Review articleProtein requirements for endurance athletes☆
Introduction
This review examines the literature on protein metabolism during endurance exercise and provides practical suggestions regarding protein needs for recreational sport enthusiasts and top sport athletes. Endurance activities can be broadly defined as those that use predominantly oxidative phosphorylation as the main energy source. There are several important aspects that determine the effect of such activities on the physiologic stress of short- and long-term exercise and the subsequent requirements for nutrients. There are issues regarding each specific exercise bout that will determine the metabolic and nutritive requirements including intensity and duration, nutrition and hydration status before and during exercise, and the background training status of the individual. When considering the nutritional needs of a person performing exercise, it is important to take all of these factors into consideration and make activity-specific, not “blanket,” recommendations. For example, a recreational athlete who is jogging four times a week at 45% of maximum oxygen consumption (Vo2max) for 1 h represents a very different physiologic scenario from a top sport athlete who may be training and competing at intensities 60% to 85% of Vo2max for 8 to 40 h/wk. Even at these levels of energy expenditure, the recreational athlete described above would metabolize about 2000 kcal/wk, whereas the competitive athlete would require anywhere from 5600 to more than 40 000 kcal/wk of energy beyond basal needs.1 It is obvious that nutritional recommendations based on modest physical activity should not be extrapolated to represent the needs of top sport or elite athletes. Given the current limited status of the literature regarding protein requirements in humans performing endurance exercise, I broadly divide the recommendations into only recreational, modest, and top sport categories.
Clearly the predominant fuels used during endurance exercise are carbohydrates and fats.2, 3, 4 Although there is no doubt that skeletal muscle oxidizes about 1% to 6% of total energy from amino acids during endurance exercise,2, 3, 4, 5 there is some controversy as to whether this alters the dietary requirement for protein.2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 The purpose of this report is to review the pathways and determinants of protein metabolism in skeletal muscle as they relate to endurance exercise. Special emphasis on sex differences, carbohydrate and energy adequacy, and timing of nutrient delivery are considered. The ultimate goal is to consider whether or not there are circumstances in which dietary protein requirements for an endurance athlete exceed that of the suggested protein requirements for the general population.
Section snippets
General aspects
Proteins are important molecules that serve structural and regulatory functions in the body. Structural proteins include cytoskeletal proteins such as dystrophin and connective tissue proteins such as collagen, and regulatory proteins include enzymes such as hexokinase or carnitine palmityl transferase. Proteins are comprised of constituent amino acids that contain an amino (-NH2), a carboxylic acid (-COOH), and a radical (different for each amino acid) group. Of the 20 amino acids, 9 are
Methods of determining protein requirements
Nitrogen balance (NBAL) is a classic method used to determine the protein requirements of humans. The technique involves quantifying all the protein that enters the body (diet, intravenous, etc.) and all the nitrogen that is excreted.74, 75 Because the body excretes nitrogenous compounds rather than whole proteins and proteins are approximately 16% nitrogen (w/w), NBAL involves measurement of total nitrogen intake (NIN) and total nitrogen excretion (NOUT = urine + feces + sweat + miscellaneous,
Conclusions and practical suggestions
It appears that low- and moderate-intensity endurance exercise does not affect dietary protein requirements. At the initiation of an endurance exercise program or during a ramp increase in training demands (such as training camp), there is a transient increase in dietary protein needs; however, the body rapidly adapts to the increase in need through an increase in efficiency. For the well-trained endurance athlete training 4 to 5 d/wk for longer than 60 min, there appears to be a very modest
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Dr. Tarnopolsky is supported in part by a Hamilton Regional Assessment Center Chair in Neuromuscular Disorders and most of the research in this laboratory was conducted with the support of NSERC Canada and the Hamilton Health Sciences Foundation.