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[ ♪INTRO ]
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Losing weight is hard.
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Like, really, really, really hard.
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The overwhelming majority of people who try to do it don't succeed or end up gaining
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back what they lose, sometimes more.
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And that's not just because pizza is amazing.
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It turns out your body actually pushes back when you attempt to slim down.
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The fat stored in your adipose tissue is a super energy-rich substance that your body
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can use in a pinch to fuel your cells.
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If you can't eat for whatever reason, or need a little extra energy to grow or reproduce,
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your body can turn to your fat — which is why, from a survival perspective, having some
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fat is actually a good thing!
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Still, you'd think that losing weight would be pretty straightforward: just eat less than
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you need, force your body use up some of its fat, then go back to eating a normal amount
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when you're the size you want to be.
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But the body doesn't want to lose its energy buffer — no matter how large or small it
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is — so when you cut calories, it reacts in ways that ultimately make it harder to
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lose weight.
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A lot of the push back is driven by changes to hormones.
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One of the most important is leptin, a hormone secreted by your fat cells.
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The larger your fat cells are, the more leptin they produce.
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So when you lose weight, leptin levels drop.
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Parts of your brain like your hypothalamus interpret less leptin as starvation, and it
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jumps in and starts telling your body to conserve energy and to eat more to rebuild those reserves.
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Other organs also use hormones to complain to your brain about the decrease in fuel intake.
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Your stomach tells your brain it's not getting filled by increasing levels of the hormone
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ghrelin.
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At the same time, your pancreas secretes less insulin, which regulates blood sugar, and
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amylin, which signals fullness.
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So when you cut calories, ghrelin levels rise and insulin and amylin levels plummet, signalling
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your brain to increase appetite — making you feel ravenous.
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In addition to changing how hungry you feel, a suite of studies have suggested your brain
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responds to these hormonal changes by making you more aware of all the food you're not
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eating, and upping the pleasure you feel if you do cave in.
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Meanwhile, the rest of your body becomes more energy-efficient.
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For example, your muscles change where they get their fuel.
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When your muscles need energy, they generally use a mix of stored fat and circulating glucose.
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But when you're on a calorie-restricted diet, they rely more heavily on glucose, so
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they end up pulling more energy from the foods you eat instead of those fat stores you're
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trying to lose.
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They also make other small changes to become more efficient — and so do other tissues
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in your body.
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Here's the really annoying thing: this hormonal starvation signal doesn't stop when you
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stop dieting.
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That makes sense for leptin, since it's based on the amount of fat you have.
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But other hormones which generally respond to food intake can stay on that slower production
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cycle even when you return to normal eating.
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And these hormones can stay altered for years.
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So even when you've stopped restricting calories, your body continues to act like
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it's being starved — which is a big part of why people who lose weight often gain it
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back.
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To make matters worse, even regaining the weight doesn't shift your body out of energy-efficient
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mode.
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In general, the smaller you are, the less energy you need to fuel everything.
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But it's not a simple, linear relationship.
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How much energy you use per kilo at any given body weight varies depending on whether you've
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ever been heavier or skinnier.
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And this effect could be clearly seen in a 2016 study which followed contestants from
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a televised weight loss competition for six years.
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In particular, the researchers looked at the participants' resting metabolic rates: the
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calories their bodies burned at rest.
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It's basically a measure of the minimum amount of energy needed to keep a person's
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cells running.
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After the 30 week contest, the 14 participants lost an average of about 58 kilograms, and
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their resting metabolic rates dropped by about 610 calories per day.
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In the years that followed, though, they gained back an average of 41 kilos, and their metabolic
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rates didn't go back up accordingly.
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They ended up burning 500 calories a day less than they should have at their final weights.
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Which means to lose weight in the future, they'd have to restrict themselves even
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more than they did the first time around.
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Lots of other studies have come to similar conclusions.
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After people lose weight, even if they gain it back, their bodies simply use fewer calories
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per kilogram than similarly sized people whose weight hasn't changed.
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And that means they have to eat less to stay at that weight than people who were never
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heavier, and they gain weight faster if they do overeat.
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It's not yet clear just how long all these anti-weight-loss changes last — or if they
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ever completely go away.
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But not everyone experiences the same degree of resistance from their bodies.
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Scientists are still trying to figure out how our person's genetics, the foods they
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eat, and other factors affect how a person responds to dieting.
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But given how fiercely the body can fight slimming down, it's no wonder so many people
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struggle with it.
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Thanks for watching this episode of SciShow!
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And thanks especially to our patrons on Patreon.
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Your continued support is what allows us to make educational videos like this one.
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If you like what we do and want us to help us keep doing it, you can learn more about
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joining our patron community at Patreon.com/SciShow
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