What happens when you fast?

12 hours = Ketosis. Your body starts to break down and burn fat, some of which is used by the liver to produce ketones for energy. Ketones produce less inflammatory bi-products than glucose, can kick-start production of the brain growth factor BDNF, reduce inflammation, and lower cellular damage and cell death in neurons
18 hours = Fat burning. Your body is now generating plenty of ketones, usually measuring between 0.05 and 0.1 mM, but now 5-7 mM. Heart-pumping exercise can further increase this. These tell your body to ramp up stress-busting pathways that reduce inflammation and repair damaged DNA
24 hours = Autophagy. Your cells are now recycling old components, breaking down mis-folded proteins linked to Alzheimer’s and other diseases, and rejuvenating cells and tissue. We all need periods of autophagy to clean house, especially with ageing. Autophagy is initiated because fasting activates AMPK signalling, which inhibits mTOR (growth) signalling. But your glucose stores need to be depleted and your insulin levels lowered
48 hours = Growth Hormones. These are now up five-fold. This helps preserve lean muscle mass, reduces fat tissue accumulation and may promote wound healing, longevity and cardiovascular health
54 hours = Insulin Sensitivity. Lowered insulin levels can reduce inflammation, make you more insulin sensitive and/or less insulin resistant and protect you from chronic lifestyle diseases like cancer
72 hours = Immunity. Old immune cells are broken down and new ones are being generated. This is via reduced circulating IGF-1 levels and PKA activity, turning down cellular survival pathways and leading to breakdown and recycling of old cells and proteins. This leads to stress resistance, self-renewal and the regeneration of stem cells. Fasting for 72 hours or more preserves healthy white blood cells in chemotherapy patients

Fasting must be done properly eg for cycling women; and may not suit everyone at all times eg pregnant/breastfeeding women, those with low blood sugar or thyroid issues.

Fasting is just one of the tools I use in my practice.