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  • [ ♪INTRO ]

  • 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 fatwhich 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 bufferno matter how large or small it

  • isso 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 appetitemaking 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 efficientand 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 starvedwhich 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 lastor 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.

  • Thanks for watching this episode of SciShow!

  • And thanks especially to our patrons on Patreon.

  • Your continued support is what allows us to make educational videos like this one.

  • If you like what we do and want us to help us keep doing it, you can learn more about

  • joining our patron community at Patreon.com/SciShow

  • [ ♪ OUTRO ]

[ ♪INTRO ]

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The Real Reason It's So Hard to Lose Weight

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    joey joey に公開 2021 年 05 月 05 日
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