The purpose of this review is to summarize the state of our knowledge on sex-based differences in muscle protein turnover in response to both feeding and contraction. What is unclear, at least in humans, is whether male or female sex steroids within a normal physiological range exert an effect on muscle protein turnover. In rodents, the administration of female sex steroids lowers MPS and thus may inhibit muscle growth (Toth et al., 2001). There are, however, data suggesting that testosterone may inhibit muscle protein breakdown (Ferrando et al., 1998). Testosterone is thought to exert its anabolic effect through post-transcriptional mechanisms that stimulate MPS (Ferrando et al., 1998). When testosterone is given exogenously in pharmacologic doses, it promotes the building of muscle or anabolism (Bhasin et al., 1996). These differences in body composition are often thought to be due to differences in sex steroids and primarily testosterone. It is well documented that women have less lean muscle mass and more body fat than men (Mingrone et al., 2001). In addition, the role of aging in this area has been poorly studied with little data available. However, an understudied question is whether there are sex-based differences in MPS in response to feeding and muscle contractions. Many studies of the regulation of MPS in humans have been carried out in young men and a variety of nutritional and contractile variables have been studied. Various stimuli have been proposed to be responsible for the feeding-induced stimulation of MPS, including insulin mediated via changes in local blood flow (Fujita et al., 2006b, 2007), and leucine via stimulation of the mammalian target of rapamycin complex 1 (mTORC1) (Crozier et al., 2005). It 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.
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