Protein Recommendations Exercise
Protein is a macronutrient often misunderstood among the athletic population and there are also misconceptions as to the optimal amount
of protein needed by athletes for muscle tissue repair and growth. Protein does much more than build muscle; rather, it has many essential functions in the body. Yet, contrary to what some athletes believe, protein is not an efficient or optimal fuel source. The following is a review of the most current research regarding optimal protein intake for athletes.
The daily recommended protein intake for athletes is an area of research that is not without controversy. There is consensus that athletes have increased protein needs above those of sedentary individuals, though s pecific amounts are disputed. Part of the uncertainty is based upon methodological limitations when studying protein needs, and some confusion comes from defining what an athlete “needs” versus what is considered optimal intake. Providing appropriate protein recommendations for athletes requires an understanding of these issues.
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As part of the DRIs, the Recommended Dietary Allowance (RDA) for protein in the United States and Canada for nonathletic adults is 0.8 grams per kilogram of body weight (0.8 g/kg). However, this is not necessarily an amount that an individual should be consuming, but rather is the amount needed to avoid the negative consequences of protein deficiency in 98 percent of the healthy population. Even with nonathletic populations, some researchers and professionals believe higher intakes would be beneficial. However, given that the AMDR for protein is 10 to 35 percent of one’s total caloric intake, this wide range can allow for higher protein intakes and still be considered “acceptable.”
Protein recommendations for athletic and nonathletic populations are typically based upon nitrogen balance studies that try to assess total body protein metabolism. The process measures nitrogen intake (by looking at protein intake from food records, since protein is the only source of dietary nitrogen) and nitrogen excretion in the urine. Positive nitrogen balance occurs during periods of growth and repair and indicates an increase in total body protein. Negative nitrogen balance occurs during periods of illness, burns, injury, and fasting when nitrogen excretion exceeds that of consumption and total body protein stores are being diminished. Nitrogen balance occurs when nitrogen intake equals the rate of nitrogen excretion and indicates that protein needs are being met. Nitrogen balance studies have become a method frequently employed to assess protein status within populations.
One challenge, however, is that using nitrogen balance as an indicator of protein adequacy does not translate well to athletes. There are numerous methodological concerns with using nitrogen balance studies as a basis for protein recommendations. Further, the goal of the RDA is to prevent deficiencies by replacing losses. This does not equate to what is optimal or desirable for athletes. Athletes are not merely trying to prevent deficiency; rather, athletes are typically striving for positive adaptations from training. These adaptations include repairing and replacing damaged proteins, remodeling protein, supporting training-induced adaptations in metabolic functioning, contributing to lean muscle mass accrual, and enhancing immune functioning (Phillips 2012).
With this in mind, research has set out to better determine what a truly optimal intake of protein is for athletes. The recommended daily intake for athletes falls in the range of 1.2 to 1.7 g/kg/day though can be as high as 2.0 g/kg/day such as with athletes with weight management goals, as will be discussed in Chapter 8. Resistance athletes benefit from higher protein recommendations because protein, and specifically the EAAs (see Table 4.4) derived from protein, are what stimulate muscle protein synthesis and inhibit muscle protein breakdown. Endurance exercise also stimulates muscle protein synthesis, though to a smaller extent than resistance exercise. Additionally, consumption of dietary protein may preserve lean muscle mass. Prolonged exercise results in increased amino acid oxidation for energy production. This includes long-duration endurance exercise (over 3 hours) that relies upon the oxidation of leucine, a branched-chain amino acid found in certain protein sources, for additional energy substrate. Consumption of exogenous protein sources can minimize the breakdown of muscle tissue for the oxidation of amino acids. Finally, after resistance and endurance training, adequate protein consumption supplies amino acids for tissue repair. See Table 4.5 outlining the recommended daily protein intake for athletes.
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