Situations can occur that increase the use of body protein for energy. Eating too few calories or fasting increases the reliance on body protein as an energy source. In these situations the level of circulating glucagon and cortisol increase. Cortisol, the stress hormone, will promote the breakdown of our body proteins to amino acids. Meanwhile, both of these hormones promote the conversion of amino acids to glucose in our liver which is released to serve as fuel. The amount of amino acids used to make glucose is related to the length and degree of caloric restriction and the intensity and duration of exercise. Simply stated, as glycogen stores in the liver and muscle become depleted, as in prolonged fasting and aerobic exercise, the reliance upon amino acids to make glucose increases.
During a longer period of fasting (for example, more than a week) the reliance on amino acids lessens as our brain adapts to utilize more ketone bodies. This is one way that our body attempts to slow the loss of protein, however the use of amino acids for energy is still greater than during more normal times. If the loss of body protein continues for months, a person can reach a critical level of body protein whereby normal function is compromised and illness can occur and over more protracted periods, death is possible. Even if the cause of death is due to an infection, the true cause is probably a failure to maintain an optimal immune defense because of poor protein status.
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