Fasting is known to cause significant hormonal and neuronal changes. These changes are thought to be responsible for some of the health effects of fasting. Fasting can change brain chemistry through a number of mechanisms and one mechanism might be the availability of particular energy substrates. During fasting glucose levels fall, and this reduces the availability of glucose to the brain, forcing a switch to the use of ketones as an energy substrate. Evidence suggests that the stress response may rely on glucose to be fully activated, and without a ready supply of glucose the pituitary-hypothalamus-adrenal axis may become blunted. The effect of this would be to lower circulating levels of the stress hormone cortisol, and this may provide some health effects where chronic stress is present through an attenuation of the cortisol response. This may explain some of the mood changes that are seen with fasting, which include reductions in anxiety and depression, both of which may be caused by cortisol from the stress response.
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