The main fuel sources of energy during exercise are known to be carbohydrates and fats. In recent years, the role of protein metabolism has steadily been gaining more attention in the literature. Today it is known that protein requirements are slightly increased in highly active individuals. However, several questions still remain as to 1) How much protein does an active person should consume? 2) What are some possible side effects caused by a high dietary protein intake? 3) Does the body metabolize and/or absorb the extra intake of protein? and 4) Is there a limit to how much protein the body can metabolize?
In order to further discuss the effects of a physically active lifestyle on dietary protein needs, it is important to further understand the role of protein in the body. Proteins are made of building blocks knows as “amino acids” which are connected by peptide bonds. There are 20 different amino acids in proteins and a few non-protein amino acids (i.e., taurine). The body maintains a “pool” of protein and free amino acids. Skeletal muscle holds the largest pool of free amino acids in the body. Out of the 20 amino acids, only 6 are metabolized in the skeletal muscle. They are known as the branched-chain amino acids (leucine, isoleucine, valine, glutamate, aspartate, and asparagines). A continuous exchange of amino acids occurs between these pools as proteins are constantly being synthesized and simultaneously being degraded (protein turnover). Normally these processes are in equilibrium such that the breakdown of body protein is replaced by protein synthesis that uses amino acids from the free pool. These amino acids are provided from the digestion of foods that are high in protein. Insufficient intake of protein may cause an imbalance in the amino acid free pool. Over time this may lead to loss of muscle mass. In contrast, excessive intakes of protein are converted to carbohydrate or fat and later stored (using the extra carbon from the amino acid). The extra nitrogen from the amino acids is excreted as urea via the urine. Some of the many functions of protein include the synthesis of new tissue (i.e., muscle and skin), synthesis of enzymes, synthesis of plasma proteins (i.e., albumin, globulin), synthesis of hormones (i.e., epinephrine and norepinephine) and serve as a minor source of energy.
Protein metabolism has typically been determined by measuring nitrogen (N) balance- difference between N dietary intake and losses. When N intake exceeds excretion it is referred as a positive N balance. Conversely, when excretion exceeds intake, it is referred as a negative N balance. Human nitrogen requirements are usually determined from the nitrogen balance. A zero N balance equals to an even N intake and loss. Nitrogen losses occur in the form of urea (urine), ammonia, creatine, and to a lesser extent from fecal and miscellaneous losses. Other methods have been used to measure protein metabolism (metabolic tracers). Thus, the Nitrogen balance method is still considered the gold standard.
It has been suggested that protein requirements are slightly increased for both endurance and strength-trained athletes. The question still remains to whether a higher intake of protein (above and beyond the recommended values) can bring about further benefits to the athlete. Thus, the general consensus is that strength athletes can increase muscle growth with supplemental protein relative to similar subjects who trained without supplement. However, there are limits to where supplementation will enhance performance. It has been suggested that protein intake at around 1.4 grams per kilogram of body weight/day appears to reach a plateau. On the other hand, this ceiling effect can be further raised if protein intake is combined with creatine. The daily protein intake recommendations for individuals involved in resistance training programs may be as high as 1.6 to 1.8 g/kg body weight per day. Thus, there is no need of extra protein or amino acid supplements, provided that the individual is consuming a diet containing adequate energy to maintain body weight.
Endurance exercise also appears to alter protein metabolism. For instance, increased Leucine oxidation has been shown with as little as 2 hours of moderate intensity exercise (55% of VO2Max). Also, increased excretion of urea and ammonia have been shown (they are end products of amino acid oxidation) with endurance exercise. Currently, protein recommendations for endurance athletes are 1.2 to 1.4grams/kg body weight/day. As with the strength athletes, there is no need for further protein supplementation (above and beyond the recommended levels), provided the individual is consuming a balance diet.
Possible side effects to increased protein intake may include the following but are not limited to:
- An increased excretion of calcium in the urine which long term can lead to bone resorption and potentially osteoporosis.
- Excessive intake of single amino acids (i.e., lysine, arginine, methionine) can be toxic, as large pools of amino acids are not tolerated. For instance, methionine metabolism has been associated with increased levels of homocysteine which has been associated with vascular disease.
- An increased stress on the kidney.
- Increased metabolic imbalances and altered neurotransmitter activity
In summary, one cannot deny the importance of proper protein intake for the highly active individual. Current research indicates that as long as energy intake is adequate a daily protein intake of 1.2 to 1.4 grams/kg of body weight for endurance athletes and 1.6 to 1.8 grams for the strength athletes should be adequate. Thus, there is no need for further supplementation above and beyond the recommended values.
Finally, further research needs to be done with physically active women, children and the elderly (currently most studies have been done with college age male subjects).
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