After i switched to a vegan diet, i cant even put on weight if i wanted to. Try eating fruits and veggies and see if you can consume more than 2000kcals a day. Its not easy. I measured 72.7 kgs today at 6 4 and iam not even trying to stay lean
The problem with an all out fast is this; you will lose a lot of lean muscle mass. Your body doesnt need fats to survive, it can consume stored fats (which is the point really). Problem is it also needs proteins and catabolizes your muscles for its daily protein needs. So intermittent fasting is the best balance.
Most people who fast usually have very poor muscle mass.