There are some military studies that showed autophagy didn't kick in until they dropped below 6% body fat. But that was with 600kcal and no change in duties. So it's not necessarily relevant. I do IF but I'm a couch potato. If I really stick to 20hrs, it seems to work very well. If I slip to even 19 I have to skip food for a day or even two to get back to feeling good (clear head functioning gut etc.). I am overweight still, but not obese. I lose about 2lbs a week when I stick with it. I am nervous about resuming excercise. I hope I can still recover well. I just do strength training 3 days a week for an hour, or two once I'm back to needing more warmup sets.