Oh, the first 3 or so days on my prep diet, I felt like death warmed over....but I just stuck to it, and then I felt better, and by week 3 I was able to function pretty well. I don't want to say 100%, as that for me would imply being able to continue to beast everyone on hikes, etc, which is impossible on a marked caloric deficit, but I was definitely feeling almost as good as on my regular 2500 calories or so.