Fasting is a challenge to your brain, and your brain responds to that challenge by adapting stress response pathways which help your brain cope with stress and risk for disease. The same changes that occur in the brain during fasting mimic the changes that occur with regular exercise. They both increase the production of protein in the brain (neurotrophic factors), which in turn promotes the growth of neurons, the connection between neurons, and the strength of synapses .
Fasting can also stimulate the production of new nerve cells from stem cells in the hippocampus. He also mentions ketones (an energy source for neurons), and how fasting stimulates the production of ketones and that it may also increase the number of mitochondria in neurons. Fasting also increases the number of mitochondria in nerve cells; this comes as a result of the neurons adapting to the stress of fasting (by producing more mitochondria).
By increasing the number of mitochondria in the neurons, the ability for nerons to form and maintain the connections between each other also increases, thereby improving learning and memory ability .
Human clinical trials were conducted using patients who were receiving chemotherapy . For long periods of time, patients did not eat, which significantly lowered their white blood cell counts. In mice, fasting cycles “flipped a regenerative switch, changing the signalling pathways for hematopoietic stem cells, which are responsible for the generation of blood and immune systems.”
This means that fasting kills off old and damaged immune cells, and when the body rebounds it uses stem cells to create brand new, completely healthy cells. (Source: Muslimvillage)