I'm a small, slight female, but for my height and weight need a lot of calories to keep me going. When I fast, as as happens from time to time, for a variety of reasons, it doesn't mean I go without. It just means I eat within a shorter window (start later or finish earlier), or I am fasting for a specific reason - such as I did a while ago for a long haul flight, where I knew the food would be poor. That fast was about 30 hours in the end, which isn't long by comparison the the fasting ninjas on here, but it just meant I fuelled well beforehand, and when I broke my fast, I did so on nutrient dense foods, with a decent amount of fat included.