eli5: why is it so difficult to go a week without eating when we have 10s of thousands of calories worth of body fat?
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Other comments focus on calories not being your body’s only need, and they are correct.
But there is one more reason you feel awful when not eating. Your body doesn’t WANT you to do that even if you’re technically capable of fasting for days.
You’re programmed to eat whenever food is available, because you never know when it will be scarce in the future. So your body is constantly yelling at you to eat something, especially if you haven’t eaten and there’s food around.
And this alarm bell does get quieter after a while. When you’re working hard, or under huge amounts of stress, you may literally forget to eat for hours or, in more extreme cases, for days.
This varies a lot between people, and depends on an individual person. But also – most people in the developed world are used to multiple large-ish meals per day, so the body gets used to that, and goes into panic mode when you don’t eat for more than 24 hours.
Anecdotally, I used to eat a *lot* once in a while for a year or two when I was in my 20s, and I would eat much, MUCH less than my body needed in the days between. There were no ill effects other than the fact that I was slowly gaining weight (because my overall long-term calorie balance was a net positive). And I’ve known a lot of people (mind you, people without eating disorders) who had to remind themselves to eat during, say, college exam period, because they’d just forget to eat. And yes, they’d rapidly lose weight.
But normally, your body wants to avoid that, because it’s not like the automatic systems that regulate it are aware there’s a stocked fridge in your house. You’re still supposed to be a hunter-gatherer in the African savannah, and with that sort of lifestyle, not eating is Bad News, so the body nudges you to eat. And then screams at you to eat.
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