Here Is What Crash-Dieting Does To Your Body

Welcome to the hunger games.

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We all have that friend who occasionally runs on a cool 700 calories a day for a week so they can ~lose weight fast~ or whatever.

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And, OK, sometimes you are that friend.

That’s a crash diet. As in, it’s pretty much bound to crash and burn.

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A crash diet is essentially a very restrictive meal plan that’s unsustainable for the long term, says Darcy Johannsen, Ph.D., R.D., assistant professor at Pennington Biomedical Research Center. It’s usually about severely restricting your calories — like cutting back to 1,000 or even as low as 500 calories per day. Or sometimes a crash diet is all about a juice cleanse or just eating one or two foods for a week. The length of time differs from diet to diet, but they always come with an end date.

Here’s the thing: Crash diets can actually wreak havoc on your body, not to mention your general disposition (because, hangry). Here are nine reasons you should probably never do them again:

1. Your metabolism slows WAY down, so you suck at burning calories.

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“When you limit calories so dramatically, your body goes into starvation mode and your metabolic rate plummets,” says Johannsen. Your body isn’t sure when it’s going to get fed again, so it does everything it can to conserve energy. So not only are you burning fewer calories doing normal activities (like digesting, standing, walking), but you’re also just moving less because you have less energy.

2. And then you start to burn off muscle.

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“If you’re being very restrictive with calories, you’re not just burning your fat as fuel,” says Johannsen. “You’re going to burn lean tissue, as well,” which is mostly muscle. In the first few weeks of your crash diet (which is about all they last anyway), about half of the fuel you’re burning comes from fat stores, while the other half comes from lean tissue mass (i.e., your biceps).

3. You're hungry ALL the time.

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Maybe it’s because all you’ve eaten today is an apple, or maybe it’s because your levels of the hormone ghrelin go up substantially when you're in starvation mode, explains Johannsen. This hormone revs up your appetite and makes you want to eat everything, so it’s basically a vicious cycle that you cannot win.

4. You become kind of a monster.

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And not just because everyone is eating cheeseburgers while you’re eating lettuce and lemon juice. “When people go on crash diets, there are changes in the neural networks and brain activity that make you very cranky and irritable,” says Johannsen. This is also anecdotally confirmed by everyone around you.

5. Moving at all sounds like a chore.

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Thanks to decreases in your thyroid hormone and epinephrine (the hormone and neurotransmitter better known as adrenaline), you’re physically and mentally exhausted. “You basically have to fight to be active,” says Johannsen.

6. Your sympathetic nervous system gets lazy.

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This can make you feel colder all the time, slow down your heart rate, or produce a drop in blood pressure that could even cause you to pass out.

7. You miss out on major nutrients.

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On most crash diets, it’s hard to ensure that you’re getting enough protein, fiber, amino acids, calcium, iron, vitamin B12, and all that other good stuff that your body needs on a daily basis. Without these, lots of things can suffer — like your bones, cognitive function, blood cells, and more, says Johannsen.

8. OK, sure, you'll probably lose weight. BUT keeping it off is no guarantee.

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“If you really adhere to this, you’re going to lose weight because your calorie expenditure is far exceeding your intake,” says Johannsen. But whether or not you’ll gain it right back is up for debate (except, yeah, you probably will). A lot of people overdo it after a strict diet and end up gaining everything back immediately, says Johannsen.

This could be related to your levels of the hormone leptin, which drop when you lose a lot of weight in a short period of time. And some research suggests that a larger decline in leptin levels is associated with regaining the weight back, says Johannsen.

9. And post-crash... no one's totally sure what might happen.

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Unfortunately, the research isn’t as clear when it comes to what happens after your crash diet, says Johannsen. For instance: Why do some people gain the weight back and others don't? And what about your hormones and insulin sensitivity? Here’s what ~could~ happen, based on what research has been done so far:

Your metabolism might be all wonky.

This drops a lot during a crash, and there’s some data to suggest that this persists even after you go back to a normal diet, says Johannsen. That means you’d be eating more calories and still burning fewer, which would obviously lead to weight gain.

Your insulin sensitivity (and risk of diabetes) might be impacted.

The initial drop in weight might improve your insulin sensitivity temporarily, but researchers don’t know what effect repeated crash diets could have on it in the long run, says Johannsen.

Fat might come back faster than muscle does.

Remember how crash diets burn through your muscle tissue? Well, they might not bounce right back after the crash. “Your body’s first impetus is to rebuild the fat stores,” says Johannsen. So when you come off the crash diet, your body will want to store fat before it works on rebuilding your muscles. Because life isn’t fair.

Bottom line: Crash diets might be a quick fix, but they can also be awful, dangerous, and come with lasting negative health effects.

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The whole basis of a crash diet is that it’s unsustainable, and that’s not what you should look for in any eating plan. Sure, you could lose weight quickly, and if you follow that up with healthier behaviors (like cutting back on sugar, eating less processed food, and working out more), the results might stick. (But honestly: How likely is that, actually?) That said, severely restricting your calories or eating only one food for any extended period of time can do some scary things to your body — and those effects might stick too.

“Metabolically, we’re probably doing more harm than good,” says Johannsen. “Even if a little bit of weight is lost during that time, it could have a lot of immediate health effects as well as longer-term consequences.”