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.

對大多數人來說,脂肪是一種負擔。無論它是以脂肪團的形式出現在我們的大腿上,還是以膽固醇的形式出現在我們的血管里,都沒有關系,反正我們就是不需要它。

但是研究表明我們的身體也產生一種獨特的脂肪組織跟其它的脂肪不一樣:它不是用來儲藏多余的脂肪的,而是用來燃燒脂肪的。

這種脂肪叫棕色脂肪(用以和懸掛在腰帶扣上的白花花的脂肪以及后背和胳膊上的脂肪相區別。)新英格蘭的醫學雜志上發表的一系列論文也第一次證實健康的成年人體內都有這種動物脂肪組織,研究人員正對這種脂肪進行研究,希望能從中找到一種新的減肥方法。

到目前為止,只在嚙齒類動物和新生兒的體內發現了這種重要的棕色脂肪,之所以叫做棕色脂肪是因為這種脂肪內含有非常高的暗色的線粒體,線粒體是用來維持細胞活動的。棕色脂肪的基本作用是調節體溫:充滿了線粒體的細胞可以燃燒大量的糖分---身體的燃料,并把這種能量作為熱量釋放掉。新生兒正是靠這種機制來給自己保溫,從而使自己適應離開了溫暖子宮保護后的生活。

然而隨著年齡的增長,人體更加擅長于體溫調節,所以棕色脂肪的含量就降低了,白色脂肪開始出現了(從生物學的角度來說,棕色脂肪的作用也在降低,因為細胞不再需要熱量,而是用ATP,另外一種由線粒體產生的化學物質來運轉)。只有那些患有癌癥和甲亢的人體內才有棕色脂肪,因為在這種狀況下會刺激棕色脂肪的生長。

瑞典Göteborg大學的Sven Enerbäck博士通過成像技術顯示,健康的成人在脖子的前后部位也有少量的棕色脂肪。這真令人驚奇,因為棕色脂肪是存在于后背,在肩胛骨的部位。Enerback和他的研究小組通過研究五名病人證實,通過基因分析,頸部周圍的細胞確實是棕色脂肪。

在某種意義上說,科學家認識到這種現象已經好幾年了。通過使用正電子發射X射線攝影技術(PTF)掃描病人發現--- PTF經常被用于檢查癌癥病人的腫瘤擴散情況。科學家很久以前就在這些病人的影像中注意到了棕色脂肪的活躍活動。只是當時不知道它為何物。

自從PET顯現出細胞里的葡萄糖燃燒活動后,癌癥患者的PET掃描顯示出了腫瘤的活躍生長。但是醫生在進行了活組織檢查后發現大多數病人頸部上的熱點根本就不是癌細胞,而是棕色脂肪。

“我們開始精準的實驗看PET掃描顯示的葡萄糖消耗區域是否就是真正的棕色脂肪組織。我想我們已經或多或少的證明了這點了。”Enerbäck博士說。他發現頸部的神秘細胞表現出了和棕色細胞同樣的蛋白質。

證實了棕色細胞的存在是一回事,使它能燃燒更多的葡萄糖是另一回事。兩名學生包括Enerbäck在新英格蘭醫學雜志上證實棕色脂肪在寒冷的環境中更活躍,也就是說什么時候受試人員需要調高體溫。Enerback讓每名志愿者都進入深度達一英尺的冰塊浴后在掃描成像中發現細胞活動變活躍了。位于荷蘭的馬薩諸塞大學醫學中心的科學家在試驗中也發現受試者在一個氣溫為16攝氏度(61華氏度)的房間里呆上兩小時后,棕色脂肪的活動也會變活躍。

PET掃描師也發現讓病人們呆在暖和點的房間里就能讓那種煩人的細胞從影像中消失。

如果我們不考慮把恒溫器關掉,或者考慮參加北極熊俱樂部的話,棕色細胞真能幫助減肥

嗎?棕色細胞可能真能調節卡洛里的攝入量和消耗量,能讓人在消耗相同的情況下燃燒更多的熱量,而不用去健身貨真把自己弄得汗流浹背。“我們不去刻意增加能量消耗,”Franceso Celi,國家衛生部糖尿病,消化道疾病,和腎病研究中心的臨床醫生說,“我們這里有一種組織就是用來燃燒能量的,”通過動物實驗,研究人員發現50g的棕色脂肪就能燃燒掉一個成年人平均20%的卡洛里攝入量。

在新英格蘭雜志的第三次研究中,研究人員發現體型較瘦的人和超重或肥胖的人相比,體內含有更多的棕色脂肪。有意思的是女性體內含有棕色脂肪的幾率是男性的兩倍,據馬薩諸塞州中心醫院Joslin糖尿病中心的Ronald Kahn博士和他的同事說。

當然,這些發現并不說明活躍的棕色脂肪能讓你的腰更細。一方面,我們的身體很善于保持平衡,這就是為什么卡洛里燃燒后我們會產生饑餓感,胖人們會吃更多的東西來彌補損失。即使制藥公司能找到一種安全的生產棕色脂肪的方法,這種脂肪的活動也會破壞新陳代謝系統,有損健康。畢竟目前為止那些體內含有棕色脂肪的人還是癌癥患者和患有甲亢的人。