Fasting has some interesting metabolic effects. For example, the fall in insulin levels seen with fasting may induce weight loss, and the fall in inflammatory markers may improve immunity. Testosterone levels are known to drop in males undergoing fasting and this could be seen as detrimental, particularly if maximising muscle mass is a concern to that person. For example, in male rats, fasting has been shown to result in a significant reduction in plasma testosterone, plasma luteinising hormone and plasma insulin, as well as liver and muscle glycogen stores. This could therefore be seen as a detrimental effect of fasting. However, in addition to a reduction in plasma testosterone are a reduction in plasma cortisol. Therefore while testosterone levels may be reduced, the testosterone to cortisol ratio is not as detrimental as it would if cortisol levels remained high. Muscle catabolism may therefore be spared somewhat by the lower levels of cortisol, despite the lower than normal testosterone levels.
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