A couple more studies that raise a red flag about fasting:
Decreased Mitochondrial Metabolic Requirements In Fasting Animals Carry An Oxidative Cost, Functional Ecology, September 2018
The reduction in total hepatic metabolic capacity in fasted fish was associated with an almost two‐fold increase in in vivo mitochondrial H2O2 [hydrogen peroxide] levels.
The resulting increase in mitochondrial ROS [reactive oxygen species], and hence potential risk of oxidative damage, provides mechanistic insight into the trade‐off between the short‐term energetic benefits of reducing metabolism in response to fasting and the potential long‐term costs to subsequent life‐history traits.
Surprisingly, IF [intermittent fasting] animals also presented an increase in oxidative damage in the brain.
Both of these studies discuss the cost of fasting: oxidative stress.
It is fair to note:
The body can repair damage from oxidation, to a degree. Free radicals, products of oxidation, can damage DNA. Damaged DNA, as you know from radiation effects, can cause cancer. The body has DNA repair genes, although these can be overwhelmed. The body also has in-house antioxidants like glutathione which can quench free radicals. These also can be overwhelmed. How much damage oxidation does depends upon a person’s general health, the presence of any chronic conditions like diabetes or liver disease, diet, and the length and frequency of fasts.
There’s something else to consider:
Anything that depletes your glycogen, a storage form of glucose, can lead to the breakdown of muscle – muscle in your heart, muscle in your arms and legs, muscle everywhere. That’s because muscle supplies the raw material for gluconeogenesis, the new making of glucose. The body sacrifices muscle to provide glucose for the brain. For an older person, this muscle loss contributes to sarcopenia which is very difficult to reverse. Fasting and low-carb diets (Keto, Paleo, Atkins) deplete your glycogen. That’s how they work.