[even_639765]

Your body requires nutrients to function. Deliberately depriving your cells of those nutrients can't possibly be good for them and likely does them harm. You evolved to survive such catastrophes of course, but the key word is "survive".

What states of mind can be induced by fasting is a separate consideration, it seems to me.

[s1artibartfast]

I don't think deprivation is the right model. Cells are still getting nutrients, just from different sources, particularly fat. Animals have 500 millions of years of evolutionary experience using this energy source.

I would be more concerned about the harm caused by not using this metabolic pathway, at least occasionally.

[even_639765]

"Particularly fat"

Fat contains lipids, not nutrients save the fat soluble ones.

[s1artibartfast]

Fat itself is a nutrient [1], providing one of the most important bodily requirements of energy. The body requires other nutrients as well, and has methods of storing them. Fasting does not mean going without them. The nutrients of most interest to fasting are probably essential amino acids which can not be synthesized and ionic electrolytes.

Free protein and amino acids only last a couple days in the body before being used for muscle or being irreversibly converted to fat. When necessary, the body can break down muscle to obtain essential amino acids.

The relevant electrolytes the body needs are sodium, potassium, and magnesium, which it maintains fairly deep stores of in the bones.

https://en.wikipedia.org/wiki/Nutrient#Macronutrients