NUTRITION – PROTEIN – SEX & AGE DIFFERENCES – AGING


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SEX-BASED DIFFERENCES IN MUSCLE PROTEIN TURNOVER AND METABOLISM IN AGING – 2013


Stuart M. Phillips
KEY POINTS (MPS – Muscle Protein Synthesis)
– Muscle proteins turn over in response to both feeding and exercise, dictated by amino acid availability and exercise loading variables
(1) Basal levels of MPS are similar in young men and women as well as older men … However, older women appear to have an elevated rate of MPS
(2) Younger men and women exhibit similar MPS responses to feeding … Older men and women show comparatively diminished MPS response, or become “resistant” to feeding … Older women in particular have a diminished MPS response to protein feeding
(3) Older men have robust MPS responses to resistance exercise … Older women are somewhat impaired in their ability to stimulate MPS with exercise
(4) With resistance exercise training, MPS is elevated in both sexes and all ages, but this does not change feeding-induced increase in MPS
(5) Younger women exhibit no difference in MPS across the menstrual cycle and very little difference in amino acid kinetics … With age, changes in sex steroids in women (likely post-menopause) may result in an elevated rate of PRO turnover, but with little response to feeding or contraction
(6) Despite marked differences in levels of testosterone, young men and women show very similar responses to resistance exercise
(7) Sex-based differences in muscle protein turnover are not observed when comparing young men and women
PRACTICAL APPLICATIONS for AGING
(1) Older women have elevated basal rates of protein synthesis compared to older men and they do not exhibit as robust a response to feeding as do older men; the reasons for these differences remain unknown
(2) While older men and women have never been directly compared as to MPS responses to resistance exercise, results from resistance training studies show relatively similar degrees of hypertrophy and strength gains
PRACTICAL APPLICATIONS for the YOUNG
(1) Young men and women exhibit very little difference with respect to MPS in the basal state, in response to feeding or resistance exercise; thus, general recommendations to optimize lean mass gains in young men and women are similar
(2) Young men have only slightly greater hypertrophic (increased muscle fiber size) responses to resistance training than young women and the relative gains in strength are similar or favor younger women
(3) Young women appear to oxidize less PRO during endurance exercise than young men; this difference is small and appears related to lean body mass


SEX BASED COMPARISONS OF MPS AFTER RESISTANCE EXERCISE IN THE FED STATE – 2012


J Appl Physiol (1985). 2012 Jun;112(11):1805-13. doi: 10.1152/japplphysiol.00170.2012. Epub 2012 Mar 1.
West DW1, Burd NA, Churchward-Venne TA, Camera DM, Mitchell CJ, Baker SK, Hawley JA, Coffey VG, Phillips SM
AIM
– We made sex-based comparisons of rates of myofibrillar protein synthesis (MPS) and anabolic signaling after a single bout of high-intensity resistance exercise
METHODS
(1) 8 men (20 ± 10 yr) and 8 women (22 ± 1.8 yr) underwent primed constant infusions of l-[ring-(13)C(6)]phenylalanine on consecutive days with serial muscle biopsies
(2) Biopsies were taken from the vastus lateralis at rest and 1, 3, 5, 24, 26, and 28 h after exercise
(3) 25 grams of whey protein was ingested immediately and 26h after exercise
(4) We also measured exercise-induced serum testosterone because it is purported to contribute to increases in myofibrillar protein synthesis (MPS) postexercise and its absence has been hypothesized to attenuate adaptative responses to resistance exercise in women
RESULTS
(1) The exercise-induced area under the testosterone curve was 45-fold greater in men than women in the early (1h) recovery period following exercise
(2) MPS was elevated similarly in men and women (2.3- and 2.7-fold, respectively) 1-5h postexercise and after protein ingestion following 24h recovery
(3) Phosphorylation of mTOR(Ser2448) was elevated to a greater extent in men than women acutely after exercise
(4) Increased phosphorylation of p70S6K1(Thr389) was not different between sexes
(5) Androgen receptor content was greater in men (main effect for sex)
(6) Atrogin-1 mRNA abundance was decreased after 5 h recovery in both men and women
(7) MuRF-1 expression was elevated in men after protein ingestion following 24h recovery
CONCLUSIONS
(1) Minor sex-based differences in signaling responses and no difference in the MPS response to resistance exercise in the fed state
(2) Interestingly, our data demonstrate that exercise-induced increases in MPS are dissociated from postexercise testosteronemia and that stimulation of MPS occurs effectively with low systemic testosterone concentrations in women


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