為什麼心情不好時愛吃垃圾食品？Why Do We Eat Junk Food When We're Anxious?

華爾街日報 Robert M. Sapolsky 在科學的萬神廟裡，有一些人為人類的健康做出了巨大貢獻，比如巴斯德(Pasteur)和索爾克(Salk)等人。神廟裡還有一個位子，正等著那位解決了這樣一個迷題的科學家：為什麼當我們感到沒人愛的時候就會吃垃圾食品？ 問這個問題並不愚蠢，至少在9月份時肯定不，因為這個月正好是“全國警惕兒童肥胖月”(National Childhood Obesity Awareness Month)。在世界各地，與肥胖有關的健康問題都普遍存在，成年型糖尿病尤為突出。而問題背後的一個重要原因，是我們在不餓時吃東西。 那我們為什麼在不餓時吃東西呢？可能是周圍的人都在吃，也可能是食品廣告說服力太強。還有可能是我們不喜歡某場聚會的東道主，想把他的薯片吃光，吃到讓他破產。 非營養性進食最容易理解的一個例子，是壓力往往導致我們吃得更多。這可以從心理學角度解釋，最容易在壓力下進食的人，就是平時最積極限制進食的那些人：當處境不順、需要善待自己的時候，進食便是他們放鬆的方式。他們更喜歡攝入脂肪和碳水化合物。如果老板是個混蛋，不如狂吃包巧克力的海象肉吧？ 但我們不能把這些習慣一股腦兒地歸到人類心理的復雜性上面，因為表現出這些習慣的不只是人類。給一只實驗鼠施加壓力（比如在它的籠子裡放一只陌生老鼠），它就會吃得更多，並且比平時更加傾向於吃高脂肪、高碳水化合物的東西。 這種現象出現在很多物種當中，這可以從進化論角度解釋。對於99%的動物來說，壓力都涉及能量消耗的大幅增加（比如說在逃命的時候）。在這之後，身體刺激食欲、特別是對高熱量的食欲，以重新積累耗盡的能量儲備。但聰明而又神經兮兮的人類因為純粹心理上的原因而不斷出現應激反應，使我們的身體反復進入重新積累能量的模式。 科學家正在開始理解壓力導致垃圾食品渴求的機理。壓力會增加大腦中某些區域“內源性阿片口”的釋放，而這些神經傳導物質的結構和成癮屬性類似於阿片口（阿片口是通過刺激受體起作用，這些受體是為了應腦部的阿口片進化而成）。這有助於理解垃圾食品在壓力時刻的巨大強化屬性。 壓力還會激活腦中的“內源性大麻素”系統。是的，大腦中有一類化學物質就像大麻中所含那種導致吸食後想吃東西的成分一樣。此外，壓力還會激活另一種名叫“神經口Y”的大腦化學物質，它可以激發人對脂肪和糖的欲望。 這種壓力效應背後的最基本機制，在於安慰食物真的讓人感到安慰。瑪麗﹒多爾曼(Mary Dallman)和加州大學舊金山分校(University of California, San Francisco)的同事利用實驗鼠首次証明，脂肪和碳水化合物會刺激大腦中的激勵機制，進而屏蔽掉身體激素的應激反應。 一種快感抵消另一種來源非常不同的不快感，看起來或許是不太可能。為什麼富含脂肪的食物會減輕老鼠對新同伴的緊張感呢？然而我們人類的跳躍常常要大得多。飽受單相思之苦？大購物常會有幫助。因懷疑人生而煩惱？聽巴赫或許有用。大腦中的激勵機制就成了“病急”之中亂投的“醫”。 但是，盡管安慰的來源各種各樣，有些安慰來源用力太猛，從而有害於我們的健康。它體現了一項我們在進化過程中的遺留物：在結束充滿壓力的一天之後，從羅伯特﹒弗洛斯特(Robert Frost)詩篇裡尋求慰藉的人，遠遠少於來一斤雙乳脂軟糖巧克力冰激凌的人。 作者是斯坦福大學(Stanford University)生物學與神經病學教授。 Robert M. Sapolsky The pantheon of science includes individuals who have made enormous contributions to human health -- the likes of Pasteur and Salk. A pedestal in that temple awaits the scientist who solves the following mystery: Why do we eat junk food when we feel unloved? This isn't a silly question, certainly not during September, which happens to be National Childhood Obesity Awareness Month. There's an epidemic of obesity-related health problems, with adult-onset diabetes leading the way throughout the world. The fact that we eat when we're not actually hungry contributes a lot to this problem. So why do we do it? It can be because everyone around us is eating. Or because food ads can be so persuasive. Or because we want to bankrupt a hated party host by eating all his Cheetos. One of the best-understood examples of non-nutritive eating is the fact that stress tends to make us eat more. It makes sense psychologically, in that the people most prone to stress eating are those most actively restricting food intake the rest of the time: When the going gets tough and they need to be nice to themselves, this is how they ease up. They prefer to eat fats and carbs. If the boss is a creep, why not run wild on the chocolate-covered walrus blubber? But we can't trace these habits merely to the complexities of the human psyche, because it's not just humans who exhibit them. Stress a lab rat by, let's say, putting an unknown rat in its cage, and it will eat more and show a stronger preference for high-fat/high-carb options than usual. This phenomenon's occurrence in many species makes evolutionary sense. For 99% of animals, stress involves a major burst of energy use as they, say, run for their lives. Afterward, the body stimulates appetite, especially for high-density calories, to rebuild depleted energy stores. But we smart, neurotic humans keep turning the stress-response on for purely psychological reasons, putting our bodies repeatedly into the restocking mode. Scientists are beginning to understand how this stress-related junk-food craving works. Stress increases the release of 'endogenous opioids' in some brain regions. These neurotransmitters resemble opiates in their structure and addictive properties (and opiates work by stimulating the receptors that evolved for responding to the brain's opioids). This helps to account for the hugely reinforcing properties of junk food at such times. Stress also activates the 'endocannabinoid' system in the brain. Yes, there's a class of chemicals in the brain that resemble the ingredient in cannabis that famously links pot to getting the munchies. And stress activates another brain chemical called neuropeptide Y that can stimulate the craving for fat and sugar. The most fundamental mechanism to explain this stress effect is that comfort food is, well, comforting. As first demonstrated by Mary Dallman and colleagues at the University of California, San Francisco, working with lab rats, fat and carbs stimulate reward systems in the brain, thereby turning off the body's hormonal stress-response. It may seem unlikely that one type of pleasure works to offset the effects of a very different source of displeasure. Why should fat-laced rat chow lessen angst about a new cage mate? Yet we regularly make much bigger leaps. Burdened with unrequited love? Shopping often helps. Roiled with existential despair? Bach might do the trick. The common currency of reward in the brain makes for all sorts of unlikely ports in a storm. But despite the varied possibilities of sources of comfort, some exert particularly strong primal pulls -- to the detriment of our health. It is a sign of our evolutionary legacy that, at the end of a stressful day, far fewer of us will seek solace in the poetry of Robert Frost than in a pint of double fudge brownie ice cream. Robert M. Sapolsky is a professor of biology and neurology at Stanford University and the author of many books.