TED英语演讲稿：为什么节食减肥没效果

简介：在美国，80%的女孩在她们10岁的时候便开始节食。神经学家Sandra

Aamodt结合自己的亲身经历，讲述大脑是如何控制我们的身体的。节食减肥为何没效果?来听听她的说法吧!

Three and a half years ago, I made one of the best decisions of my life. As

my New Year's resolution, I gave up dieting, stopped worrying about my weight,

and learned to eat mindfully. Now I eat whenever I'm hungry, and I've lost 10

pounds.

This was me at age 13, when I started my first diet. I look at that picture

now, and I think, you did not need a diet, you needed a fashion consult.

(Laughter) But I thought I needed to lose weight, and when I gained it back, of

course I blamed myself. And for the next three decades, I was on and off various

diets. No matter what I tried, the weight I'd lost always came back. I'm sure

many of you know the feeling.

As a neuroscientist, I wondered, why is this so hard? Obviously, how much

you weigh depends on how much you eat and how much energy you burn. What most

people don't realize is that hunger and energy use are controlled by the brain,

mostly without your awareness. Your brain does a lot of its work behind the

scenes, and that is a good thing, because your conscious mind -- how do we put

this politely? -- it's easily distracted. It's good that you don't have to

remember to breathe when you get caught up in a movie. You don't forget how to

walk because you're thinking about what to have for dinner.

Your brain also has its own sense of what you should weigh, no matter what

you consciously believe. This is called your set point, but that's a misleading

term, because it's actually a range of about 10 or 15 pounds. You can use

lifestyle choices to move your weight up and down within that range, but it's

much, much harder to stay outside of it. The hypothalamus, the part of the brain

that regulates body weight, there are more than a dozen chemical signals in the

brain that tell your body to gain weight, more than another dozen that tell your

body to lose it, and the system works like a thermostat, responding to signals

from the body by adjusting hunger, activity and metabolism, to keep your weight

stable as conditions change. That's what a thermostat does, right? It keeps the

temperature in your house the same as the weather changes outside. Now you can

try to change the temperature in your house by opening a window in the winter,

but that's not going to change the setting on the thermostat, which will respond

by kicking on the furnace to warm the place back up.

Your brain works exactly the same way, responding to weight loss by using

powerful tools to push your body back to what it considers normal. If you lose a

lot of weight, your brain reacts as if you were starving, and whether you

started out fat or thin, your brain's response is exactly the same. We would

love to think that your brain could tell whether you need to lose weight or not,

but it can't. If you do lose a lot of weight, you become hungry, and your

muscles burn less energy. Dr. Rudy Leibel of Columbia University has found that

people who have lost 10 percent of their body weight burn 250 to 400 calories

less because their metabolism is suppressed. That's a lot of food. This means

that a successful dieter must eat this much less forever than someone of the

same weight who has always been thin.

From an evolutionary perspective, your body's resistance to weight loss

makes sense. When food was scarce, our ancestors' survival depended on

conserving energy, and regaining the weight when food was available would have

protected them against the next shortage. Over the course of human history,

starvation has been a much bigger problem than overeating. This may explain a

very sad fact: Set points can go up, but they rarely go down. Now, if your

mother ever mentioned that life is not fair, this is the kind of thing she was

talking about. (Laughter) Successful dieting doesn't lower your set point. Even

after you've kept the weight off for as long as seven years, your brain keeps

trying to make you gain it back. If that weight loss had been due to a long

famine, that would be a sensible response. In our modern world of drive-thru

burgers, it's not working out so well for many of us. That difference between

our ancestral past and our abundant present is the reason that Dr. Yoni

Freedhoff of the University of Ottawa would like to take some of his patients

back to a time when food was less available, and it's also the reason that

changing the food environment is really going to be the most effective solution

to obesity.

Sadly, a temporary weight gain can become permanent. If you stay at a high

weight for too long, probably a matter of years for most of us, your brain may

decide that that's the new normal.

Psychologists classify eaters into two groups, those who rely on their

hunger and those who try to control their eating through willpower, like most

dieters. Let's call them intuitive eaters and controlled eaters. The interesting

thing is that intuitive eaters are less likely to be overweight, and they spend

less time thinking about food. Controlled eaters are more vulnerable to

overeating in response to advertising, super-sizing, and the all-you-can-eat

buffet. And a small indulgence, like eating one scoop of ice cream, is more

likely to lead to a food binge in controlled eaters. Children are especially

vulnerable to this cycle of dieting and then binging.