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What happens to your brain when you go on a diet

This is why it's so hard to lose weight.

You’d think we’d be dieting pros by now. There are countless experts telling you they know the secret to getting slim by eating this or avoiding that. And then there’s the old “calories in versus calories out” equation that seems so simple at the surface, but is much harder when put in practice. And with 45 million Americans dieting each year and most people failing at those diets, there much be so much more going on underneath the calls to snack on kale chips and forget the calls of Little Debbie.

At the heart of the matter is a little thing called “set point weight,” says Stephan Guyenet, PhD, author of The Hungry Brain. “Body weight is regulated by the brain. If you don’t know that, you’re going to be surprised when your brain and body start fighting back against weight loss,” he says.

This is at the heart of why diets don’t work, says Sandra Aamodt, PhD, (appropriately) the author of Why Diets Make Us Fat. “Whenever your weight changes too much, your brain will intervene to push it back to what it thinks is the correct weight for you. And you might not prefer the same weight your brain prefers. Many of us don’t,” she says.

Guyenet likens the entire process to a thermostat. In your house, this device measures the temperature in your home and helps regulate it so that it stays stable. When the temperature goes up, the heat goes down (or the ac turns on), and vice versa. In your brain, your thermostat is in the hypothalamus. “Your hypothalamus will activate physiologic and behavioral responses to maintain your body temperature. For instance, if you’re cold, you may shiver or put on a sweater,” Guyenet explains.

It's Not Just Body Weight, It's Body Fat, Too

A similar thing happens in your brain when it comes to how much body fat you’re carrying. “Your brain measures the level of body fat using leptin, a hormone that’s secreted in your bloodstream in proportion to the amount of fat you carry,” says Guyenet. Higher levels of leptin in your bloodstream mean more fat on your body. You may have heard of leptin before, called one of the “hunger” hormones, along with ghrelin. In terms of leptin, it decreases your hunger.

Whenever your weight changes too much, your brain will intervene to push it back to what it thinks is the correct weight for you.

But here’s where things get tricky. Everyone has a certain level of body fat that their bodies are happiest at. “Your brain will defend this amount just like it defends your body temperature,” explains Guyenet. As you lose weight, the amount of leptin in your bloodstream drops — and that’s where the trouble starts. It sends a signal to your brain to help you fight to bring that fat back. Of course, that’s the exact opposite effect that you’re going for. But it’s hard to beat out biology.

Guyenet calls this a classic starvation response. Your brain responds by upping hunger, making those doughnuts in the break room that’ve been sitting out for four hours look actually yummy, and ensures cravings are impossible to ignore. Physiologically, your metabolic rate slows so you can conserve energy and send it right back into building up fat stores, he says. As a 2011 study in the New England Journal of Medicine shows, these “compensatory mechanisms” driving weight regain last for at least one year, a long time to spend battling your body.

Your Brain Wants You to Cheat On Your Diet

The hunger is what may really do you in. “Sometimes you don’t even realize you’re eating more. But when you’re below your set point, you end up eating more to maintain the same hunger level as you did before. That’s why it’s possible to maintain a healthy diet and still gain weight back,” says Aamodt.

You can imagine why it’s so hard to lose weight — and even harder to maintain that weight loss. You can do it for a time, but eventually your brain will win out and you’ll stand in your kitchen eating straight from the container of Ben & Jerry's.

That’s where binging comes in. Whereas you might never have considered polishing off a sleeve of Oreos in the past, your dieting self feels like it’s necessary. “In lab experiments, when scientists want to induce rodents to binge eat, the most reliable method for doing it is to reduce food intake until they’re at a lower weight and then expose them to super tasty food, like Cocoa Puffs or Oreos,” Aamodt explains. She adds that in human research, some studies that look at the brain show that this type of junk food activates reward centers even more fiercely in those who have lost weight. And, she says, animal research may suggest that repeated dieting makes the brain more vulnerable to binging behavior even after the diet is donezo.

It's Called Yo-Yo Dieting for a Reason

It turns out dieting is one of the best ways to … gain weight. “If you follow people over the long term, dieters are more likely than equivalent non-dieters to end up gaining weight over the next two to 15 years after the diet,” says Aamodt. As she explains, dieting is stressful, and it triggers an increase in stress hormones, which are linked to weight gain (particularly visceral, or belly, fat). It also makes you suppress and ignore your hunger. Over time, that can make you less responsive to your natural hunger cues, making it harder to listen to your body and allow it to regulate your weight. “You become more at risk of emotional eating, eating out of boredom, and are more vulnerable to environmental cues that tell you to eat more than your body actually wants,” she continues.

Animal research may suggest that repeated dieting makes the brain more vulnerable to binging behavior even after the diet is donezo.

Cue post-diet weight gain (and then some). In fact, though the risks of yo-yo dieting have been contested, one 2016 study found that repeatedly going through these starvation cycles prompts your body to gain more weight.

Want to Lose Weight? Make Your Brain More Comfortable

If you want to drop a few pounds (or 50) or your doctor recommended it, this doesn’t mean that you have to abandon your get-healthy pursuits. (Though it’s worth saying that being slim doesn’t automatically equal being healthy.) What you have to know is that the answer is simply not to eat less. Do that and you’ll butt up against that starvation response — and you will lose.

“An easier and more sustainable method is to do things that make your brain more comfortable at a lower weight,” says Guyenet. Ultimately, you can lower your set point weight so that your body is happy carrying around less fat. He points out that animal and human research suggests that eating a diet of unrefined, lower calorie-density, and simple foods is key. That’s fresh, whole foods enjoyed as nature intended. So rather than ordering fried zucchini and calling it a veggie, you eat that zucchini grilled with a little salt. Instead of eating candied nuts, you eat plain nuts.

If it sounds boring and far less flashy than most diets, it might be! But this also involves skipping the calorie counting (so many of us are so bad at it any way) and letting those unconscious circuits in your brain take cues from your diet and exercise habits and naturally lower your weight for you.

Other important non-diet factors key to weight loss include regular physical activity, managing your stress, and getting the right amount of sleep, adds Guyenet. (That’s seven to eight hours for most adults. Sacrificing sleep in an effort to pack in more in your day will work against your fat burning ability.)

It’s also important to ask yourself if losing weight is a reasonable expectation, says Aamodt. “Sometimes it’s no. You may overestimate how much health danger your weight actually poses or you may not even need to lose any in the first place,” she says.

Aamodt advocates for a style of eating called intuitive eating. It's is a non-dieting approach that allows you to listen to your body and nourish it with what it wants (sometimes kale, sometimes a brownie) in the amounts it wants (more or less depending on your hunger). One limit: “it probably isn’t going to get you down to your fantasy weight,” she says, adding this silver lining: “We’d all be healthier if we focus our efforts on healthy behaviors rather than weight.”


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