Does a high protein diet decrease muscle endurance? by Jerry Brainum

Even those who profess to know nothing at all about nutrition still seem to be aware of the importance of consuming a high protein diet in order to build muscle. Many mistakenly believe that the need for increased protein is related to muscles being made out of protein. In fact, muscle tissue is only 22 percent protein. The majority of muscle, similarly to other body tissues, is composed mainly of water. You don't have to have a physiology degree to know that consuming a lot of water won't build big muscles. The structural and contractile proteins in muscle are what determines muscular size and strength (with strength also having a neural component). Until recently, many nutrition authorities advised those engaged in weight-training that their requirement for protein was the same as a person who did nothing more strenuous than changing the channels on a TV. This seemed intuitively wrong, and it turned out that it was. The requirement for dietary protein does indeed increase for those seeking added muscle mass.
    It's still a matter of debate, however, as to just how much extra protein is needed to build muscle. The consensus of published research puts the range as between 1.6 and 2.7 grams of protein per kilogram of bodyweight. In reality, most competitive bodybuilders consume far more protein than this recommended range. According to the scientists, excess protein will simply be oxidized in the liver, thus not contributing to muscle building progress. On the other hand, other research has shown that protein requirements increase for anyone under dieting or caloric restriction regimes. If you lower your total calorie or carbohydrate intake, you need to boost protein intake to prevent the loss of excess lean mass, which includes muscle. A loss of muscle from excessive dieting results in a lowered resting metabolic rate, which practically ensures the regain of lost weight. Indeed, a just published review of the protein needs of those engaged in weight-training under calorie-restricted conditions concluded that the range of protein needed is between 2.3 to 3.1 grams per kilogram of fat-free mass. The less calories consumed, the greater the need for a higher protein intake to preserve muscle tissue.
    Various critiques have been offered over the years about consuming a high protein diet. These range from a higher protein diet promoting dehydration (possible, but never happens in the real world); loss of calcium (only if pure protein minus phosphate, a common mineral in high protein foods is consumed); and perhaps the silliest of all: a high protein diet can make you fat. The latter effect is based on the fact that that a gram of protein contains four calories, and excess calories from any source can be converted into body fat. The problem with this is that in those who are active and exercise, any excess protein is not converted into fat, but is rather oxidized in the liver. Certain amino acids can be converted into glucose, but recent research shows this conversion produces only small amounts of glucose, much less than previously believed.
    One of the most outrageous critiques of a high protein diet was recently published by a group of Japanese researchers. This involved a study in which mice were provided a high protein diet starting at 8 weeks of age, then followed for 20 to 50 weeks. During this time, the mice engaged in endurance exercise, presumably running on mouse-sized treadmills. The mice were closely watched for any changes in mitochondrial function and muscular endurance. The mitochondria are cell structures where energy is produced as ATP, and fat is oxidized. They are closely associated with endurance, and endurance exercise is known to boost the number of mitochondria in cells.
    In the new mouse study, some of the mice consumed a high protein diet, while others consumed normal mouse chow. At the end of 50 weeks, the high protein mice showed a decreased number of mitochondria, less mitochondrial activity, along with reduced running distance.The high protein mice did also show greater muscle mass and grip power compared to their normal eating littermates. The study authors suggest that these negative changes in mitochondria number and function, as well as the loss of endurance in the high protein mice, resulted from the high protein intake stimulating a protein called mTOR. This protein, which is stimulated by amino acids, particularly the branched-chain amino acid, leucine, sets into motion a cascade that culminates in increased muscle protein synthesis. When you lift weights and consume a high protein diet, you promote the activity of mTOR. This results in bigger and stronger muscles. Recent research even shows that anabolic steroids produce their notable effects in muscle through this same mechanism.
    When you engage in endurance exercise, another protein, called AMPK is increased. This protein is also increased under low calorie intake, or low carb intake. In the body, AMPK acts as an energy sensor, boosting glucose uptake into muscle and also promoting fat oxidation in muscle. But AMPK also blocks the activity of mTOR. This is the basis of the long-held notion that doing endurance or aerobic exercise leads to either a loss of muscle mass, or a prevention of muscle mass gains. In the study with the mice, the high protein diet led to a dominant mTOR effect, while AMPK levels remained stable. This, in turn, led to reduced mitochondrial activity in the rodents, although they gained muscle mass. Based on this result, the Japanese researchers suggest that consuming a high protein diet will lead to a loss of muscle endurance.
   The problem with this, however, is first, what occurs in mice isn't necessarily replicated in humans.In fact, recent evidence shows that weight-training is capable of boosting mitochondrial activity, and also boosts the levels of a protein called PGC-1A, which stimulates the development of new mitochondria. Several nutrients, including resveratrol, also are known to boost the activity of PGC-1A. There is no human evidence that consuming a chronic high protein diet will adversely affect muscular endurance. Indeed, endurance athletes are advised to consume more protein than even strength athletes because some of the protein can be used for energy, and also to help repair damaged muscle tissue that is common after long distance running events.Some evidence does show that doing an extensive amount of aerobic exercise following a weight-training routine may interfere with muscle and strength gains through the AMPK mechanism. However, this only pertains to aerobic sessions that are over one hour in length. Shorter sessions have little or no effect on muscle growth. There is one exception to this, however. If you engage in intense aerobic activity under zero carbohydrate conditions, there is an outside chance that you may sacrifice some muscle because of elevated cortisol levels that may result.But even in this instance, a higher protein intake will act as a buffer against muscle loss, especially the branched-chain amino acids and a leucine metabolite called HMB.

Mitsuishi, M, et al. Dietary protein decreases exercise endurance through rapamycin-sensitive suppression of muscle mitochondria.Am J Physiol Endocrin Metabol 2013: in press

©,2013 Jerry Brainum. Any reprinting in any type of media, including electronic and foreign is expressly prohibited.

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