eli5: why is it so difficult to go a week without eating when we have 10s of thousands of calories worth of body fat?


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

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.

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.

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.

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.