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新的肥胖理论

A New Theory of Obesity
新的肥胖理论
3560字
2019-09-26 21:57
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新的肥胖理论

Nutrition researcher Kevin Hall strives to project a Zen-like state of equanimity. In his often contentious field, he says he is more bemused than frustrated by the tendency of other scientists to “cling to pet theories despite overwhelming evidence that they are mistaken.” Some of these experts, he tells me with a sly smile, “have a fascinating ability to rationalize away studies that don’t support their views.”

营养研究员凯文·霍尔努力呈现出一种禅宗般的平静心态。身处常常引起争议的领域,他说他对其他科学家“尽管有压倒性的证据表明他们是错误的,但仍然坚持自己喜欢的理论”的这种倾向感到困惑,而非沮丧。他狡猾地笑着对我说,其中一些专家“有一种迷人的能力,能把不支持他们观点的研究合理化。”

Among those views is the idea that particular nutrients such as fats, carbs or sugars are to blame for our alarming obesity pandemic. (Globally the prevalence of obesity nearly tripled between 1975 and 2016, according to the World Health Organization. The rise accompanies related health threats that include heart disease and diabetes.) But Hall, who works at the National Institute of Diabetes and Digestive and Kidney Diseases, where he runs the Integrative Physiology section, has run experiments that point fingers at a different culprit. His studies suggest that a dramatic shift in how we make the food we eat—pulling ingredients apart and then reconstituting them into things like frosted snack cakes and ready-to-eat meals from the supermarket freezer—bears the brunt of the blame. This “ultraprocessed” food, he and a growing number of other scientists think, disrupts gut-brain signals that normally tell us that we have had enough, and this failed signaling leads to overeating.

在这些观点中,有一种观点认为,脂肪、碳水化合物或糖等特定的营养素是造成现今普遍且令人担忧的肥胖现象的罪魁祸首。(根据世界卫生组织的数据,从1975年到2016年,全球肥胖患病率几乎增加了两倍。与此同时,包括心脏病和糖尿病在内的相关威胁健康的疾病也在增多。霍尔是美国国家糖尿病、消化和肾脏疾病研究所下属的综合生理学部门的负责人。他的研究表明,我们制作食物的方法发生了巨大的变化---把食材拆开,然后重新组合成像糖霜点心蛋糕和超市冰箱里的即食食品那样的东西,这种转变是造成肥胖现象泛滥的首要原因。他和越来越多的科学家认为,这种“超加工”食物会破坏内脏脑信号,而这些信号通常会告诉我们已经吃饱了,而信号传导失败会导致我们暴饮暴食。

Hall has done two small but rigorous studies that contradict common wisdom that faults carbohydrates or fats by themselves. In both experiments, he kept participants in a hospital for several weeks, scrupulously controlling what they ate. His idea was to avoid the biases of typical diet studies that rely on people’s self-reports, which rarely match what they truly eat. The investigator, who has a physics doctorate, has that discipline’s penchant for precise measurements. His first study found that, contrary to many predictions, a diet that reduced carb consumption actually seemed to slow the rate of body fat loss. The second study, published this year, identified a new reason for weight gain. It found that people ate hundreds more calories of ultraprocessed than unprocessed foods when they were encouraged to eat as much or as little of each type as they desired. Participants chowing down on the ultraprocessed foods gained two pounds in just two weeks.

霍尔做了两项严谨的小研究,其结果与碳水化合物或脂肪本身就是错误的普遍观点相矛盾。在这两个实验中,他让参与者在医院里呆了几周,严格控制他们的饮食。他的想法是避免典型的饮食研究的偏见,这些研究依赖于人们的自我报告,很少与他们真正吃的食物相匹配。这位拥有物理学博士学位的研究人员对学科研究的精确测量十分讲究。他的第一项研究发现,与许多预测相反,减少饮食中碳水化合物摄入实际上似乎减缓了身体脂肪的消耗率。今年发表的第二项研究发现了体重增加的新原因。研究发现,当人们被鼓励按照自己的意愿吃多少每种食物时,他们摄入的超加工食品产生的卡路里比未加工食品多出数百卡。参与者在短短两周内大量食用了超加工食品,体重增加了两磅。

“Hall’s study is seminal—really as good a clinical trial as you can get,” says Barry M. Popkin, a professor of nutrition at the University of North Carolina at Chapel Hill, who focuses on diet and obesity. “His was the first to prove that ultraprocessed foods are not only highly seductive but that people tend to eat more of them.” The work has been well received, although it is possible that the carefully controlled experiment does not apply to the messy way people mix food types in the real world.

位于教堂山上的北卡罗莱纳大学的营养学教授巴里·波普金专门研究饮食和肥胖问题,他说:“霍尔的研究是半正式的,是一项非常好的临床试验。”他首次证明了超加工食品不仅本身非常诱人,而且人们往往会吃得更多。这项研究得到了广泛的认可,尽管这项精心控制的实验可能并不适用于现实世界中人们把食物混合起来的做法。

The man who designed the research says he is not on a messianic mission to improve America’s eating habits. Hall admits that his four-year-old son’s penchant for chicken nuggets and pizza remains unshakable and that his own diet could and probably should be improved. Still, he believes his study offers potent evidence that it is not any particular nutrient type but the way in which food is manipulated by manufacturers that plays the largest role in the world’s growing girth. He insists he has no dog in any diet wars fight but is simply following the evidence. “Once you’ve stepped into one camp and surrounded yourself by the selective biases of that camp, it becomes difficult to step out,” he says. Because his laboratory and research are paid for by the national institute whatever he finds, Hall notes that “I have the freedom to change my mind. Basically, I have the privilege to be persuaded by data.”

设计这项研究的人说,他并不是在执行救世主改善美国饮食习惯的使命。霍尔承认,他四岁的儿子对鸡块和披萨的嗜好仍然不可动摇,他自己的饮食应该需要改善而且他可以做到这一点。尽管如此,他相信他的研究提供了有力的证据,证明在全球人类日益增长的腰围中起最大作用的并不是某种特定的营养类型,而是食品制造商操纵食品的方式。他坚称,他在任何减肥战中都没有养狗,只是在遵从数据证明。他说:“一旦你进入一个阵营,被这个阵营的选择性偏见所包围,你就很难走出这个阵营。”因为无论他的实验室和研究发现了什么都是由国家研究所资助的,霍尔指出:“我有改变主意的自由。基本上,我有被数据说服的特权。”

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The carb test

碳水化合物测验

Hall once had great sympathy for the theory that specific nutrients—in particular carbs—were at fault for our collective losing battle with body weight. “I knew that consumption of carbohydrates increases insulin levels in the blood and that insulin levels affect fat storage and fat cells,” he says. “So it was certainly plausible that consumption of carbohydrates versus other macronutrients could have a deleterious effect on body weight. But while plausible, it wasn’t certain, so I decided to test it.”

霍尔曾经非常赞同这样一种理论,即特定的营养物质——尤其是碳水化合物——是我们集体减肥失败的罪魁祸首。他说,“我知道碳水化合物的摄入会提升血液中的胰岛素水平,而胰岛素水平会影响脂肪储存和脂肪细胞。”因此,碳水化合物与其他大量营养素的摄入对体重产生有害影响,这固然是合理的。不过,虽然这听起来有道理,但并不能确定,所以我决定测试一下。”

In Hall’s carb study, 10 men and nine women, all obese, were sequestered in a hospital ward at the National Institutes of Health and fed a high-carbohydrate/low-fat diet for two weeks. Then they left for a short time and returned to repeat another two-week stint. For the first five days of each stay, the balance was kept at 50 percent carbohydrate, 35 percent fat and 15 percent protein, with calorie intakes matched to their energy expenditure—measured in a specially constructed metabolic chamber—to ensure they neither gained nor lost weight. Over the next six days of each stay, they ate a diet with 30 percent fewer calories from the carb category.

在霍尔的碳水化合物研究中,10名男性和9名女性都患有肥胖症,他们被隔离在美国国立卫生研究院(National Institutes of Health)的一所医院病房,连续两周吃高碳水化合物/低脂肪食物。然后他们离开了一小段时间,又回来继续进行为期两周的训练。每次在入住的前五天,他们的体重都保持在50%的碳水化合物、35%的脂肪和15%的蛋白质的平衡,卡路里的摄入量与他们的能量消耗相匹配(在一个特别建造的代谢室中测量得出),以确保他们既没有增重也没有减重。在接下来的六天里,他们的饮食中碳水化合物类的卡路里减少了30%。

“We were not surprised to find that when you manipulate the level of carbohydrates versus fats, you do see very different insulin levels,” Hall says. He had expected the low-carb diet would reduce insulin activity. “But what did surprise us was that we did not see a significant effect of the sharply lower insulin levels on the rate of calories burned over time or on body fat.” Typically lowered insulin affects the way fat cells burn calories. Yet, Halls says, “we found that the reduced-carbohydrate diet slightly slowed body fat loss.” It also slightly increased the loss of lean body mass. A year later Hall and his colleagues did a similar experiment over a longer, eight-week period. This time they cut carbohydrates to very low levels. In the end, they found no meaningful difference in body fat loss or calorie expenditure between the very low-carb diet and a baseline high-carb/high-sugar diet. The scientists published the first results in 2015 in the journal Cell Metabolism and the second set in 2016 in the American Journal of Clinical Nutrition.

霍尔说:“我们发现,当你控制碳水化合物和脂肪的含量时,你会发现胰岛素的含量非常不同,这一结果并没有让我感到惊讶。”他原以为低碳水化合物饮食会降低胰岛素的活性。“但让我们惊讶的是,我们并没有看到胰岛素水平的急剧下降对长期燃烧的卡路里率或身体脂肪产生显着的影响。“胰岛素水平降低通常会影响脂肪细胞燃烧卡路里的方式。然而,霍尔斯说:“我们发现,低碳水化合物饮食略微减缓了身体脂肪的流失。“它还使低体脂身体体重的小幅度地减低了。一年后,霍尔和他的同事做了一个类似的实验,这个实验时间更长,历时8周。这次他们把碳水化合物的含量降到了很低的水平。最后,他们发现极低碳水化合物饮食和基线高碳水化合物/高糖饮食在身体脂肪减少或热量消耗方面没有显着差异。2015年,科学家们在《细胞代谢》(Cell Metabolism)杂志上发表了第一组结果,2016年在《美国临床营养学杂志》(American journal of Clinical Nutrition)上发表了第二组结果。

If it’s not carbohydrates, what is to blame for our global obesity problem? Sure, meal portions today are larger, food more abundant, and many of us are eating more calories than people did decades ago. But with temptations so plentiful, almost all Americans could be overeating—yet a good number do not. That, Hall thinks, is the real nutrition mystery: What factors, for some people, might be acting to override the body’s inborn satiety mechanisms that otherwise keep our eating in check?

如果不是碳水化合物,我们的全球肥胖问题该归咎于什么?当然,现代人的饭量更大,现在的食物更丰富了,我们中的许多人比几十年前摄入了更多的卡路里。由于诱惑如此之多,几乎所有的美国人都有可能吃得过多——然而很多人并没有出现这种情况。霍尔认为,这才是真正的营养之谜:对一些人来说,是什么因素在起作用,超越了身体天生的饱腹感机制,否则我们的饮食就会受到控制?

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Processed calories

加工过的卡路里

Hall likes to compare humans to automobiles, pointing out that both can operate on any number of energy sources. In the case of cars, it might be diesel, high-octane gasoline or electricity, depending on the make and model. Similarly, humans can and do thrive on any number of diets, depending on cultural norms and what is readily available. For example, a traditional high-fat/low-carb diet works well for the Inuit people of the Arctic, whereas a traditional low-fat/high-carb diet works well for the Japanese. But while humans have evolved to adapt to a wide variety of natural food environments, in recent decades the food supply has changed in ways to which our genes—and our brains—have had very little time to adapt. And it should come as no surprise that each of us reacts differently to that challenge.

霍尔喜欢把人类比作汽车,他指出人类和汽车都可以随意使用能源。就汽车而言,它可能是柴油,高辛烷值汽油或电力,这取决于制造和型号。同样地,人类可以并且确实在各种各样的饮食下茁壮成长,这取决于文化规范和约定俗成。例如,传统的高脂肪/低碳水化合物饮食适合北极的因纽特人,而传统的低脂肪/高碳水化合物饮食适合日本人。然而,尽管人类已经进化到能够适应各种各样的自然食物环境,但最近几十年来,食物供应的方式发生了变化,我们的基因和大脑几乎没有时间去适应。我们每个人对这一挑战的反应都不一样,这不足为奇。

At the end of the 19th century, most Americans lived in rural areas, and nearly half made their living on farms, where fresh or only lightly processed food was the norm. Today most Americans live in cities and buy rather than grow their food, increasingly in ready-to-eat form. An estimated 58 percent of the calories we consume and nearly 90 percent of all added sugars come from industrial food formulations made up mostly or entirely of ingredients—whether nutrients, fiber or chemical additives—that are not found in a similar form and combination in nature. These are the ultraprocessed foods, and they range from junk food such as chips, sugary breakfast cereals, candy, soda and mass-manufactured pastries to what might seem like benign or even healthful products such as commercial breads, processed meats, flavored yogurts and energy bars.

19世纪末,大多数美国人生活在农村地区,近一半的人在农场生活,在那里,新鲜食物或轻微加工的食品是常态。今天,大多数美国人住在城市里,直接购买食物而不是自己种植出来,越来越多的是即食食品。据估计,我们消耗的58%的热量和近90%的所有添加糖都来自工业食品配方,这些配方大部分或全部由营养素、纤维或化学添加剂组成,而这些成分在自然界中并没有以类似的形式和组合形式存在。这些都是超加工食品,从垃圾食品如薯片、含糖早餐麦片、糖果、苏打水和大量生产的糕点,到看似无害甚至健康的产品,如商业面包、加工肉类、调味酸奶和能量棒,也都属于超加工食品。

Ultraprocessed foods, which tend to be quite high in sugar, fat and salt, have contributed to an increase of more than 600 available calories per day for every American since 1970. Still, although the rise of these foods correlates with rising body weights, this correlation does not necessarily imply causation. There are plenty of delicious less processed foods—cheese, fatty meats, vegetable oil, cream—that could play an equal or even larger role. So Hall wanted to know whether it was something about ultraprocessing that led to weight gain. “Basically, we wondered whether people eat more calories when those calories come from ultraprocessed sources,” he says.

自1970年以来,高糖、高脂肪和高盐的超加工食品使每个美国人每天多摄入了600多卡路里的可用热量。然而,尽管这些食物的增加与体重的增加有关,但这种相关性并不一定意味着因果关系。还有许多美味的低加工食品——奶酪、肥肉、植物油、奶油——也可以发挥同样甚至更大的作用。所以霍尔想知道是否是超加工导致了体重增加。他说:“基本上,我们想知道的是,当这些热量来自超加工食品时,人们是否摄入了更多的热量。”

Tackling that question is not simple. The typical nutritional study, as noted earlier, relies on self-reports of individuals who keep food diaries or fill out questionnaires from memory. But Hall knew that in the case of ultraprocessed foods, that approach would fail to provide convincing evidence either way. For one thing, nutrition study participants are notorious for cheating on dietary surveys—claiming more broccoli and fewer Double Stuf Oreos than they actually eat or “forgetting” drinking that third beer with friends. For another, with such a large percentage of the American diet coming from ultraprocessed foods, it would be hard to find a group of people with a markedly different diet for comparison.

解决这个问题并不简单。如前所述,典型的营养研究依赖于那些写食物日记或凭记忆填写问卷的人的自我报告。但霍尔知道,对于超加工食品,无论哪种方法都无法提供令人信服的证据。首先,营养研究的参与者在饮食调查中作弊是出了名的——声称吃了更多的西兰花,吃了更少的奥利奥,或者“忘记”和朋友一起喝了第三杯啤酒。另一方面,美国人的饮食中有很大一部分来自超加工食品,很难找到一群饮食习惯明显不同的人进行比较。

To avoid these and related problems, in 2018 Hall turned once again to the metabolic ward, where he randomly assigned 20 ad/ult volunteers to receive either ultraprocessed or unprocessed diets for two weeks. Then people switched: if they had been on one diet, they went on the alternative one for two more weeks. (Clearly, 20 is not a large enough sample size from which to draw conclusions that apply to the public as a whole, but this pilot study was meant as a “proof of concept” on which to build future, larger studies. Subjecting more people to the strict study regimen at this preliminary stage, Hall says, “would be unethical.”) Dietitians scrupulously matched the ultraprocessed and processed meals for calories, energy density, fat, carbohydrate, protein, sugars, sodium and fiber. They also made sure that the research subjects had no taste preference for one category of food over the other. On both diets, participants were instructed to eat as much or as little of the meals and snacks as they liked.

为了避免这些及其他相关的问题,霍尔在2018年再次将研究方向转向代谢病房,他随机指定了20名志愿者,接受为期两周的超加工或未经加工的饮食。然后人们改变了饮食习惯:如果他们只吃一种食物,他们会在另外两周内继续吃另一种食物。(显然,20个样本不足以得出适用于整个公众的结论,但这项试点研究旨在作为一个“概念证明”并在此基础上建立未来更大规模的研究。霍尔说,让更多的人在这个初级阶段接受严格的研究方案这是不道德的”)营养师严格搭配超加工和加工食品的热量,能量密度,脂肪,碳水化合物,蛋白质,糖,钠和纤维。他们还确保研究对象对某一类食物没有偏好。在这两种饮食方式中,参与者都被要求尽可能多吃或尽量少吃正餐和零食。

This past spring, in his office, Hall showed me color photographs of each of the meals and snacks. The ultraprocessed meals included food such as canned ravioli, hot dogs, burgers topped with processed cheese, white bread, margarine and packaged cookies. Breakfast in this category had foods such as turkey bacon, sugared cereals, egg substitutes, Tater Tots, fruit-flavored drinks (most sweetened with artificial sweetener) and Spam. The unprocessed meals had dinners with roast beef, rice pilaf, couscous and pasta and breakfasts with nuts, vegetable omelets fried in oil, hash browns cooked with butter, and full-fat yogurt.

今年春天,在霍尔的办公室里,他给我看了每顿饭和小吃的彩色照片。超加工食品包括罐装馄饨、热狗、汉堡、奶酪、白面包、人造黄油和包装好的饼干。这类早餐包括火鸡培根、加糖谷物、鸡蛋替代品、土豆泥、水果味饮料(大多数用人造甜味剂增甜)和垃圾邮件。未加工的食物包括烤牛肉、米饭饭饭饭、蒸粗麦粉和意大利面,以及坚果早餐、油煎蔬菜煎蛋卷、黄油煎土豆饼和全脂酸奶。

Roast beef, pasta and fried eggs are very appealing to many of us, and it would not have been shocking if people ate more of these than they ate, say, ultraprocessed Spam. But that’s not what happened. Hall’s results, published earlier this year in Cell Metabolism, showed that on the ultraprocessed diet people ate about 500 extra calories every day than they did when eating the unprocessed diet, an increase that caused them to gain about two pounds in two weeks. “What was amazing about Hall’s findings was how many extra calories people eat when they are faced with ultraprocessed foods,” says Carlos Augusto Monteiro, a physician and professor of nutrition and public health at the School of Public Health at the University of S?o Paulo in Brazil.

烤牛肉、意大利面和煎蛋对我们很多人都很有吸引力,如果人们吃的这些东西比他们吃的(比如)加工过的垃圾邮件还要多,这也不会令人感到震惊。但事实并非如此。霍尔今年早些时候发表在《细胞代谢》杂志上的研究结果显示,食用超加工食品的人每天比食用未经加工食品的人多摄入约500卡路里,这导致他们在两周内增重约两磅。巴西圣保罗大学公共卫生学院营养与公共卫生教授、内科医生卡洛斯?奥古斯托?蒙泰罗表示:“霍尔的研究结果发现了人们在面对超加工食品时,他们摄入了多少额外的卡路里,这才是令人惊奇的。”

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A Gut-Brain disconnect

肠脑脱节

Why are more of us tempted to overindulge in egg substitutes and turkey bacon than in real eggs and hash brown potatoes fried in real butter? Dana Small, a neuroscientist and professor of psychiatry at Yale University, believes she has found some clues. Small studies the impact of the modern food environment on brain circuitry. Nerve cells in the gut send signals to our brains via a large conduit called the vagus nerve, she says. Those signals include information about the amount of energy (calories) coming into the stomach and intestines. If information is scrambled, the mixed signal can result in overeating. If “the brain does not get the proper metabolic signal from the gut,” Small says, “the brain doesn’t really know that the food is even there.”

为什么我们中有更多的人会忍不住吃鸡蛋替代品和火鸡培根,而不是真正的鸡蛋和用真正的黄油炸的土豆泥呢?耶鲁大学精神病学教授、神经学家达纳斯莫尔认为,她已经找到了一些解答这一问题的线索。Small研究了现代食物环境对大脑回路的影响。她说,肠道中的神经细胞通过一个叫做迷走神经的大管道向我们的大脑发送信号。这些信号包括进入肠胃的能量(卡路里)的信息。如果信息被打乱,混合的信号会导致暴饮暴食。斯莫尔说,如果“大脑没有从肠道获得适当的代谢信号”,“大脑甚至不知道食物在那里。”

Neuroimaging studies of the human brain, done by Small and others, indicate that sensory cues—smells and colors and texture—that accompany foods with high-calorie density activate the striatum, a part of the brain involved in decision-making. Those decisions include choices about food consumption.

Small等人对人脑进行的神经成像研究表明,伴随着高热量食物而来的感官线索——气味、颜色和纹理——会激活纹状体,即大脑中参与决策的部分。这些决定包括关于食物消费的选择。

And that is where ultraprocessed foods become a problem, Small says. The energy used by the body after consuming these foods does not match the perceived energy ingested. As a result, the brain gets confused in a manner that encourages overeating. For example, natural sweeteners—such as honey, maple syrup and table sugar—provide a certain number of calories, and the anticipation of sweet taste prompted by these foods signals the body to expect and prepare for that calorie load. But artificial sweeteners such as saccharin offer the anticipation and experience of sweet taste without the energy boost. The brain, which had anticipated the calories and now senses something is missing, encourages us to keep eating.

斯莫尔说,这就是过度加工食品的问题所在。人体在食用这些食物后所消耗的能量与所感知到的能量并不匹配。结果,大脑在某种程度上变得混乱,导致暴饮暴食。例如,天然甜味剂——如蜂蜜、枫糖浆和蔗糖——提供了一定数量的卡路里,而这些食物所带来的甜味预示着身体对这些卡路里的期待和准备。但是像糖精这样的人工甜味剂,在没有能量提升的情况下提供了甜味的预期和体验。大脑原本预计到了卡路里的摄入,现在却感觉少了什么,于是鼓励我们继续吃下去。

To further complicate matters, ultraprocessed foods often contain a combination of nutritive and nonnutritive sweeteners that, Small says, produces surprising metabolic effects that result in a particularly potent reinforcement effect. That is, eating them causes us to want more of these foods. “What is clear is that the energetic value of food and beverages that contain both nutritive and nonnutritive sweeteners is not being accurately communicated to the brain,” Small notes. “What is also clear is that Hall has found evidence that people eat more when they are given highly processed foods. My take on this is that when we eat ultraprocessed foods we are not getting the metabolic signal we would get from less processed foods and that the brain simply doesn’t register the total calorie load and therefore keeps demanding more.”

更复杂的是,超加工食品往往含有营养和非营养甜味剂的组合,斯莫尔说,这些甜味剂会产生令人惊讶的代谢作用,产生极强的强化作用。也就是说,一旦吃了它们会使我们想要更多的这些食物。斯莫尔指出:“很明显,含有营养和非营养甜味剂的食物和饮料的能量价值并没有准确地被传达给大脑。”同样清楚的是,霍尔发现的证据表明,当人们食用高度加工食品时,他们会吃得更多。我的观点是,当我们吃超级加工食品时,我们不会从加工程度较低的食品中获得代谢信号,而大脑根本没有记录下总热量负荷,因此会不断要求更多的热量。”

Small says that animal studies bear out the theory that ultraprocessed foods disrupt the gut-brain signals that influence food reinforcement and intake overall. “We’ve gone in with this cavalier attitude, that a calorie is a calorie, but a lot of foods have unintended consequences,” she says. “For example, in the natural world, carbohydrates almost always come packaged with fiber, whereas in ultraprocessed foods, fiber is either not there at all or included in a form not found in nature. And it is rare to find carbohydrates and fat in the same food in nature, but ultraprocessed foods tend to have both in one package. We’ve created all these hyperpalatable foods filled with fat, sugar, salt and additives, and we clearly prefer these foods. But these foods don’t necessarily provoke satiety. What they seem to provoke is cravings.”

斯莫尔说,动物研究证实了一种理论,即过度加工的食物会破坏内脏脑信号,从而影响食物的强化和整体摄入量。她说:“我们采取了一种漫不经心的态度,认为卡路里就是卡路里,但很多食物都有意想不到的后果。”例如,在自然界中,碳水化合物几乎总是与纤维一起包装的,而在超加工食品中,纤维要么根本不存在,要么以一种自然界中不存在的形式存在。在自然界中,很难在同一种食物中发现碳水化合物和脂肪,但经过超级加工的食物往往同时含有这两种成分。我们创造了所有这些超级美味的食物充满了脂肪,糖,盐和添加剂,我们显然更喜欢这些食物。但这些食物并不一定会引起饱腹感。它们似乎引发的是欲望。”

Small and other scientists speculate that ultraprocessed foods in some sense resemble addictive drugs, in that consuming them leads not to satisfaction but to a yearning for more. Neuroscientist Ann Graybiel of the Massachusetts Institute of Technology, a recognized expert on habit formation, says that external cues—like the mere sight of a candy bar—can provoke a reflexive response that causes the brain to encourage a behavior almost automatically. “Part of what’s happening when habits form is ‘chunking,’” she says. “You learn the behavior pattern, and your brain packages the whole sequence, including the beginning and the end markers, so you don’t have to think about it further.” (Certain neurons in the striatum are responsible for grouping behaviors into a single, habitual routine.)

斯莫尔和其他科学家推测,在某种意义上,超加工食品类似于成瘾药物,因为食用它们不会带来满足感,而是会产生对更多东西的渴望。麻省理工学院的神经学家Ann Graybiel是公认的习惯形成专家,她说,外部的暗示,比如仅仅是看到一根棒棒糖,就能激起一种反射性的反应,使大脑几乎自动地鼓励一种行为。“习惯形成的部分原因是‘分块’,”她说。“你学习了行为模式,你的大脑将整个序列打包,包括开始和结束的标记,所以你不必再去想它。”(纹状体中的某些神经元负责将行为组合成一个单一的、习惯性的程序。)

Eating large amounts of ultraprocessed foods may actually change brain circuitry in ways that increase sensitivity to food cues, adds Kent Berridge, a professor of psychology and neuroscience at the University of Michigan. He has shown this effect in rodents. “When you give rats junk-food diets, some gain weight, but others do not. In those that became obese, their dopamine systems changed, and they became hypersensitive to food cues—they became superfocused on that one reward. They showed no more pleasure, but they did show more wanting, and that wanting led to more actions—that is, more food-seeking behavior.”

密歇根大学(University of Michigan)心理学和神经科学教授肯特·贝里奇(Kent Berridge)补充说,大量食用超加工食品实际上可能会改变大脑回路,增加对食物线索的敏感度。他已经在啮齿动物身上证明了这种效果。“当你给老鼠喂食垃圾食品时,一些老鼠会增重,而另一些不会。在那些变胖的人身上,他们的多巴胺系统发生了变化,他们对食物的暗示变得非常敏感——他们变得非常专注于那一种奖励。他们没有表现出更多的快乐,但他们确实表现出了更多的渴望,而这种渴望导致了更多的行为——也就是更多的觅食行为。”

But this is not a uniform reaction, Berridge emphasizes, and he does not think it will turn out to be the only cause of overeating. “It’s very plausible that altering foods (through ultraprocessing) could trigger this response in some of us, but my guess is that we aren’t going to find that it affects all of us in the same way. My guess is that in the case of obesity, we are going to find subgroups—that is, that there are different avenues to becoming obese depending on one’s genes.”

但贝里奇强调,这并不是一种统一的反应,他也不认为这是导致暴饮暴食的唯一原因。“(通过超加工)改变食物很有可能在我们当中的一些人身上引发这种反应,但我的猜测是,我们不会发现它会以同样的方式影响我们所有人。”我的猜测是,在肥胖的情况下,我们会找到亚群,也就是说,根据一个人的基因,有不同的途径导致肥胖。”

Food fight

食物战争

Not all researchers agree that Hall’s avenue—the ultraprocessed one—is the major road leading to obesity. Rick Mattes, a professor of nutrition science at Purdue University and the incoming head of the American Society of Nutrition, says that he is concerned that Hall is damning a whole food category without sufficient cause. “He’s saying that ultraprocessed foods result in overeating, but there is no [large] body of evidence to support that claim. My view is that how items are manipulated may not be the primary driver of our response to them but that it is the nutrient composition that is the more relevant factor.”

并不是所有的研究人员都同意霍尔的研究方法,超加工食品是导致肥胖的主要原因。普渡大学(Purdue University)营养科学教授、即将上任的美国营养学会(American Society of nutrition)会长里克·马特斯(Rick Mattes)表示,他担心霍尔在没有充分理由的情况下,对整个食品类别进行了谴责。他说,过度加工食品会导致暴饮暴食,但没有(大量)证据支持这一说法。我的观点是,如何处理食品可能不是我们对它们做出反应的主要驱动因素,但更相关的因素是营养成分。”

Hall points out that he did match the nutritional composition of the diets, but Mattes has several other objections. Perhaps the most serious is that the participants were offered only ultraprocessed or unprocessed foods in each leg of the study. “In the real world, people would mix” different food types, he wrote in an e-mail. “This is not a fault with the study, but it is a serious issue when attempting to extrapolate the findings to free-living people.”

霍尔指出,他确实匹配了这些饮食的营养成分,但马特斯有几个其他的反对意见。也许最严重的是,在研究的每一段时间里,参与者只吃了经过加工或未经加工的食物。“在现实世界中,人们会混合”不同的食物类型,他在一封电子邮件中写道。“这不是这项研究的错误,但当试图将研究结果推广到自由生活的人们身上时,这是一个严重的问题。”

Another possible factor driving overconsumption of ultraprocessed foods is that they are eaten quickly, so people could devour a lot before any satiation mechanisms kick in to slow them down. Ultraprocessed foods tend to be energy-dense and pack a relatively large number of calories into a relatively small package. This, too, might encourage rapid consumption that bypasses satiety mechanisms. Still, fast eating does not explain why people continued to eat more ultraprocessed food at their next meal, when, at least in theory, they should have been less hungry.

另一个可能导致超加工食品过度消费的因素是,它们吃得很快,所以人们可能会在任何饱腹感机制发挥作用、使其慢下来之前狼吞虎咽。超加工食品往往能量密集,在一个相对较小的包装中含有相对大量的卡路里。这也可能鼓励快速消费,从而绕过饱腹感机制。尽管如此,快速进食并不能解释为什么人们在下一餐继续吃更多的超加工食品,而至少在理论上,他们本应该不那么饿。

If ultraprocessed foods are indeed a big problem, the question is what, if anything, we can and should do about them. When I asked Hall, he was reluctant to call for stringent measures such as a tax on these foods. “I worry that because almost 60 percent of our calories come from ultraprocessed foods, taxing those foods might add to some people’s food insecurity,” he says. “We’ve found an association of ultraprocessed foods and overeating, and there are many hypotheses about the causal mechanism. But until you fully understand the mechanism, it’s too early to intervene. It could be that the additives and artificial flavoring are having an impact or that ultraprocessed foods have micronutrient deficiencies that the body senses and responds to by overeating. There are likely other factors as well. We just don’t know—yet.”

如果超加工食品确实是个大问题,那么问题是我们能做什么,应该做什么。当我问霍尔时,他不愿呼吁对这些食品征税等严厉措施。他说:“我担心,因为我们近60%的卡路里来自超加工食品,对这些食品征税可能会增加一些人的食品安全。”“我们发现了超加工食品和暴饮暴食之间的联系,关于其因果机制有很多假说。但在你完全理解其机制之前,现在进行干预还为时过早。可能是添加剂和人工调味剂产生了影响,也可能是超加工食品存在微量营养素不足,身体会感觉并通过过量进食做出反应。可能还有其他因素。我们只是还不知道。”

At the same time, he does think the available evidence on ultraprocessed foods is a reason to worry about them: “We can change our diet to minimize the damage. And for now I think that’s where we need to set our sights.” The food industry can help, perhaps by designing more foods with less processing, but people have to show they want such food by buying more of it. “I’m no evangelist,” Hall asserts, “but I do think that the public demand on the food system is more powerful than any government regulation.” His job in all this, he says, is to get the science right.

与此同时,他认为现有的关于超加工食品的证据是担心它们的一个理由:“我们可以改变我们的饮食,把危害降到最低。就目前而言,我认为这是我们需要瞄准的地方。食品工业或许可以提供帮助,比如设计更多加工更少的食品,但人们必须通过购买更多的食品来表明他们想要这些食品。霍尔说:“我不是传福音的人,但我确实认为,公众对食品系统的需求比任何政府监管都要强大。他说,他的工作就是搞清楚科学。

This article was originally published with the title "Obesity on the Brain" in Scientific American 321, 4, 38-45 (October 2019)

这篇文章最初发表在《科学美国人》321,4,38-45(2019年10月)上,标题为“肥胖对大脑的影响”。

doi:10.1038/scientificamerican1019-38

doi:10.1038/scientificamerican1019-38

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探索更多信息

Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss Than Carbohydrate Restriction in People with Obesity. Kevin D. Hall et al. in Cell Metabolism, Vol. 22, No. 3, pages 427–436; September 1, 2015.

就卡路里而言,饮食中脂肪的限制比碳水化合物的限制更容易导致肥胖患者体内脂肪的减少。出自《细胞代谢》,第22卷第3期,第427-436页;2015年9月1日。

Energy Expenditure and Body Composition Changes after an Isocaloric Ketogenic Diet in Overweight and Obese Men. Kevin D. Hall et al. in American Journal of Clinical Nutrition, Vol. 104, No. 2, pages 324–333; August 2016.

超重和肥胖的男性的能量消耗和身体组成在等热量生酮饮食后发生变化。出自凯文·霍尔等人《美国临床营养学杂志》第104卷第2期,第324-333页;2016年8月。

Processed Foods and Food Reward. Dana M. Small and Alexandra G. DiFeliceantonio in Science, Vol. 363, pages 346–347; January 25, 2019.

加工食品和食品奖励。出自戴娜·斯莫尔和亚历山德拉·迪费利斯·安东尼奥的《科学》,第363卷,第346-347页;2019年1月25日。

Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake. Kevin D. Hall et al. in Cell Metabolism, Vol. 30, No. 1, pages 67–77 and e1–e3; July 2, 2019.

一项研究住院病人随意摄入食物的情况的随机对照试验发现超加工食品会导致热量摄入过多和体重增加出自《细胞代谢》,第30卷,第1期,第67-77页和e1-e3页;2019年7月2日。

FROM OUR ARCHIVES

我们的档案有此记载

The Food Addiction. Paul J. Kenny; September 2013.

食物上瘾。Paul j . Kenny;2013年9月。

ABOUT THE AUTHOR(S)

关于作者

Ellen Ruppel Shell

艾伦·鲁佩尔·谢尔(Ellen Ruppel Shell)

Ellen Ruppel Shell is a professor of science journalism at Boston University and author of The Job: Work and Its Future in a Time of Radical Change (Crown Publishing, 2018). She writes frequently on medical issues and is author of The Hungry Gene: The Inside Story of the Obesity Industry (Grove Press, 2002). She wrote about the controversy over lead poisoning in Flint, Mich., in the July 2016 issue of Scientific American.

艾伦·鲁佩尔·谢尔是波士顿大学科学新闻学教授,着有《工作:巨变时期的工作及其未来》(Crown Publishing, 2018)。她经常就医学问题撰文,着有《饥饿基因:肥胖产业的内幕》(Grove Press, 2002)一书。她写了关于密歇根州弗林特市铅中毒事件的争议问题,该文发表在2016年7月的《科学美国人》杂志上。

Credit: Nick Higgins

资料来源:尼克·希金斯

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