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The Real Reason It's So Hard to Lose Weight 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.绝大多数尝试减肥的人都以失败告终或是体重反弹，有时甚至更胖了。这可不仅仅是因为披萨太好吃了。事实证明，当你尝试减肥时，你的身体其实会拖后腿。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!储存在脂肪组织里的脂肪是一种富含能量的物质，使身体可以在必要时为细胞提供能量。如果你出于什么原因不能吃东西，或者需要一点点额外的能量来生长或繁殖，你的身体就会分解脂肪，那就是为什么从生存的角度来说，有些脂肪其实是好事！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.但是，你可能会认为减肥很简单，只需要吃得比身体需要的少，强迫身体消耗一些脂肪，达到理想的体型后，再回到正常的饮食量。但是身体可不想失去能量储备，无论多少，所以，当你减少卡路里摄入时，身体会做出反应，反而使你更难减肥。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.重新长胖很多时候是由激素导致的。其中最重要的是瘦素，一 种由脂肪细胞分泌的激素。脂肪细胞越大，产生的瘦素就越多。所以，减肥的时候，瘦素水平会下降。大脑中的某些部分，比如下丘脑，会把瘦素减少解读为饥饿，然后它会介入，告诉你的身体，要节约能量，多吃点东西，来重建能量储备。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.其他器官也会向大脑抱怨能量摄入的减少。胃通过提高饥饿素的指标告知大脑，它尚未充满。同时，胰腺减少分泌可调节血糖的胰岛素和可提示饱腹的胰淀素。所以，减少卡路里摄入时，胃释放的饥饿素水平上升，而胰岛素和胰淀素水平迅速下降，这会让大脑增加食欲，让你感觉饿了。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.除了改变饥饿感之外，一系列研究表明，大脑对于这些激素变化做出的反应是，让你更清楚地感到自己没有吃到食物，如果你就此放弃减肥，反而会更加愉悦。同时，你身体的其他部分会变得更加节能。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.例如，肌肉会改变获得能量的方式。肌肉需要能量时，通常会同时消耗储存的脂肪和循环的葡萄糖。但节食时，它们更依赖于葡萄糖，所以最终会从摄入食物中获取更多能量，而不是从那些你尝试消耗的脂肪中。它们还会做出其他节能调整——其他身体组织也是如此。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.真正令人恼火的是，当你停止节食，激素释放的饥饿信号不会停止。那使瘦素水平很有意义，因为它是基于你所具有的脂肪量而产生的。但其他激素受摄入食物影响，会保持在一个较低的分泌状态， 即使你已恢复正常饮食。这些激素可能多年保持较低的分泌状态。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.因此，即使你不再限制卡路里摄入，你的身体仍会表现得如同它依然饥饿，这是减肥的人经常反弹的一个重要原因。更糟糕的是，即使重新胖回来，也不会让你的身体摆脱节能模式。In general, the smaller you are, the less energy you need to fuel everything. But it’s not a simple, linear relationship. 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.一般来说，体重越小，需要提供的能量越少。每消耗1公斤体重所需的能量会根据曾经更瘦还是更胖的身体状况而变化。这个结果在2016年的一项研究中清晰可见，该研究持续6年跟踪记录了一项电视减肥比赛的选手。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.研究人员尤其关注选手的静息代谢率，也就是他们在休息时身体会消耗多少卡路里。这基本上是人体维持细胞正常运转的最低能量代谢量。经过30周的比拼，这14位选手平均减掉了约58千克，他们的静息代谢率每天下降610卡路里。然而，之后几年里， 他们体重平均回增41公斤，不过静息代谢率却未能相应回调。最终，他们每天消耗的热量比最终体重时应消耗的热量少500卡路里。这意味着，如果未来想减肥，他们必须要比第一次减肥时更严格约束自己。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.其他很多研究也得出了类似结论。人们减肥后，即使又胖了回来，他们每公斤体重消耗的卡路里比体重没有变化的相似身材的人少。这意味着相比从没胖过的人，他们必须吃更少来保持现在的体重，如果吃多了，他们的体重会更容易增加。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.现在还不清楚，这些抵抗减重的能量减耗会持续多久，或者这个变化是否会完全消失。但是，不同的人，身体会遇到不同程度的阻力。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.科学家仍在尝试弄清人体基因、摄入食物及其他因素如何影响人们对节食的反应。但是，考虑到身体如此强烈地阻止减肥，难怪这么多人要和减肥苦苦斗争。
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