8. Myth: Fasting increases cortisol.
Truth
Cortisol is a steroid hormone that maintains blood pressure, regulates the immune system and helps break down proteins, glucose and lipids. It's a hormone that's gotten quite a bad rep in the fitness and health community but we have it for a reason. The morning peak in cortisol makes us get out of bed and get going. A blunted morning cortisol peak is associated with lethargy and depression. Cortisol is elevated during exercise, which helps mobilize fats, increase performance and experience euphoria after and during workouts. Trying to suppress acute elevations of cortisol during exercise, or the normal diurnal rhythm, is foolish. Chronically elevated levels of cortisol, resulting from psychological and/or physiological stress, is another thing and unquestionably bad for your health; it increases protein breakdown, appetite and may lead to depression.
Short-term fasting has no effect on average cortisol levels and this is an area that has been extensively studied in the context of Ramadan fasting. Cortisol typically follows a diurnal variation, which means that its levels peak in the morning at around 8 a.m. and decline in the evenings. What changes during Ramadan is simply the cortisol rhythm, average levels across 24 hours remain unchanged.
In one Ramadan study on rugby players, subjects lost fat and retained muscle very well. And they did despite training in a dehydrated state, without pre-workout or post-workout protein intake, and with a lower protein intake overall nonetheless. Quoting directly from the paper:
"Body mass decreased significantly and progressively over the 4-week period; fat was lost, but lean tissue was conserved..."
"...Plasma urea concentrations actually decreased during Ramadan, supporting the view that there was no increase of endogenous protein metabolism to compensate for the decreased protein intake."
In one study on intermittent fasting, the fasting group even saw "significant decrease in concentrations of cortisol." However, this study should be taken with a grain of salt as it had some flaws in study design.
In conclusion, the belief that fasting increases cortisol, which then might cause all kinds of mischief such as muscle loss, has no scientific basis whatsoever.
Origin
Prolonged fasting or severe calorie restriction causes elevated baseline levels of cortisol. This occurs in conjunction with depletion of liver glycogen, as cortisol speeds up DNG, which is necessary to maintain blood sugar in absence of dietary carbs, protein, or stored glycogen. Again, it seems someone looked at what happens during starvation and took that to mean that short-term fasting is bad.
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