In the quest to fight obesity¹, scientists are looking at an intriguing² question: Is it possible for adults to lose weight by having more baby fat?

Babies have lots of brown adipose³ tissue, or brown fat, so called because of its color. It is critical to the body's heat production. Unlike white fat cells, which store energy from the food we eat, brown fat consumes calories to generate heat. Revving⁴ up this process, research has shown, may help us grow leaner by burning more of the white fat.

Until recently, experts believed that only babies and children had brown fat, to help keep them warm before their young bodies develop techniques like shivering to help them cope with cold temperatures. A discovery last year that adults still have at least some brown fat has spawned⁵ hope among scientists and drug developers that the calorie-burning tissue may provide one solution to curbing⁶ obesity.

Researchers at Harvard Medical School have identified a protein in the body that appears to spur production of brown fat, including by converting some white fat cells into brown ones, and are now working to develop a drug that would encourage that process. They expect their work could lead to a new approach to treating obesity within a few years.

Other researchers are seeking ways to prompt the brown fat we already have to become more active, thereby⁷ prompting our bodies to generate more heat and consume more calories. One technique being investigated: exposing people to colder temperatures, which appears to trigger brown fat to turn up the body's heat.

Brown fat is able to burn calories because of the cells' abundance of mitochondria, the engines of the cellular⁸ world. White fat cells, by contrast, are like fat-filled balloons that store energy and help insulate the body.

Brown fat is thought to have evolved to protect babies and children from hypothermia. Babies and smaller organisms are at greater risk because they tend to cool more easily than adults. And, in prehistoric⁹ times, once people became adults, the more important issue wasn't keeping warm, it was to make sure they had enough energy stored to survive times without food. But today, with food so plentiful¹⁰, white fat cells' energy-storage ability has led to obesity as a significant public health issue.

Wouter van Marken Lichtenbelt, a professor of human biology at the Maastricht University Medical Centre in the Netherlands, estimates that boosting the body's heat-generating ability, a process known as thermogenesis, could help people burn as much as 5% to 10% additional calories. 'That can be a lot,' he says. However, whether we lose weight or not depends on whether we overcompensate by eating more food. 'We shouldn't eat too much,' he says.

Researchers at various institutions are seeking ways to increase the body's heat-generating ability by taking advantage of the fact that brown fat is activated¹² by the cold. Studies with obese¹³ mice show that putting them in the cold for a few days activates¹⁴ their heat-production system significantly. Within a week, an obese mouse could burn off half of its white fat, even while eating 1.5 times as much as it normally does, according to Leslie Kozak, a professor of molecular¹⁵ genetics at Pennington Biomedical Research Center in Baton¹⁶ Rouge¹⁷, La. 'This is a very powerful system,' Dr. Kozak says.

While there are data showing that people who live in colder climates tend to be leaner than people from warmer places, losing weight isn't as easy as taking a vacation in Alaska, for example. For one thing, people often compensate¹¹ in colder environments by eating more; a similar phenomenon occurs when people begin to exercise more and their metabolism¹⁸ speeds up. It's also unclear how long people need to be exposed to cold in order to boost their heat generation.

Dr. Kozak has been investigating whether activation¹⁹ of brown fat can be increased through training people to better tolerate the cold. The idea here is that if people get used to cold, it's because their internal furnace is working better, he says.

Dr. Kozak says it's unclear how much cooler an environment needs to be to increase brown fat's activity level. He says he would like to test whether a school classroom, for instance, set at 65 degrees, a few degrees below normal, could increase heat generation in the students' bodies.

在与肥胖作斗争的探索过程中，科学家正面对一个有趣的问题：成年人是否可以通过增加婴儿脂肪来减肥？

婴儿有大量的褐色脂肪组织，这个名称是因其颜色而得名的。褐色脂肪组织对身体产生热量很重要。与存贮从饮食中吸取的能量的白色脂肪细胞不同，褐色脂肪消耗热量而产热。研究发现，这个过程若加速，可能有助于人类通过燃烧更多白色脂肪而变瘦。

直到最近，专家还相信只有婴幼儿有褐色脂肪，以使其年幼的躯体在具有打寒战等技能以驱寒前使身体保暖。而去年的一项发现表明，成年人仍有很少一部分褐色脂肪，这给了科学家及药品研发者一种希望，即热量燃烧组织可能提供一种方式来遏制肥胖。

哈佛大学医学院(Harvard Medical School)的研究者已确定人体内的一种蛋白似乎可刺激褐色脂肪的产生，包括将某些白色脂肪细胞转化为褐色脂肪。他们正在研制一种可激发这种过程的药品，并希望这项工作能在几年内带来治疗肥胖的新方法。

另一些研究者正在设法使人体已有的褐色脂肪活动得更为积极，由此使人体产生更多热量、消耗更多热能。正在被研究的一项技术是，将人们置身于温度更低的环境内，这似乎能引发人体内褐色脂肪消耗掉人体内的热量。

褐色脂肪能燃烧热量是因为这种细胞内富含线粒体这种分子世界的发动机。而白色脂肪细胞，却像充满了脂肪的气球，贮存着能量并对人体有隔离保暖作用。

褐色脂肪的进化功能据信是保护婴幼儿免于体温过低。婴儿及小型有机体因比成年人更易冷却因而面临体温过低风险更大。在史前时期，人们一旦成年，最重要的事并非保暖，而是确保贮存充足的能源以在缺乏食物时得以存活。但今天，因食物非常充足，白脂肪细胞的能量贮存功能已导致肥胖成为一个显著的公共健康问题。

荷兰马斯特里赫特大学医学中心(Maastricht University Medical Centre)的人体生物学教授李赫腾贝尔特（Wouter van Marken Lichtenbelt）估计，增加人体的产热能力(这一过程称为生热)，有助于人体额外燃烧5%至10%的热能。他说，这个量很大。但体重是否能下降还决取于是否会吃更多的食物造成补偿过度。他认为人们不应吃得太多。

既然褐色脂肪通过寒冷激活，各机构的研究者就利用这个事实来寻求增加人体产热能力的办法。路易斯安那州立大学Pennington 生物医学研究中心(Pennington Biomedical Research Center)的分子遗传学教授科扎比(Leslie Kozak)对肥胖老鼠的研究显示，将老鼠放到寒冷的环境里过几天，其体内的产热系统就明显激活。在一周内，一个肥胖的老鼠可燃烧掉其体内一半的白脂肪，即便在此期间它的食量是平常的1.5倍。他说，这个产热系统非常强大。

尽管数据显示生活在气候寒冷地区的人往往比温暖地区的人瘦，但要想在阿拉斯加度个假来减肥可并不容易。原因之一，寒冷环境中人们通常吃得更多而补偿过度；当人们开始加强锻炼而新陈代谢加速时也是同样。还有一点是，不清楚要在寒冷环境中待多久才能增加人体产热。

科扎比一直在研究，是否可通过训练人体更耐寒来增强褐色脂肪的活动。他说，这种想法是基于这种理念，如果人体习惯于寒冷，是因为其内部热能燃烧机制运作得更好。

科扎比说，尚不清楚周围环境需要冷却多少才能增加褐色脂肪的活动水平。他说，他要进行一个测试，比方说，学校的一间教室，设在比正常温度略低几度的18.3摄氏度，是否会增加学生体内产生的热量。