If carbohydrates are removed from the diet they must be replaced by either fat or protein, if energy balance is to be maintained (alcohol could also replace carbohydrate, and sometimes does in lesser amounts, but for the purpose of this discussion alcohol will be ignored). Although some do choose to eat high fat diets, such diets are generally not palatable if the high fat component is devoid of both carbohydrate and protein. Therefore when the term low carbohydrate diet is used, what is often being referred to is a low carbohydrate high protein diet, with protein replacing most of the energy from carbohydrates. The amount of fat in such diets is usually dictated by the choice of protein, red meat being high in fat and therefore increasing the energy contribution from fat significantly, while chicken and fish is lower in fat. Low carbohydrate high protein diets have some interesting physiological effects that makes them useful in weight loss. One of the most significant of these beneficial effects being the muscle sparing effect they possess.
Skeletal muscle is in a constant state of turnover, which is a combination of the catabolic breakdown of muscle tissue with the anabolic building of tissue, processes that occur simultaneously. The change to the size of the muscle pool is determined by whether the catabolic or anabolic processes dominate. Dietary protein provides the building blocks for new muscle growth because it contains the amino acids required to build muscle tissue. Without adequate protein, anabolic processes slow and any increase in the size of the skeletal muscle pool is not possible. However, a minimum amount of protein, defined by its nitrogen content, is required to maintain the current pool of skeletal muscle. Low intakes of protein that fail to provide this minimum threshold allow catabolism to proceed. Low energy diets that provide inadequate protein therefore cause a loss of skeletal muscle, and this skeletal muscle loss can account for significant amounts of weight lost during low energy diets.
Higher protein diets have a muscle sparing effect because they supply adequate nitrogen for maintaining current skeletal muscle levels. Studies have demonstrated the protein sparing effects of high protein intakes in individuals undertaking energy restricted diets. For example in one study1, overweight women were fed an isoenergetic diet that provided either 68 grams of protein or 125 grams of protein, both versions of the diet supplying 30 % dietary fat. Following ten weeks of the diet, the high carbohydrate low protein group had lost 7 kg of weight and 4 kg of this was body fat. However the low carbohydrate high protein group had lost 8 kg of weight and 6 kg of this was body fat. The low carbohydrate high protein group therefore lost most weight than the high carbohydrate low protein group, and more of that weight was fat. The fact that the diets were isoenergetic shows that energy reductions are not needed to cause weight loss. Both groups likely lost weight because dietary improvements in carbohydrate choice were made.
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