Abstract
Protein intake recommendations to optimally stimulate muscle protein synthesis (MPS) are derived from dose-response studies examining the stimulatory effects of isolated intact proteins (e.g., whey, egg) on MPS in healthy individuals during energy balance. Those recommendations may not be adequate during periods of physiological stress, specifically the catabolic stress induced by energy deficit. Providing supplemental intact protein (20–25 g whey protein, 0.25–0.3 g protein/kg per meal) during strenuous military operations that elicit severe energy deficit does not stimulate MPS-associated anabolic signaling or attenuate lean mass loss. This occurs likely because a greater proportion of the dietary amino acids consumed are targeted for energy-yielding pathways, whole-body protein synthesis, and other whole-body essential amino acid (EAA)-requiring processes than the proportion targeted for MPS. Protein feeding formats that provide sufficient energy to offset whole-body energy and protein-requiring demands during energy deficit and leverage EAA content, digestion, and absorption kinetics may optimize MPS under these conditions. Understanding the effects of protein feeding format-driven alterations in EAA availability and subsequent changes in MPS and whole-body protein turnover is required to design feeding strategies that mitigate the catabolic effects of energy deficit. In this manuscript, we review the effects, advantages, disadvantages, and knowledge gaps pertaining to supplemental free-form EAA, intact protein, and protein-containing mixed meal ingestion on MPS. We discuss the fundamental role of whole-body protein balance and highlight the importance of comprehensively assessing whole-body and muscle protein kinetics when evaluating the anabolic potential of varying protein feeding formats during energy deficit.
Highlights
The dose-dependent relationship between supplemental dietary protein and resting and post-resistance exercise skeletal muscle protein synthesis (MPS) is well described [1,2,3,4,5]
We summarized how supplemental free-form essential amino acid (EAA), supplemental intact protein, and protein-containing mixed meal formats moderate MPS during energy balance
Knowledge gaps remain, including the optimal dose of EAA during the catabolic stress of energy deficit, as well as how whole-food-containing mixed meals directly compare to supplemental free-form EAA and isolated intact proteins
Summary
The dose-dependent relationship between supplemental dietary protein and resting and post-resistance exercise skeletal muscle protein synthesis (MPS) is well described [1,2,3,4,5]. These well-designed dose-response studies clearly demonstrate that consuming 0.25–0.3 g and 0.4 g of protein/kg per dose maximally stimulates MPS in young and older adults, respectively [4,5,7] As a result, these doses are promoted as effective post-exercise dietary strategies that optimize muscle adaptations to exercise when coupled with habitual protein intakes nearly twice the recommended dietary allowance [11,12,13]. Whether these recommendations derived from studies in healthy individuals during energy balance are adequate for individuals undergoing physiological stressors, including the catabolic stress of energy deficit, remains in question Our work suggests these protein doses are not effective for mitigating lean mass (muscle) loss during military training that often results from energy deficit-mediated changes in MPS. The objectives of this review are to summarize what is known regarding the effects of supplemental free-form EAA, supplemental intact protein, and protein-containing mixed meals on MPS during energy balance and present the importance of simultaneous measures of MPS and whole-body protein balance during energy deficit
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