This is just my experience - I did aggressive IF fasts - 16 off/8on with calories consumed around 1000-1100. In my experience, the more you have to lose, the "safer" you can eat minimally as long as you doing some physical movement - like light weight lifting to keep your muscles active. You have to be careful as you will absolutely hit a brick wall and you have to start eating more to keep your body from crashing. Angus Barbieri had a lot of fat stored up and so he was able to go a long time without eating as long as you incorporate some sort of muscle resistance exercises. Since losing weight, I have to eat more as my metabolism is a lot higher and I'm way more active but it just gives me a little leeway to eat more. I still CICO as aggressively as I can.