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Losing weight is hard. Like, really, really, really hard.

The overwhelming majority of people who try to do it don't succeed or end up gaining back what they lose, sometimes more. And that's not just because pizza is amazing. It turns out your body actually pushes back when you attempt to slim down.

The fat stored in your adipose tissue is a super energy-rich substance that your body can use in a pinch to fuel your cells. If you can't eat for whatever reason, or need a little extra energy to grow or reproduce, your body can turn to your fat — which is why, from a survival perspective, having some fat is actually a good thing! Still, you'd think that losing weight would be pretty straightforward: just eat less than you need, force your body use up some of its fat, then go back to eating a normal amount when you're the size you want to be.

But the body doesn't want to lose its energy buffer — no matter how large or small it is — so when you cut calories, it reacts in ways that ultimately make it harder to lose weight. A lot of the push back is driven by changes to hormones. One of the most important is leptin, a hormone secreted by your fat cells.

The larger your fat cells are, the more leptin they produce. So when you lose weight, leptin levels drop. Parts of your brain like your hypothalamus interpret less leptin as starvation, and it jumps in and starts telling your body to conserve energy and to eat more to rebuild those reserves.

Other organs also use hormones to complain to your brain about the decrease in fuel intake. Your stomach tells your brain it's not getting filled by increasing levels of the hormone ghrelin. At the same time, your pancreas secretes less insulin, which regulates blood sugar, and amylin, which signals fullness.

So when you cut calories, ghrelin levels rise and insulin and amylin levels plummet, signalling your brain to increase appetite — making you feel ravenous. In addition to changing how hungry you feel, a suite of studies have suggested your brain responds to these hormonal changes by making you more aware of all the food you're not eating, and upping the pleasure you feel if you do cave in. Meanwhile, the rest of your body becomes more energy-efficient.

For example, your muscles change where they get their fuel. When your muscles need energy, they generally use a mix of stored fat and circulating glucose. But when you're on a calorie-restricted diet, they rely more heavily on glucose, so they end up pulling more energy from the foods you eat instead of those fat stores you're trying to lose.

They also make other small changes to become more efficient — and so do other tissues in your body. Here's the really annoying thing: this hormonal starvation signal doesn't stop when you stop dieting. That makes sense for leptin, since it's based on the amount of fat you have.

But other hormones which generally respond to food intake can stay on that slower production cycle even when you return to normal eating. And these hormones can stay altered for years. So even when you've stopped restricting calories, your body continues to act like it's being starved — which is a big part of why people who lose weight often gain it back.

To make matters worse, even regaining the weight doesn't shift your body out of energy-efficient mode. In general, the smaller you are, the less energy you need to fuel everything. But it's not a simple, linear relationship.

How much energy you use per kilo at any given body weight varies depending on whether you've ever been heavier or skinnier. And this effect could be clearly seen in a 2016 study which followed contestants from a televised weight loss competition for six years. In particular, the researchers looked at the participants' resting metabolic rates: the calories their bodies burned at rest.

It's basically a measure of the minimum amount of energy needed to keep a person's cells running. After the 30 week contest, the 14 participants lost an average of about 58 kilograms, and their resting metabolic rates dropped by about 610 calories per day. In the years that followed, though, they gained back an average of 41 kilos, and their metabolic rates didn't go back up accordingly.

They ended up burning 500 calories a day less than they should have at their final weights. Which means to lose weight in the future, they'd have to restrict themselves even more than they did the first time around. Lots of other studies have come to similar conclusions.

After people lose weight, even if they gain it back, their bodies simply use fewer calories per kilogram than similarly sized people whose weight hasn't changed. And that means they have to eat less to stay at that weight than people who were never heavier, and they gain weight faster if they do overeat. It's not yet clear just how long all these anti-weight-loss changes last — or if they ever completely go away.

But not everyone experiences the same degree of resistance from their bodies. Scientists are still trying to figure out how our person's genetics, the foods they eat, and other factors affect how a person responds to dieting. But given how fiercely the body can fight slimming down, it's no wonder so many people struggle with it.

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