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Losing weight is hard. Like, really, really, really hard.
减肥很难。真的真的真的很难。
The overwhelming majority of people who try to do it don't succeed or end up gaining back what they lose, sometimes more.
大部分想减肥的人要么是没成功，要么最后反弹甚至更胖。
And that's not just because pizza is amazing. It turns out your body actually pushes back when you attempt to slim down.
这不仅仅是因为披萨诱人。事实证明，当你想要减肥时，你的身体是会反抗的。
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.
储存在脂肪组织中的脂肪是超级高能量的物质，紧要关头你的身体可以利用它给细胞补充能量。
If you can't eat for whatever reason, or need a little extra energy to grow or reproduce,
如果你不进食或是需要一点额外能量帮助生长和再生，
your body can turn to your fat which is why, from a survival perspective, having some fat is actually a good thing!
那么你的身体就会求助于你的脂肪，因此，从生存角度来看，有些脂肪其实是一件好事！
Still, you'd think that losing weight would be pretty straightforward: just eat less than you need,
如果你还认为减肥相当简单：只要少吃一点
and 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.
强迫身体消耗一些脂肪，然后达到满意体重后就恢复正常饮食量就行了。
But the body doesn't want to lose its energy buffer no matter how large or small it is
但不论多少，身体其实不想失去它的能量缓冲器
so when you cut calories, it reacts in ways that ultimately make it harder to lose weight.
所以当你减少热量时，身体的反应最终会让减肥变得更加困难。
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.
很多反抗是由激素的变化所引起的。其中最重要一种是瘦素，脂肪细胞分泌的一种激素。
The larger your fat cells are, the more leptin they produce. So when you lose weight, leptin levels drop.
脂肪细胞越大，它们产生的瘦素就越多。所以当你减肥时，瘦素水平下降。
Parts of your brain like your hypothalamus interpret less leptin as starvation,
大脑的某些区域，比如下丘脑会将低水平的瘦素解读为饥饿，
and it jumps in and starts telling your body to conserve energy and to eat more to rebuild those reserves.
它会跳起来告诉你的身体要节能并补充更多食物重新储备能量。
Other organs also use hormones to complain to your brain about the decrease in fuel intake.
其他器官也利用激素向你的大脑抱怨能量吸收的下降。
Your stomach tells your brain it's not getting filled by increasing levels of the hormone ghrelin.
你的胃会通过增加胃饥饿素水平的方式告诉大脑能量不够了。
At the same time, your pancreas secretes less insulin, which regulates blood sugar, and amylin, which signals fullness.
同时，你的胰腺会分泌更少的胰岛素，胰岛素可以调节血糖以及提示饱足感的胰淀素。
So when you cut calories, ghrelin levels rise and insulin and amylin levels plummet,
所以当你减少热量时，胃饥饿素水平上升，胰岛素和胰淀素水平下降，
signalling your brain to increase appetite making you feel ravenous.
以提示你的大脑增进食欲让你狼吞虎咽。
In addition to changing how hungry you feel, a suite of studies have suggested your brain responds to these hormonal changes
除了改变饥饿感外，一系列研究表明大脑会对这些激素变化产生反应
by making you more aware of all the food you're not eating, and upping the pleasure you feel if you do cave in.
让你更加留意所有你不吃的食物并且如果屈服，你的快乐感会上升。
Meanwhile, the rest of your body becomes more energy-efficient. For example, your muscles change where they get their fuel.
同时，身体其他部位变得更节能。例如，你的肌肉会改变它们吸收能量的地方。
When your muscles need energy, they generally use a mix of stored fat and circulating glucose.
当肌肉需要能量时，它们通常会使用储存脂肪和循环葡萄糖的混合。
But when you're on a calorie-restricted diet, they rely more heavily on glucose,
但当你节食时，它们会更多的依靠葡萄糖
so they end up pulling more energy from the foods you eat instead of those fat stores you're trying to lose.
所以它们最终会从你所吃的食物而非你试图减掉的脂肪储存中获取更多能量。
They also make other small changes to become more efficient and so do other tissues in your body.
它们还会做出其他小改变以变得更高效，而你身体中的其他组织也一样。
Here's the really annoying thing: this hormonal starvation signal doesn't stop when you stop dieting.
真正讨厌的事在这里：这种激素饥饿信号在你停止节食后也不会停止。
That makes sense for leptin, since it's based on the amount of fat you have.
对瘦素而言，这是有道理的，因为它是以你的脂肪为基础。
But other hormones which generally respond to food intake can stay on that slower production cycle even when you return to normal eating.
但是其他对食物吸收产生反应的激素可以继续维持那种缓慢的生产周期，即便你恢复正常饮食。
And these hormones can stay altered for years.
这些激素的改变可以维持好几年。
So even when you've stopped restricting calories, your body continues to act like it's being starved
所以当你停止限制热量时，你的身体会继续像饥饿时一样运作
which is a big part of why people who lose weight often gain it back.
这就是减肥会反弹的主要原因。
To make matters worse, even regaining the weight doesn't shift your body out of energy-efficient mode.
更糟糕的是，即便肉又长了回来也无法改变身体的节能模式。
In general, the smaller you are, the less energy you need to fuel everything. But it's not a simple, linear relationship.
总之，越瘦所需补充的能量就越少。但是这并非是一种简单的线性关系。
How much energy you use per kilo at any given body weight varies depending on whether you've ever been heavier or skinnier.
在某给定体重下，每公斤所消耗的能量多少取决于你曾经是更胖还是更瘦。
And this effect could be clearly seen in a 2016 study which followed contestants from a televised weight loss competition for six years.
可以从2016年的一项研究清晰的看到这种作用，该研究花了6年追踪一档电视减肥比赛的参赛者。
In particular, the researchers looked at the participants' resting metabolic rates: the calories their bodies burned at rest.
那些研究人员特别观察了参与者的静止代谢率：他们在休息时身体消耗的热量。
It's basically a measure of the minimum amount of energy needed to keep a person's cells running.
就是测量维持个人细胞运转所需的最少能量。
After the 30 week contest, the 14 participants lost an average of about 58 kilograms,
经过30周比赛后，14名选手平均减重约58公斤，
and their resting metabolic rates dropped by about 610 calories per day.
他们的静止代谢率每天下降约610卡路里。
In the years that followed, though, they gained back an average of 41 kilos, and their metabolic rates didn't go back up accordingly.
随后几年，他们体重平均反弹41公斤，他们的静止代谢率并没有因为回升，
They ended up burning 500 calories a day less than they should have at their final weights.
止步每天消耗500卡路里，低于他们最终体重应该的消耗量。
Which means to lose weight in the future, they'd have to restrict themselves even more than they did the first time around.
这意味着以后要减肥的话，他们需要比第一次更加克制。
Lots of other studies have come to similar conclusions. After people lose weight, even if they gain it back,
很多其他研究也得出了类似的结论。减肥后，即便体重反弹
their bodies simply use fewer calories per kilogram than similarly sized people whose weight hasn't changed.
他们的身体还是比那些和他们体型相似但体重没变的人消耗更少的热量。
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.
这意味着他们需要吃得更少才能维持体重，并且如果吃太多，他们体重增加更快。
It's not yet clear just how long all these anti-weight-loss changes last or if they ever completely go away.
目前尚不清楚这些抗减肥变化会维持多久或者说永远就这样了。
But not everyone experiences the same degree of resistance from their bodies.
但是并不是每个人都会经历同一程度的身体抵抗。
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.
科学家们仍在尝试弄清楚人类的遗传、他们所吃的食物以及其他因素如何影响人们对节食的反应。
But given how fiercely the body can fight slimming down, it's no wonder so many people struggle with it.
但是鉴于身体对抗减肥的激烈程度，难怪有很多人难以减肥。
Thanks for watching this episode of SciShow! And thanks especially to our patrons on Patreon.
感谢收看本期《科学秀》！特别感谢Patreon赞助人。
Your continued support is what allows us to make educational videos like this one.
大家一直以来的支持让我们能够制作像这样有教育意义的视频。
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
如果你喜欢我们的视频并想帮助我们，你可以登陆Patreon.com/SciShow了解更多。
