Abstract
Background: The anabolic effect of resistance exercise is enhanced by the provision of dietary protein.
Objectives: We aimed to determine the ingested protein dose response of muscle (MPS) and albumin protein synthesis (APS) after resistance exercise. In addition, we measured the phosphorylation of candidate signaling proteins thought to regulate acute changes in MPS.
INTRODUCTION
The provision of an exogenous source of amino acids, likely only the essential amino acids (EAAs), enhances the anabolic effect of resistance exercise on muscle. Muscle protein synthesis (MPS), not breakdown, is the more responsive variable to a variety of anabolic stimuli in healthy individuals and is stimulated in a dose-dependent manner with increasing amounts of ingested EAAs at rest. Data from a series of studies suggest that a similar dose response of protein synthesis to amino acids exists after resistance exercise. No study to date has directly measured a dose-response relation between ingested whole protein and muscle protein synthetic rates after resistance exercise.
The acute synthesis of muscle proteins is primarily regulated at the level of messenger RNA translation via the activation of a variety of intracellular signaling proteins, especially those of the mammalian target of rapamycin (mTOR) signaling cascade. Stimulation of MPS in humans after feeding or resistance exercise is accompanied by enhanced phosphorylation, and presumably activity, of the mTOR signaling pathway, including the 70-kDa ribosomal protein S6 kinase (S6K1) and its target ribosomal protein S6 (rpS6). In addition, global rates of protein synthesis are tightly regulated by the activity of the guanine nucleotide exchange factor eukaryotic initiation factor 2B (eIF2B), which is responsive to both amino acids and exercise and is under the control of glycogen synthase kinase-3b in rats and cultured myoblasts. Collectively, these observations suggest that changes in MPS in response to feeding and exercise may be regulated by specific downstream target proteins of mTOR signaling such as S6K1, rpS6, and eIF2B.
Albumin is a major hepatic-derived plasma protein that is unaffected by resistance exercise in young men. However, albumin synthesis is stimulated by an increased availability of amino acids. It has been suggested that dietary amino acids may be incorporated into albumin protein in an effort to minimize their irreversible oxidation. Thus, a feeding induced increase in albumin protein synthesis may serve as a mechanism to ‘‘store’’ excess amino acids from the diet until they are needed during periods of reduced supply. The present study was designed to yield a dose-response relation for ingested protein in the stimulation of muscle and albumin protein synthesis after resistance exercise. We also wished to characterize other fates of amino acids when the amount of protein ingested might exceed the capacity for amino acids to be used in MPS, namely irreversible oxidation. We hypothesized that mixed MPS would demonstrate a dose response to dietary protein after resistance exercise and that the maximal effective (ie, maximally stimulatory for MPS) dose would be similar to what has previously been reported to be maximal at rest (7). In addition, we hypothesized that plasma albumin protein synthesis would display a similar dose response to dietary protein as mixed muscle protein. However, above an ingested dose of protein that maximally stimulated muscle and albumin protein synthesis, we hypothesized that amino acid oxidation would increase.
Design: Six healthy young men reported to the laboratory on 5 separate occasions to perform an intense bout of leg-based resistance exercise. After exercise, participants consumed, in a randomized order, drinks containing 0, 5, 10, 20, or 40 g whole egg protein. Protein synthesis and whole-body leucine oxidation were measured over 4 h after exercise by a primed constant infusion of [1-(13)C]leucine.
Results: MPS displayed a dose response to dietary protein ingestion and was maximally stimulated at 20 g. The phosphorylation of ribosomal protein S6 kinase (Thr(389)), ribosomal protein S6 (Ser(240/244)), and the epsilon-subunit of eukaryotic initiation factor 2B (Ser(539)) were unaffected by protein ingestion. APS increased in a dose-dependent manner and also reached a plateau at 20 g ingested protein. Leucine oxidation was significantly increased after 20 and 40 g protein were ingested.
Conclusions: Ingestion of 20 g intact protein is sufficient to maximally stimulate MPS and APS after resistance exercise. Phosphorylation of candidate signaling proteins was not enhanced with any dose of protein ingested, which suggested that the stimulation of MPS after resistance exercise may be related to amino acid availability. Finally, dietary protein consumed after exercise in excess of the rate at which it can be incorporated into tissue protein stimulates irreversible oxidation.