eli5: why is it so difficult to go a week without eating when we have 10s of thousands of calories worth of body fat?
In: 89
Fat may provide energy, but it does not provide all your nutritional needs.
You *can* live on fat and vitamin supplements (and water) pretty much indefinitely. As there was one guy who didn’t eat for 200+ days. And came out the other end just fine, he had some trouble adjusting back to food, but no long term health complications.
But it’s also incredibly unpleasant. You’re not supposed to *want* to live off fat reserves. Going back to hunter gatherer times your fat reserves were what kept you alive over winter or in the event you got hurt/sick and couldn’t hunt for a while. Evolutionarily, you’re not supposed to be losing fat. It means you’re failing at feeding yourself. But inevitably there will be times that you’d go hungry, so fat reserves.
That food scarcity is hardly an issue in our modern time, and so that’s why it’s so hard to overcome our nature and keep ourselves at a calorie deficit.
Just burning the fat reserves you have **might** keep you alive a bit longer, but it certainly isn’t going to do anything when it comes to replenishing the vitamins and minerals that your body depends on. No sodium or potassium? Goodbye muscle functions.
Your body keeps telling you that it’s hungry because it depends on a lot more than just calories to survive.
Because your fat reserves are *reserves*. That’s the last-ditch fuel your body burns to stave off starvation. In “normal” mode, your metabolism fuels your body mainly off your regular calorie intake.
The fat reserves take a different chemical/hormonal pathway (and extra water) to access the energy stored there.
This analogy’s not perfect, but retooling to a fat-based metabolism is a bit like trying to install and hook up a diesel generator *after* the regular power goes down. You can do it, but it’ll be damned inconvenient to everybody in the house until it’s up and going. No computers (brain no thinky so good on a carb deficit), regular lights don’t work, even if you can do a lot of day-to-day stuff by natural light or candle/lamplight…
Even once you make the switch and the lights are back on, you can have trouble with current regulation, and you definitely can’t run on backup power forever. Diesel exhaust is pretty toxic, for one thing, and eventually, your reserve fuel will just run out.
ETA: That kind of describes the issues people have when they try to switch to a keto diet – you basically stop taking in carbs until your body gets the message and retools to burn mainly fat instead.
I did that once and yes, it’s possible, but definitely not nice. Fat is a long-term storage of energy and just as it is difficult to build it (*) it is difficult to burn it. The easy way to do it is to have some carbs + exercise, to turn up your metabolism. When you’re in this “high performance” mode for more than around 30 minutes, your organism naturally switch to burning fat and do it even after you stop exercising if you don’t consume more carbs.
But if you just want to stop eating, that’s another story. Yourbody quickly use up stored carbs and then it starts to feed off your fat… but also your muscles. Muscles are protein and these are easier to turn into energy. So you risk that you will actually lose your muscles but not as much fat as you would like. And because there are no carbs, you need to deal with lower sugar in blood, which result in feeling tired, mood swings, and slower thinking. It’s tough on the psyche and that’s why dieting is difficult – you need to handle precise intake of carbs and protein to shield your muscles from being digested instead of your fat tissue, while your brain is just one step from throwing a tantrum.
People answer the *how* but not the *why.*
Our bodies didn’t evolve in the modern age. Immense food availability is something that’s barely 100 years old, with the invention of producing synthetic ammonia (fertilizer).
Hominids are pretty unique in how pervasively we store fat (though there are others), and much of it comes down to migratory behavior and a very calorie hungry brain. The brain can’t store energy, so the body needs to constantly provide it, via consuming food or tapping into reserves. Way back when, food availability wasn’t constant, so the body developed the insulin response to trigger the rapid storing of blood sugar as body fat when food was plentiful. In later times of sparse food, the body fat would be consumed and converted to a usable form to produce ATP (body’s raw “energy” unit).
So in short, while we may have plenty of fat reserves, there’s no connection between quantity of fat reserves and the instinct to consume, as there would be little to reinforce such a physiological development – those that consumed more had more biological resources and had more offspring.
A nice reminder that all of us don’t have that much fat. I hit 31 this year, still underweight, still can’t gain weight no matter what I do. Docs said “the level of fat in your body is so low that it can be dangerous”. I didn’t choose it, shit just happens.
Guess I’d be among the first to die if a famine comes.
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.
You probably wouldn’t starve to death in a week, but evolution has 500 million years of practice telling your ancestors “go eat you idiot, you need it to survive”. Of course it’s hard to go against your most basic instincts.
By the way, you shouldn’t try to do this, just because you are likely not going to starve to death doesn’t mean it’s good for your health. If this is some sort of “weight loss experts hate this one trick” then doubly so you shouldn’t try this. Rather take up exercise, seriously and every single day, it’ll be both easier and much more effective than starving yourself.
For largely the same reasons why your common sense (hopefully) tells you to keep going to your job every day even though you (hopefully) have money in your bank account.
You don’t want to use up your reserves unless you absolutely have to do so.
I have gone for a week without eating on two occasions. Once as a religious fast, another as part of testing the waters of longer than intermittent fasting. Keeping in mind that I did research ahead of time to know how to do it while staying healthy, I mostly just took an electrolyte supplement, and can say that it really is not hard at all. Day two is kind of annoying, but after that day 3 was easier than late day 1, and it really didn’t ever feel any harder. I looked forward to breaking the fast, but I did not feel hungrier than a normally long wait between meals. There’s even someone who [fasted for over a year](https://en.wikipedia.org/wiki/Angus_Barbieri%27s_fast).
Latest Answers