It's no secret that stress isn't good for you. But what's less clear is how social stressors like a high-pressure job or a failing marriage affect your physical well-being.

    Researchers at Wake Forest University who study stress in monkeys think they may have discovered a clue: fat. More specifically, the particular form of fat called visceral fat that tends to build up in the abdomen （those dreaded beer bellies and love handles）. Researchers believe this abdominal fat lodges deep within visceral organs, such as the heart, liver and blood vessels, and may be an indicator of increased heart attack risk. In a study of 42 female monkeys, the scientists found that those with the most social stress - in the monkeys' case, that meant being at the bottom of the social hierarchy - packed away the most fat around the middle.

    "For years now there has been a recognition that the pattern in which people lay down fat is associated more with health than the absolute amount of fat," says study co-author Carol Shively, a pathologist at Wake Forest. "Fat cells that live in the visceral depot behave differently than cells that live in other areas of the body."

    Recent evidence suggests that visceral fat cells are active, unlike the fat cells found elsewhere in the body just under the skin, known as subcutaneous fat. Those fat cells are essentially just storage sinks for calories. But visceral fat cells actively secrete hormones and other agents that affect the metabolism of sugar and the way the body burns calories. In people, visceral fat has been linked to metabolic changes, such as higher blood pressure and blood sugar levels, that increase risk for diabetes and heart disease.

    Shively and her colleagues also knew that people who produce excessive amounts of the stress hormone cortisol tend to have bulky waistlines; they have apple-shaped bodies, rather than pear-shaped. So the researchers wanted to examine all these factors - stress, abdominal fat and health risk - in one study. The problem, of course, is that measuring the relationship between stress and visceral fat in people in a controlled fashion isn't easy. So, the team turned to monkeys. For nearly two and a half years, she and her team fed the animals a typical Western diet, with 40% of calories coming from fat, measured their cortisol levels and used CT scans to calculate the amount of visceral fat each monkey carried.

    The monkeys were housed in groups of four, automatically prompting them to establish a linear hierarchy of dominance. The dominant monkey in each group experienced the least stress, according to researchers. "They were groomed more than the subordinates, and they would get relaxed. Their eyes would roll up, sort of like they were getting a massage," says Shively. Monkeys further down the power chain, however, appeared more stressed-out. They were more vigilant, constantly scanning their environment for potentially aggressive threats from the leader. They also spent more time alone, out of contact with the other monkeys.

    CT scans showed that group leaders and the second most dominant monkeys showed lower amounts of visceral fat than their subordinates, who carried the bulk of their body fat in their guts. In human populations, something similar happens: Studies have linked lower social status to a higher incidence of metabolic syndrome - the condition whose symptoms include being overweight and having high blood pressure and high glucose levels - which promotes heart disease.

    Together with Shively's findings, says Dr. David Katz, director and co-founder of the Yale Prevention Research Center, the human data suggests a possible cause-and-effect link: Stress may promote accumulation of visceral fat, which in turn causes metabolic changes in the body that contribute to heart disease and other health problems.

    "This study shows that psychological stress, which we know can affect stress hormone levels, can have a fairly rapid influence on where extra calories go," he says. "I'm generally quite cautious about animal research but here I think we're seeing something that has direct relevance to human health as well."

    眾所周知，壓力不是件好事。但是，社會壓力如何影響身體健康，比如工作壓力大，婚姻失敗等，就不太清楚了。

    Wake Forest大學的猴子研究人員認為，他們可能找到了一條線索：脂肪。更具體的說是，一種特定的往往在腹部堆積的內臟脂肪（導致那些可怕的啤酒肚和游泳圈）.研究人員認為，這種腹部脂肪蓄積在內臟器官的深處，比如心、肝、血管等，可能表明心臟病的幾率更大。通過研究42只母猴，科學家們發現那些社會壓力最大的猴子，即最底層的猴子，腹部脂肪最多。

    研究的合伙作者Carol Shively是Wake Forest的一名病理學家。他說："多年來，人們認識到脂肪增加的模式和健康的關聯，大于和絕對脂肪量的關系。""內臟器官中的脂肪細胞在不同的身體里活動也不一樣。"

    最近的發現表明，內臟脂肪細胞是活性的，不同于身體中其他部位的皮膚下面的脂肪細胞，也就是皮下脂肪。這些皮下脂肪細胞只是存儲卡路里的容器。但是活性的內臟脂肪細胞分泌影響糖代謝和身體燃燒熱量的激素和其他因素。就人類而言，內臟脂肪和新成代謝的變化有關，如血壓血糖增加，進而增加了糖尿病和心臟病的風險。

    Shively及其同事都知道，分泌過多的壓力荷爾蒙皮質醇的人往往大腹便便，身材更像蘋果，而不是梨子。因此，研究人員希望檢查以下因素，例如壓力、腹部脂肪和健康風險，并將結果集中到一份研究里。當然，問題在于用控制的方式測算壓力和內臟脂肪的關系并非易事。所以，他們用了猴子。近兩年半的時間里，Shively和她的團隊讓猴子們吃典型的西餐，其中四成熱量來自脂肪。他們測算猴子的皮質醇水平，利用CT掃描，計算每只猴子體內的內臟脂肪數量。

    猴子們四只一組，可以促使他們自發的形成上下等級關系。研究人員說，每組中占據統治地位的猴子壓力最小。Shively說，"上級的猴子被梳理毛發的次數超過下屬，他們可以很放松，眼睛上翻，就像接受按摩一樣。"但是那些越底層的猴子似乎壓力越大。他們更警覺，常常左顧右盼，防止來自首領的潛在威脅。他們獨處的時間也更多，不和其他猴子交流。

    CT掃描的結果表明，首領們和次級首領猴子的內臟脂肪低于他們的下屬，下屬們都拖著肥肥的肚子。在人類的世界里，情況有些類似。研究發現社會地位低往往導致代謝問題的發病率更高，例如超重，高血壓和高血糖，這些都會導致心臟病。

    除了Shively的發現，耶魯預防研究中心的主任兼合伙創辦人，David Katz說，人類的實驗數據中存在一種可能的因果關系。壓力可能會促進內臟脂肪的堆積，進而導致代謝變化，引發心臟病和其他健康問題。

    他說，"我們都知道心理壓力影響壓力荷爾蒙的水平，而這份研究表明心理壓力也會迅速影響額外熱量的走向。""對于動物的研究，我一般都很謹慎。但是，我想這份研究顯示出某種直接影響人類健康的因素。"