Losing weight is hard.減肥難。Like, really, really, really hard.真的、真的、真的難。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.絕大多數(shù)嘗試減肥的人都以失敗告終或是體重反彈，有時甚至更胖了。這可不僅僅是因為披薩太好吃了。事實證明，當你嘗試減肥時，你的身體其實會拖后腿。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!儲存在脂肪組織里的脂肪是一種富含能量的物質(zhì)，使身體可以在必要時為細胞提供能量。如果你出于什么原因不能吃東西，或者需要一點點額外的能量來生長或繁殖，你的身體就會分解脂肪，那就是為什么從生存的角度來說，有些脂肪其實是好事！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，你可能會認為減肥很簡單，只需要吃得比身體需要的少，強迫身體消耗一些脂肪，達到理想的體型后，再回到正常的飲食量。但是身體可不想失去能量儲備，無論多少，所以，當你減少卡路里攝入時，身體會做出反應(yīng)，反而使你更難減肥。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.重新長胖很多時候是由激素導致的。其中最重要的是瘦素，一 種由脂肪細胞分泌的激素。脂肪細胞越大，產(chǎn)生的瘦素就越多。所以，減肥的時候，瘦素水平會下降。大腦中的某些部分，比如下丘腦，會把瘦素減少解讀為饑餓，然后它會介入，告訴你的身體，要節(jié)約能量，多吃點東西，來重建能量儲備。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.其他器官也會向大腦抱怨能量攝入的減少。胃通過提高饑餓素的指標告知大腦，它尚未充滿。同時，胰腺減少分泌可調(diào)節(jié)血糖的胰島素和可提示飽腹的胰淀素。所以，減少卡路里攝入時，胃釋放的饑餓素水平上升，而胰島素和胰淀素水平迅速下降，這會讓大腦增加食欲，讓你感覺餓了。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.除了改變饑餓感之外，一系列研究表明，大腦對于這些激素變化做出的反應(yīng)是，讓你更清楚地感到自己沒有吃到食物，如果你就此放棄減肥，反而會更加愉悅。同時，你身體的其他部分會變得更加節(jié)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，肌肉會改變獲得能量的方式。肌肉需要能量時，通常會同時消耗儲存的脂肪和循環(huán)的葡萄糖。但節(jié)食時，它們更依賴于葡萄糖，所以最終會從攝入食物中獲取更多能量，而不是從那些你嘗試消耗的脂肪中。它們還會做出其他節(jié)能調(diào)整——其他身體組織也是如此。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.真正令人惱火的是，當你停止節(jié)食，激素釋放的饑餓信號不會停止。那使瘦素水平很有意義，因為它是基于你所具有的脂肪量而產(chǎn)生的。但其他激素受攝入食物影響，會保持在一個較低的分泌狀態(tài)， 即使你已恢復正常飲食。這些激素可能多年保持較低的分泌狀態(tài)。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攝入，你的身體仍會表現(xiàn)得如同它依然饑餓，這是減肥的人經(jīng)常反彈的一個重要原因。更糟糕的是，即使重新胖回來，也不會讓你的身體擺脫節(jié)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.一般來說，體重越小，需要提供的能量越少。每消耗1公斤體重所需的能量會根據(jù)曾經(jīng)更瘦還是更胖的身體狀況而變化。這個結(jié)果在2016年的一項研究中清晰可見，該研究持續(xù)6年跟蹤記錄了一項電視減肥比賽的選手。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.研究人員尤其關(guān)注選手的靜息代謝率，也就是他們在休息時身體會消耗多少卡路里。這基本上是人體維持細胞正常運轉(zhuǎn)的最低能量代謝量。經(jīng)過30周的比拼，這14位選手平均減掉了約58千克，他們的靜息代謝率每天下降610卡路里。然而，之后幾年里， 他們體重平均回增41公斤，不過靜息代謝率卻未能相應(yīng)回調(diào)。最終，他們每天消耗的熱量比最終體重時應(yīng)消耗的熱量少500卡路里。這意味著，如果未來想減肥，他們必須要比第一次減肥時更嚴格約束自己。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類似結(jié)論。人們減肥后，即使又胖了回來，他們每公斤體重消耗的卡路里比體重沒有變化的相似身材的人少。這意味著相比從沒胖過的人，他們必須吃更少來保持現(xiàn)在的體重，如果吃多了，他們的體重會更容易增加。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.現(xiàn)在還不清楚，這些抵抗減重的能量減耗會持續(xù)多久，或者這個變化是否會完全消失。但是，不同的人，身體會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.科學家仍在嘗試弄清人體基因、攝入食物及其他因素如何影響人們對節(jié)食的反應(yīng)。但是，考慮到身體如此強烈地阻止減肥，難怪這么多人要和減肥苦苦斗爭。
