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.

I know it's technically true but it's seemed weird to me to lump skipping breakfast in with what I'd consider actual fasting, ever since I first encountered people using it that way in the context of diet fads.

Someone tells me they're fasting, I don't expect they mean 8 non-contiguous waking hours without eating (with sleep in the middle). That's, like... pretty close to a normal day. It's easy to have 16 hour "fasts" just by accident. "I only ate two meals yesterday because I was fasting". LOL WUT? Again, I know, technically true, but not a useful way to refer to it, IMO.