Protein requirements and supplementation in strength sports

Abstract

Daily requirements for protein are set by the amount of amino acids that is irreversibly lost in a given day. Different agencies have set requirement levels for daily protein intakes for the general population; however, the question of whether strength-trained athletes require more protein than the general population is one that is difficult to answer. At a cellular level, an increased requirement for protein in strength-trained athletes might arise due to the extra protein required to support muscle protein accretion through elevated protein synthesis. Alternatively, an increased requirement for protein may come about in this group of athletes due to increased catabolic loss of amino acids associated with strength-training activities. A review of studies that have examined the protein requirements of strength-trained athletes, using nitrogen balance methodology, has shown a modest increase in requirements in this group. At the same time, several studies have shown that strength training, consistent with the anabolic stimulus for protein synthesis it provides, actually increases the efficiency of use of protein, which reduces dietary protein requirements. Various studies have shown that strength-trained athletes habitually consume protein intakes higher than required. A positive energy balance is required for anabolism, so a requirement for “extra” protein over and above normal values also appears not to be a critical issue for competitive athletes because most would have to be in positive energy balance to compete effectively. At present there is no evidence to suggest that supplements are required for optimal muscle growth or strength gain. Strength-trained athletes should consume protein consistent with general population guidelines, or 12% to 15% of energy from protein.

Introduction

Body proteins are constantly and simultaneously being made (synthesized) and degraded. This constant turnover provides for a mechanism of steady maintenance of potentially damaged and dysfunctional proteins. In skeletal muscle, protein turnover is also ongoing and provides the basis for skeletal muscle's plasticity in response to the degree of imposed high-intensity loading (resistance exercise). A schematic representation of skeletal muscle protein turnover and other muscle-specific metabolic fates of amino acids is shown in Figure 1. The extent to which the amino acids, liberated as a result of muscle proteolysis, are reused is extensive. This intracellular recycling, however, is not 100% efficient and amino acids are lost from skeletal muscle, often in appreciable quantities. The amino acids that are lost from skeletal muscle have numerous fates, but generally speaking are oxidized or converted to glucose via gluconeogenesis, with the amino nitrogen yielding urea. Obviously, the lack of efficiency in reusing amino acids from proteolysis means that we have a daily requirement to ingest protein.