Limits of Human Endurance

L. J. C. van Loon, R. Meeusen

• Publisher: S. Karger
• Year: 2013
• ISBN:
  9783318024098
• Topic: Medicina
• Subtopic: Enfermería

Nutrition is one of the key factors that modulate exercise performance. In this book, a group of expert scientists discuss the ergogenic properties of various nutritional interventions and present research to show that dietary strategies can be applied to extend the limits of human endurance, lower the risk of illness or injury, and speed recovery rates. More specifically, they discuss recent findings on topics such as caffeine and its effect on the brain, carnitine and fat oxidation, ergogenic properties of beta-alanine, dietary protein and muscle reconditioning, nutrition and immune status, and the importance of proper hydration. This publication will provide the reader with many novel insights into the complex interaction between nutrition and exercise, allowing them to define more effective dietary strategies to improve health and performance. Moreover, while focusing on elite athletes, it is interesting to note that some of the discoveries can be applied beyond this niche, for example to improve performance outcomes in the elderly.

van Loon LJC, Meeusen R (eds): Limits of Human Endurance.
Nestlé Nutr Inst Workshop Ser, vol 76, pp 85-102, (DOI: 10.1159/000350261)
Nestec Ltd., Vevey/S. Karger AG., Basel, © 2013

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The Role of Amino Acids in Skeletal Muscle Adaptation to Exercise

Nick Aguirre^a · Luc J.C. van Loon^b · Keith Baar^a

^a University of California Davis, Davis, CA, USA; ^b Department of Human Movement Sciences, NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands

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Abstract

The synthesis of new protein is necessary for both strength and endurance adaptations. While the proteins that are made might differ, myofibrillar proteins following resistance exercise and mitochondrial proteins and metabolic enzymes following endurance exercise, the basic premise of shifting to a positive protein balance after training is thought to be the same. What is less clear is the contribution of nutrition to the adaptive process. Following resistance exercise, proteins rich in the amino acid leucine increase the activation of mTOR, the rate of muscle protein synthesis (MPS), and the rate of muscle mass and strength gains. However, an effect of protein consumption during acute post-exercise recovery on mitochondrial protein synthesis has yet to be demonstrated. Protein ingestion following endurance exercise does facilitate an increase in skeletal MPS, supporting muscle repair, growth and remodeling. However, whether this results in improved performance has yet to be demonstrated. The current literature suggests that a strength athlete will experience an increased sensitivity to protein feeding for at least 24 h after exercise, but immediate consumption of 0.25 g/kg bodyweight of rapidly absorbed protein will enhance MPS rates and drive the skeletal muscle hypertrophic response. At rest, ~0.25 g/kg bodyweight of dietary protein should be consumed every 4-5 h and another 0.25-0.5 g/kg bodyweight prior to sleep to facilitate the postprandial muscle protein synthetic response. In this way, consuming dietary protein can complement intense exercise training and facilitate the skeletal muscle adaptive response.

Copyright © 2013 Nestec Ltd., Vevey/S. Karger AG, Basel

Introduction

Muscular adaptation to resistance (muscle hypertrophy) and endurance (mitochondrial biogenesis and angiogenesis) type exercise is dependent on the de novo synthesis of myofibrillar and mitochondrial proteins, respectively [1]. The phenotype of bigger muscles in the strength athlete is the result of the rate of myofibrillar protein synthesis exceeding the rate of myofibrillar protein breakdown, whereas the endurance phenotype of more mitochondrial proteins occurs when this subset of proteins is synthesized faster than they are broken down. Therefore, protein balance (the sum of protein synthesis and breakdown) is key to determining the phenotypic adaptation to exercise training. Even though the rate of muscle protein breakdown increases following an acute bout of exercise [2], the corresponding increase in muscle protein synthesis (MPS) is 3- to 5-fold greater [2, 3]. Together with the methodological difficulties in measuring protein breakdown, this means that most research has focused on the regulation of protein synthesis after exercise.

In the fasted state, protein balance is negative. Resistance type exercise in the fasted state results in an increase in both protein synthesis and degradation [3]. Since the increase in synthesis is greater than the increase in degradation, net balance becomes less negative. However, in order for protein balance to become positive, an individual needs to consume a source of amino acids [4]. Tipton et al. [4] showed that when subjects consumed 40 g of either a mixed amino acid solution (containing both essential, EAAs, and non-essential amino acids) or an EAA solution (also containing arginine), MPS increased equally between supplemented groups and to a greater degree than in the fasted state, resulting in a positive protein balance. Therefore, resistance type exercise combined with provision of sufficient EAAs can shift net protein balance to positive. The effects of protein supplementation on mitochondrial protein synthesis have not been studied extensively. However, the early reports suggest that amino acid supplementation does not affect mitochondrial protein synthesis during the acute stages of post-exercise recovery [5]. This chapter will explore the mechanisms underlying the effects of amino acids on skeletal muscle adaptations and provides some practical suggestions based on these mechanisms in an effort to maximize the effects of training.

Adaptations to Strength Exercise

Resistance exercise, forcing a muscle to work close to or above its maximal isometric force to failure, results in an acute increase in MPS that can last more than 24 h [6, 7]. When repeated at a sufficient frequency, this transient increase in protein synthesis leads to an increase in muscle mass and strength. A number of cellular and molecular processes have been identified that transduce mechanical load into an increase in protein synthesis. Chief among these is the activation of the mTOR. In every model studied to date, loading results in the activation of mTOR, and the acute activation of mTOR is predictive of the gain in muscle mass and strength following more prolonged exercise training [8, 9]. Further, inhibiting mTOR with the macrolide antibiotic rapamycin blocks the acute increase in protein synthesis in people [10] and the increase in muscle mass in rodents [11, 12], suggesting that mTOR activation is required for muscle hypertrophy. Interestingly, mTOR is initially activated by both resistance and endurance type exercise. However, only after resistance exercise is the activation of mTOR maintained for longer periods [1]. In fact, following resistance type exercise, mTOR can remain active for upwards of 18 h [13], suggesting that mTOR might drive the protein synthesis response to resistance type exercise.

mTOR

mTOR is a serine/threonine protein kinase with structural similarities to PI-3 kinase (phosphatidylinositol-3 kinase). On its own, mTOR has no catalytic activity. In order for mTOR to become a functional kinase, it has to form a complex with other proteins that stabilize its structure, move it to specific regions of the cell, and regulate its binding to target proteins [14]. There are two known complexes of mTOR (complex 1 and 2). Both of the complexes contain the Gprotein β-subunit-like protein (GβL; also known as lsT8) and the DEP domain-containing mTOR-interacting protein (DEPTOR) [15] that positively and negatively regulate mTOR, respectively. mTORC2 is targeted to membranes through its interaction with mammalian stress-activated map kinase-interacting protein 1 (mSIN1) [16], and is held together by protein observed with rictor (PROTOR) 1 or 2 [17]. The two mTOR complexes phosphorylate different proteins due to the presence of either the regulatory-associated protein of mTOR (raptor) or rapamycin-insensitive companion of mTOR (rictor) [18]. In mTORC1, raptor binds to proteins that contain a TOS (TOR signaling) motif such as eukaryotic initiation factor (eIF) 4E binding protein-1 (4E-BP1) [19], the 70-kDa ribosomal protein S6 kinase (S6K1) [20], hypoxia-inducible factor-1 (HIF-1) [21], and proline-rich Akt substrate of 40 kDa (PRAS40) [22, 23]. In contrast, rictor directs mTOR towards akt/PKB (protein kinase B), serum- and glucocorticoid-induced protein kinase (SGK), and protein kinase C (PKC) [24].

The most studied targets of mTOR are 4E-BP1 and S6K1. Phosphorylation of 4E-BP1 by mTOR changes its shape and prevents it from binding to eIF4E [25]. Free eIF4E can then bind to two other initiation factors, eIF4G and eIF4A, to form eIF4F and promote translation initiation. When S6K1 is activated by mTOR, it phosphorylates and turns on eIF4B [26] and phosphorylates and turns off eukaryotic elongation factor 2 kinase (eEF2K) [27]. Activation of eIF4B increases the unwinding of secondary structure in mRNA so that they can be translated. This specifically increases the rate of translation of genes that are important in cell growth, such as fibroblast growth factor (bFGF) [28]. Inactivation of eEF2K prevents the phosphorylation of elongation factor 2 (eEF2), accelerating the elongation step and promoting protein synthesis. In this way, S6K1 contributes to the regulation of both translation initiation and elongation. Even though these are the most studied targets of mTOR, there is developing evidence that other targets of mTOR are more important for muscle hypertrophy [MacKenzie et al. unpubl. obs.]. To that end, the recent discovery of two novel kinases downstream of mTOR that lead to ribosome biogenesis and an increase in the capacity for protein synthesis is extremely exciting [29].

Activation of mTOR

mTOR activity is regulated by growth factors, nutrients, stress, and mechanical loading [30]. Of particular importance to this review is the role of amino acids in regulating the activation of mTOR. Amino acids can directly stimulate mTOR activity in the absence of changes in metabolic stress, growth factors, or mechanical loading. Amino acids mediate this effect by uniting mTOR and its activator Rheb (ras homologue enriched in brain) within the cell. Over the last few years, the mechanism underlying this complex process has been elucidated, and a number of novel molecular actors have been identified including: the vacuolar protein sorting-34 [31], the leucyl tRNA synthase (LRS) [3...

Table of contents

• Cover Page
• Front Matter
• Caffeine, Exercise and the Brain
• Carnitine and Fat Oxidation
• Hydration during Intense Exercise Training
• Intense Exercise Training and Immune Function
• Physiological and Performance Adaptations to High-Intensity Interval Training
• Effect of β-Alanine Supplementation on High-Intensity Exercise Performance
• Dietary Protein for Muscle Hypertrophy
• The Role of Amino Acids in Skeletal Muscle Adaptation to Exercise
• National Nutritional Programs for the 2012 London Olympic Games: A Systematic Approach by Three Different Countries
• Concluding Remarks: Nutritional Strategies to Increase Performance Capacity
• Subject Index