Losing weight is 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!
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Losing weight is not as straight forward
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. The body however 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.
It’s all about those stubborn hormones
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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, signaling your brain to increase appetite — making you feel ravenous.
Your brain is helpless against hormones
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.
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