For most people, fat is a burden. It doesn't really matter whether it appears as cellulite on our thighs or cholesterol in our veins — we just don't want it.

But it turns out that our bodies also make a unique form of fat tissue that behaves remarkably unlike any other: Rather than storing excess energy, this fat actually burns through it.

It's called brown fat (as opposed to the more familiar white fat that hangs over belt buckles and swings from the backs of arms) and a series of papers published in the New England Journal of Medicine confirm for the first time that healthy adults have stores of this adipose tissue, which researchers hope to study further as a potential new weight loss treatment.

Until now, only rodents and human newborns have been known to have any significant deposits of brown fat, so called because of its abnormally high concentration of dark-colored mitochondria, the engines that sustain cell activity. The primary purpose of brown fat is to regulate body temperature: The mitochondria-packed cells are designed to burn high quantities of sugar, the body's fuel, and release that energy as heat — a mechanism that newborns, fresh from the warm confines of the womb, rely on to keep them toasty.

As people age, however, the body becomes more adept at regulating temperature, so brown fat stores shrink and white fat starts to emerge. (From a biological perspective, also, brown fat is highly inefficient, since cells don't need heat to run; rather, they use ATP, another chemical produced by mitochondria.) Adults with appreciable amounts of brown fat are usually those who have certain types of cancer or hyperthyroidism, conditions that stimulate the growth of brown fat.

But Dr. Sven Enerbäck at the University of Göteborg in Sweden has shown, using the latest imaging technologies, that healthy adults retain a sizable amount of brown fat in the front and of back of the neck. (That was a surprise, since in rodents, the depots tend to be along the back, around the shoulder blades.) Enerback and his team studied five patients and confirmed, using genetic analysis, that the cells around the neck were indeed brown fat.

In a sense, scientists have known this for years. While scanning patients with positron emission tomography (PET), an imaging technique often used in cancer patients to detect tumor spread, scientists have long noted the excess activity of brown fat cells in their images. They just didn't realize what they were looking at.

Since PET picks up glucose-burning activity in cells, hot spots on PET scans of cancer patients generally indicate actively growing tumors. But after doing biopsies, doctors found that hot spots in the necks of most of their patients weren't cancerous at all. These turned out to be brown fat deposits.

"We set forth to actually pinpoint whether the PET glucose-uptake areas corresponded to true brown fat tissue, and I think we more or less proved the case," says Enerbäck, who found that those mystery cells in the neck expressed the same proteins as brown fat.

Identifying the presence of brown fat is one thing, but activating it to burn more glucose is another. Two studies in the New England Journal of Medicine, including Enerbäck's, confirmed that brown fat cells become more active in the cold — that is, when study participants needed to boost their body temperature. Enerback saw increased activity when he plunged one foot of each volunteer into an ice bath while in the scanner, and in a separate study, scientists at Maastricht University Medical Center in the Netherlands also saw upticks in brown fat activity in subjects who had been chilling in a 16-degree-C (61-degree-F) room for two hours. (PET technicians have also long known that putting patients in warmer rooms tended to keep that bothersome extra activity from showing up on their images.)

But before we all turn our thermostats down or consider joining the Polar Bear Club, can brown fat actually cause weight loss? Brown fat may indeed shift the balance of calorie intake and expenditure — allowing a person to burn more calories for the same amount of consumption — without the chore of going to the gym or sweating through a workout. "We have very few interventions aimed at increasing energy expenditure," says Dr. Franceso Celi, a clinician at the National Institute of Diabetes and Digestive and Kidney Diseases at the National Institutes of Health. "And here we have a tissue that works exactly with the purpose of burning energy." Based on animal models, researchers calculate that 50 g of brown fat — less than what the scientists in the current series of papers documented in their human volunteers — could burn about 20% of an average person's daily caloric intake.

In the third study in the New England Journal, researchers also found that leaner people tended to have more brown fat deposits than overweight or obese individuals. Interestingly, women were twice as likely to have active brown fat than men, according to the study conducted by Dr. Ronald Kahn and his colleagues at the Joslin Diabetes Center, Massachusetts General Hospital, Beth Israel Deaconess Medical Center and Brigham and Women's Hospital.

Still, the findings don't necessarily mean that activating brown fat leads to a trimmer waist. For one thing, the body is uncommonly good at maintaining equilibrium, which is why a boost in calorie-burning can often lead to a trigger of the hunger signal and prompt people to eat more to make up for the loss. And even if drug companies could find a way to activate brown fat safely, that excess activity could throw off other metabolic systems that could damage your health. After all, the people who have the most active brown fat so far are those with cancer and hyperthyroidism.

对大多数人来说,脂肪是一种负担。无论它是以脂肪团的形式出现在我们的大腿上,还是以胆固醇的形式出现在我们的血管里,都没有关系,反正我们就是不需要它。

但是研究表明我们的身体也产生一种独特的脂肪组织跟其它的脂肪不一样:它不是用来储藏多余的脂肪的,而是用来燃烧脂肪的。

这种脂肪叫棕色脂肪(用以和悬挂在腰带扣上的白花花的脂肪以及后背和胳膊上的脂肪相区别。)新英格兰的医学杂志上发表的一系列论文也第一次证实健康的成年人体内都有这种动物脂肪组织,研究人员正对这种脂肪进行研究,希望能从中找到一种新的减肥方法。

到目前为止,只在啮齿类动物和新生儿的体内发现了这种重要的棕色脂肪,之所以叫做棕色脂肪是因为这种脂肪内含有非常高的暗色的线粒体,线粒体是用来维持细胞活动的。棕色脂肪的基本作用是调节体温:充满了线粒体的细胞可以燃烧大量的糖分---身体的燃料,并把这种能量作为热量释放掉。新生儿正是靠这种机制来给自己保温,从而使自己适应离开了温暖子宫保护后的生活。

然而随着年龄的增长,人体更加擅长于体温调节,所以棕色脂肪的含量就降低了,白色脂肪开始出现了(从生物学的角度来说,棕色脂肪的作用也在降低,因为细胞不再需要热量,而是用ATP,另外一种由线粒体产生的化学物质来运转)。只有那些患有癌症和甲亢的人体内才有棕色脂肪,因为在这种状况下会刺激棕色脂肪的生长。

瑞典Göteborg大学的Sven Enerbäck博士通过成像技术显示,健康的成人在脖子的前后部位也有少量的棕色脂肪。这真令人惊奇,因为棕色脂肪是存在于后背,在肩胛骨的部位。Enerback和他的研究小组通过研究五名病人证实,通过基因分析,颈部周围的细胞确实是棕色脂肪。

在某种意义上说,科学家认识到这种现象已经好几年了。通过使用正电子发射X射线摄影技术(PTF)扫描病人发现--- PTF经常被用于检查癌症病人的肿瘤扩散情况。科学家很久以前就在这些病人的影像中注意到了棕色脂肪的活跃活动。只是当时不知道它为何物。

自从PET显现出细胞里的葡萄糖燃烧活动后,癌症患者的PET扫描显示出了肿瘤的活跃生长。但是医生在进行了活组织检查后发现大多数病人颈部上的热点根本就不是癌细胞,而是棕色脂肪。

“我们开始精准的实验看PET扫描显示的葡萄糖消耗区域是否就是真正的棕色脂肪组织。我想我们已经或多或少的证明了这点了。”Enerbäck博士说。他发现颈部的神秘细胞表现出了和棕色细胞同样的蛋白质。

证实了棕色细胞的存在是一回事,使它能燃烧更多的葡萄糖是另一回事。两名学生包括Enerbäck在新英格兰医学杂志上证实棕色脂肪在寒冷的环境中更活跃,也就是说什么时候受试人员需要调高体温。Enerback让每名志愿者都进入深度达一英尺的冰块浴后在扫描成像中发现细胞活动变活跃了。位于荷兰的马萨诸塞大学医学中心的科学家在试验中也发现受试者在一个气温为16摄氏度(61华氏度)的房间里呆上两小时后,棕色脂肪的活动也会变活跃。

PET扫描师也发现让病人们呆在暖和点的房间里就能让那种烦人的细胞从影像中消失。

如果我们不考虑把恒温器关掉,或者考虑参加北极熊俱乐部的话,棕色细胞真能帮助减肥

吗?棕色细胞可能真能调节卡洛里的摄入量和消耗量,能让人在消耗相同的情况下燃烧更多的热量,而不用去健身货真把自己弄得汗流浃背。“我们不去刻意增加能量消耗,”Franceso Celi,国家卫生部糖尿病,消化道疾病,和肾病研究中心的临床医生说,“我们这里有一种组织就是用来燃烧能量的,”通过动物实验,研究人员发现50g的棕色脂肪就能燃烧掉一个成年人平均20%的卡洛里摄入量。

在新英格兰杂志的第三次研究中,研究人员发现体型较瘦的人和超重或肥胖的人相比,体内含有更多的棕色脂肪。有意思的是女性体内含有棕色脂肪的几率是男性的两倍,据马萨诸塞州中心医院Joslin糖尿病中心的Ronald Kahn博士和他的同事说。

当然,这些发现并不说明活跃的棕色脂肪能让你的腰更细。一方面,我们的身体很善于保持平衡,这就是为什么卡洛里燃烧后我们会产生饥饿感,胖人们会吃更多的东西来弥补损失。即使制药公司能找到一种安全的生产棕色脂肪的方法,这种脂肪的活动也会破坏新陈代谢系统,有损健康。毕竟目前为止那些体内含有棕色脂肪的人还是癌症患者和患有甲亢的人。