With respect to energy, that’s apparently true.
However, metabolism has non-energy needs which are supplied through food: vitamins, minerals, protein, and some fats that cannot be synthesized iirc. Though these needs are greatly reduced when fasting, they do not go to zero.
Google “angus barbieri” for the most famous, though not unique, example for medically documented long term fasting.
Re:Why cancer cells waste so much energy
by ThePadawan on Friday January 15, 2021 @11:51AM
(#25792903)
It certainly has been done: https://en.wikipedia.org/wiki/Angus_Barbieri%27s_fast
If you are 350 pounds, can't you just not eat until you are 150 and be fine? (I am not suggesting that this is true or that anyone should try it; I don't know and I am asking the question.)