The effect of a protein-polyphenol nutritional intervention on the skeletal muscle metabolic and functional response to eccentric exercise and resistance exercise training
Date: 26 April 2021
University of Exeter
Doctor of Philosophy
Skeletal muscle is a dynamic tissue providing key mechanical and metabolic functions and is the principal reservoir for amino acid storage in the body. Ingesting dietary proteins increases the rate of amino acid deposition into skeletal muscle, creating net positive protein balance. Strategies targeting the synthesis of myofibrillar ...
Skeletal muscle is a dynamic tissue providing key mechanical and metabolic functions and is the principal reservoir for amino acid storage in the body. Ingesting dietary proteins increases the rate of amino acid deposition into skeletal muscle, creating net positive protein balance. Strategies targeting the synthesis of myofibrillar proteins in particular have been demonstrated to accelerate recovery from muscle damage or enhance adaptations to a period of resistance-type exercise training. However, evidence that myofibrillar protein synthesis per se dictates these outcomes is largely circumstantial due to a lack of interventional studies using direct measures of muscle metabolism. The studies included in this thesis aimed to investigate the effects of a commercially available protein and polyphenol nutritional intervention (PPB) on recovery from muscle-damaging eccentric exercise and adaptations to resistance-type exercise training. The overarching hypothesis was that PPB would accelerate recovery and adaptations to resistance-type exercise training, and that by using such models, myofibrillar protein synthesis and a dampening of inflammation would be identified as essential for recovery and adaptation. Firstly, the anabolic response to PPB was characterised in 20 recreationally active males and females compared to an isocaloric carbohydrate placebo (PLA). Myofibrillar fractional synthesis rate (myoFSR) increased from 0.019 ± 0.007 and 0.013 ± 0.003 %·h-1 during the basal period to 0.028 ± 0.006 and 0.026 ± 0.003 %·h-1 following consumption of PLA and PPB respectively (P < 0.05). Versus PLA, PPB increased postprandial plasma amino acid concentrations (P < 0.001) and induced positive net protein balance (P < 0.001). This postprandial anabolic milieu was subsequently hypothesised to promote recovery from 300 maximal unilateral eccentric contractions, as previous work has identified that eccentric exercise damages contractile proteins and increases rates of myofibrillar protein synthesis versus concentric exercise, implying a greater demand for amino acids. Corrected to the contralateral control leg, eccentric exercise impaired muscle function for 5 days in PLA, which was completely prevented by PPB (interaction; P < 0.05). However, contrary to the hypothesis, these data are the first to show that myoFSR measured following 2H2O consumption was unaffected by PPB intervention over a postprandial, overnight and early stage of recovery, corresponding to 24 – 27 h, 27 – 36 h, and 24 – 72 h after eccentric exercise. Gene ontology and cluster analysis indicated that inflammatory and regenerative signalling pathways were upregulated following muscle damage, but this was unaffected by PPB. Interestingly, myoFSR was ~35% greater with PPB versus PLA (P < 0.05) only during the latter stages of the investigation when muscle damage had largely resolved. Applying this intervention to a model of unilateral resistance-type exercise training, the third study in this thesis aimed to characterise the time course of training adaptations with PPB relative to a time-matched, untrained contralateral leg, for the first time. Extending on the results of the previous study, it was hypothesised that PPB would promote myoFSR over 48 h following a single training session and accelerate adaptations to resistance-type exercise training. Following the onset of training, myoFSR was significantly greater with PPB (2.01 ± 0.15 versus 1.51 ± 0.16 %·d-1, pooled across leg, P < 0.05). Relative to the untrained leg (%U), PPB increased muscle function (PLA: 102.6 ± 3.9 %U pre-training to 100.8 ± 2.4 %U at session 10; PPB: 99.9 ± 1.8 %U pre-training to 107.2 ± 2.4 %U session 10; time x group interaction P < 0.05), whereas maximal isometric contraction strength increased in both groups. Following 30 sessions, training increased muscle strength (P < 0.05) and function (P < 0.01) by 9.6 ± 5.7% and 9.4 ± 4.9% respectively in PLA, with no additional effect of PPB (8.4 ± 3.8% and 14.0 ± 5.6% increase in strength and function, respectively). Type II, but not type I, fibre cross-sectional area increased with PPB (time x group interaction P < 0.05). By detailing the time-course of recovery and prolonged training, these data show for the first time that accelerated recovery from muscle damage is not explained by myofibrillar protein synthesis or a dampening of inflammation. Using powerful unilateral study designs allowing for intra-individual control, this thesis demonstrates that only once recovery is resolved does protein-polyphenol intervention improve myofibrillar protein synthesis, whereby it accelerates the early functional improvements during resistance-type exercise training and increases type II fibre hypertrophy.
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