Measuring synthesis rates of different proteins – clues to training adaptations

Abstract

Over the past 15–20 years several laboratories have been investigating the regulation of muscle protein synthesis by diet and exercise. It is clear that resistance exercise stimulates muscle protein synthesis. The greatest stimulation comes from resistance exercise plus a source of amino acids (for review see Tipton & Ferrando, 2008). Interestingly, similar increases in muscle protein synthesis are observed following endurance exercise (Tipton et al. 1996); protein ingestion also enhances this response (Howarth et al. 2009). Given the differences in adaptations to these types of exercise, this similarity of response of muscle protein synthesis may seem somewhat paradoxical. The explanation may lie in the methodology. The response of mixed muscle protein synthesis was determined in these studies (Tipton & Ferrando, 2008). Measurement of mixed muscle protein synthesis is essentially a weighted average of the rates of synthesis of all proteins in the muscle, i.e. it does not distinguish the responses of different proteins. Intuitively, given the differences in adaptations to different types of training, it is not difficult to accept that resistance and endurance exercise, for example, should differentially influence different proteins. Recent advances in methodology have allowed us to begin to address these questions. The acute response to exercise and nutritional interventions may be measured and extrapolated to represent the potential for long-term adaptive changes. Adaptations to any exercise training regime result from changes in the type, quantity and activity of proteins in various tissues. For example, muscle growth in response to resistance exercise training is determined by increases in the myofibrillar proteins. Muscle protein synthesis rate has been shown to be predictive of long-term changes in muscle (Tipton, 2008). For example, a recent metabolic study demonstrated that milk ingestion resulted in greater muscle protein synthesis than soy following resistance exercise (Wilkinson et al. 2007). A follow-up longitudinal study by the same research group demonstrated that milk supplementation resulted in greater increases in muscle mass and strength than soy supplements (Hartman et al. 2007). Thus, it is clear that the acute metabolic response to various exercise and nutritional combinations is a useful way to determine optimal training and diet regimes. The results of a study published in this issue of The Journal of Physiology by Moore et al. (2009) offer interesting and important advances toward our understanding of the influence of nutrition and exercise on different types of proteins. This study helps pave the way for future studies that will further our understanding of training adaptations. Moore et al. (2009) used a unilateral exercise model to simultaneously measure protein synthesis rates in response to whey protein ingestion at rest and following resistance exercise. Moreover, the time course of the response to each was determined. The results demonstrate a differential response of the two protein types to feeding. The rate of sarcoplasmic protein synthesis was increased following protein ingestion to the same extent in both the resting and the exercised leg. On the other hand, myofibrillar protein synthesis was increased to a greater extent and the response lasted longer in the exercised leg. These novel findings illustrate that the response of proteins to exercise and nutrition is not uniform. Moreover, and perhaps more importantly, these results underline the value of measuring the response of various protein fractions to nutritional and exercise interventions. The present study examined the response only to resistance exercise (Moore et al. 2009). However, another recent study by this research group provides additional information on differential responsiveness of various protein fractions. Wilkinson et al. (2008) report that myofibrillar protein synthesis and mitochondrial protein synthesis rates both increased in response to resistance exercise. Only mitochondrial protein synthesis increased in response to endurance exercise. Furthermore, exercise training changed these responses (Wilkinson et al. 2008). These data were gathered with a background of hyperaminoacidaemia for all conditions, so the interactive effect of exercise type and nutrition on the different types of protein could not be determined. Clearly, these types of studies provide us with the ability to investigate the potential for exercise and nutrition to influence training results. Further investigation of the impact of different types of exercise on the synthesis of different protein fractions and the interaction with nutritional interventions must follow. Furthermore, studies should delineate responses in various populations. These studies will provide important information for athletes practicing various types of sports. More importantly, these types of studies will offer information critical to designing optimal nutrition and exercise interventions to diminish muscle wasting and metabolic disease.