Abstract
Maximizing the post-exercise increase in muscle protein synthesis, especially of the contractile myofibrillar protein fraction, is essential to facilitate effective muscle remodeling, and enhance hypertrophic gains with resistance training. MPS is the primary regulated variable influencing muscle net balance with dietary amino acid ingestion representing the single most important nutritional variable enhancing post-exercise rates of muscle protein synthesis. Dose-response studies in average (i.e., ~80 kg) males have reported an absolute 20 g dose of high quality, rapidly digested protein maximizes mixed, and myofibrillar protein synthetic rates. However, it is unclear if these absolute protein intakes can be viewed in a “one size fits all” solution. Re-analysis of published literature in young adults suggests a relative single meal intake of ~0.31 g/kg of rapidly digested, high quality protein (i.e., whey) should be considered as a nutritional guideline for individuals of average body composition aiming to maximize post-exercise myofibrillar protein synthesis while minimizing irreversible amino acid oxidative catabolism that occurs with excessive intakes of this macronutrient. This muscle-specific bolus intake is lower than that reported to maximize whole body anabolism (i.e., ≥0.5 g/kg). Review of the available literature suggests that potential confounders such as the co-ingestion of carbohydrate, sex, and amount of active muscle mass do not represent significant barriers to the translation of this objectively determined relative protein intake. Additional research is warranted to elucidate the effective dose for proteins with suboptimal amino acid compositions (e.g., plant-based), and/or slower digestion rates as well as whether recommendations are appreciably affected by other physiological conditions such endurance exercise, high habitual daily protein ingestion, aging, obesity, and/or periods of chronic negative energy balance.
Highlights
Lean body tissues, including skeletal muscle, are constantly being remodeled through the continuous and simultaneous processes of protein synthesis, and protein breakdown
Since the first observations that skeletal muscle protein turnover is elevated in response to resistance exercise and that exogenous amino acids augment the increase in net protein balance of this tissue [2, 3], studies have investigated the nutritional factors that contribute to the optimal enhancement of post-exercise anabolism
Recommendation of absolute meal protein intakes is at odds with daily recommendations for this macronutrient, which are almost universally prescribed relative to body mass. To address these types of generalizability concerns, an unsystematic review was performed in Pubmed from its inception to July 1, 2019 consisting of keywords related to this review topic such as “whey,” “myofibrillar protein synthesis,” and “exercise.” As maximizing post-exercise myofibrillar protein synthesis would be essential for those interested in enhancing muscle growth and potentially strength with training, studies investigating the synthesis of this muscle fraction were selected to increase homogeneity as well as reflect the greater contractile and nutrient sensitivity of this protein fraction [12]
Summary
Lean body tissues, including skeletal muscle, are constantly being remodeled through the continuous and simultaneous processes of protein synthesis, and protein breakdown (collectively referred to as “turnover”). The algebraic difference between synthesis and breakdown determines net protein balance of a given tissue (e.g., muscle) and, whether it is gaining or losing mass. To this end, resistance exercise increases muscle protein turnover for up to 48 h in the fasted state [1]. The characteristic increase in muscle protein breakdown that occurs after resistance exercise in the fasted state is negated by the provision of exogenous amino acids, which subsequently supports greater rates of muscle protein synthesis, and an increased (and positive) net protein balance [3]. Potential biological (e.g., sex, age, body composition, active muscle mass), and nutritional (e.g., macronutrient co-ingestion, habitual protein intake, food matrix) confounders will be discussed to explore potential translational issues with recommending a per meal relative protein intake based on a preponderance of studies in young adults utilizing an isolated protein source (i.e., whey)
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