feeding and exercise and these responses are dictated by amino acid availability and exercise loading variables.As suggested in several publications, a bolus of 15-20 g protein (from skimmed milk or whey proteins) and carbohydrate (± 30 g maltodextrine) drinks is needed immediately after stopping exercise to stimulate muscle protein and tendon collagen turnover within 1 h. Muscular activities promote a cascade of signals leading to the stimulation of eukaryotic initiation of myofibrillar protein synthesis. Nitrogen balance (difference between protein intake and protein degradation) for athletes is usually balanced when the intake of protein reaches 1.2 g Endurance exercise induces a greater oxidative capacity (enzymes) compared to resistance exercise, which induces fiber hypertrophy (myofibrils). Individuals who exercise respond differently when resistance and endurance types of contractions are compared. Strenuous exercise provokes increased proteolysis and decreased protein synthesis, the opposite occurring during the recovery period. There are different fractional synthesis rates in skeletal muscle and tendon tissues, but there is no major difference between collagen and myofibrillar protein synthesis. Stable isotopic tracers ((13)C-lysine, (15)N-glycine, ²H5-phenylalanine) and arteriovenous differences have been used in studies of skeletal muscle and collagen tissues under resting and exercise conditions. Noninvasive and invasive techniques have been applied to determine amino acid catabolism and muscle protein building at rest, during exercise and during the recovery period after a single experiment or training sessions. As for any cell or tissue, total muscle protein reflects a dynamic turnover between net protein synthesis and degradation. Skeletal muscle is the major deposit of protein molecules.
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