Fasting can be beneficial to the health. The purpose of a fast is to purify the body of toxins through the enhanced elimination of metabolic waste and exogenous chemicals. One of the effects of fasting is a rapid weight loss, and this weight loss can include substantial amounts of adipose tissue. As well as being a store of energy, the adipose tissue is used to contain toxins that are unable to be processed by the liver or kidneys. Exogenous chemicals such as pesticides and drug residues accumulate in adipose tissue and can remain stored for years. During fasting, these chemicals are often released to circulation as the adipose tissue is used as a source of triglycerides for energy and ketone production. Therefore the fasted state increases the plasma levels of certain toxins and this increases the chance that the toxins will be excreted. However, as with all low calorie diets, fasting results in a loss of skeletal muscle mass that can can detrimentally reduce resting metabolic rate. Of the weight lost during a fast around one third to a half can be from skeletal muscle, which is catabolised as a source of energy.
Modified fasts are a solution to the catabolic effect of fasting. Protein modified fasting has been devised as a method to supply the body with essential amino acids and nitrogen, in an attempt to prevent the catabolic effect of fasting. Studies show that fasting in combination with protein supplied at a concentration of 1.0 to 1.5 grams per kilogram of body weight per day is able to significantly reduce the loss of muscle mass. This tissue sparing effect of dietary protein is well reported and many studies have investigated the reasons for the ability of dietary protein to prevent the decrease the skeletal muscle mass seen during fasting. For example, the ability of dietary protein to spare muscle tissue has been hypothesized to originate from the insulin stimulating effects of protein. Insulin is the primary anabolic hormone and fasting and other forms of energy restriction lower insulin secretion causing a marked catabolic state to develop. Certain amino acids are insulin secretagogues and may therefore produce an anabolic response.
However, studies have shown that the insulin levels of subjects fed protein are not significantly different compared to fasting controls1. Therefore the insulin releasing effects of protein are not likely to be responsible for the protein sparing effects of modified protein fasts. More likely the amino acids supplied by the protein spare the skeletal muscle by supplying carbon skeletons for energy and ketone production, and this then negates some of the reliance on endogenous protein. Urea nitrogen levels are increased on a protein modified fast compared to traditional fasting, suggesting that it is the nitrogen balancing effects of protein that are more important than the insulin stimulating effects with regards preventing muscle loss. This is likely the same reason that high protein diets are more effective than lower protein alternatives for weight loss. By sparing skeletal muscle the high protein diet allows maintenance of a higher resting metabolic rate and this facilitates the less efficient utilisation of energy which increases the loss of body fat.
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