Don't do something like this. A 7 day fast of just water isn't healthy. Your body starts burning more than just your fat. Even when doing something like a highly-restricted-calorie diet such as 960Cal/day you don't want to do for more than 3 days without medical supervision.
While it is true that you do burn some muscle, the increased HGH levels immediately replace it when you refeed. This has been studied significantly. And for me, I don’t give a rip about muscle loss right now. I’m just trying to lose weight.
Your body isn’t stupid. You won’t turn into a ball of fat if you fast regularly. Cavemen and even animals have long feast/famine cycles. Our bodies are designed to store fat and burn it when food is scarce.
Now if folks are already in a healthy weight, then yea, extended fasting isn’t necessarily something you want to do a lot of, but for obese people, we have plenty of fat to keep things rolling as long as other essential nutrients are supplemented via multivitamins.
