Unfortunately, "fasting" is not specific enough. For some people, it means not eating for 6 hours or so ("breakfast" breaks the fast even if you've eaten late l

Unfortunately, "fasting" is not specific enough. For some people, it means not eating for 6 hours or so ("breakfast" breaks the fast even if you've eaten late last night). For some people, it means >24 hours. Some people add the qualifier "intermittent" to signify that it is less than 24 hours (but done repeatedly). You can make up for it afterwards, but usually don't. Most of the people eat way more than they need in general, and thus there is no need to "make up" for a few missed meals. If you've gotten used to it, after a fast - whether 18 hours or 240 hours - you just (slowly) resume regular eating. You do need to be aware of refeeding syndrome, which may be triggered by jumping back to regular amounts of food after a prolonged fast though; If you haven't eaten for a few days, you shouldn't just resume your regular feeding schedule and food - you need to start with small, easily digestible (preferably, liquid/pouridge) and build up back to normal over a few days.