Whether because exercise makes us hungry or because we want to reward ourselves, many people eat more - and eat more junk food, like doughnuts - after going to the gym.

    As I write this, tomorrow is Tuesday, which is a cardio day. I'll spend five minutes warming up on the VersaClimber, a towering machine that requires you to move your arms and legs simultaneously. Then I'll do 30 minutes on a stair mill. On Wednesday a personal trainer will work me like a farm animal for an hour, sometimes to the point that I am dizzy - an abuse for which I pay as much as I spend on groceries in a week. Thursday is "body wedge" class, which involves another exercise contraption, this one a large foam wedge from which I will push myself up in various hateful ways for an hour. Friday will bring a 5.5-mile run, the extra half-mile my grueling expiation of any gastronomical indulgences during the week.

    I have exercised like this - obsessively, a bit grimly - for years, but recently I began to wonder: Why am I doing this? Except for a two-year period at the end of an unhappy relationship - a period when I self-medicated with lots of Italian desserts - I have never been overweight. One of the most widely accepted, commonly repeated assumptions in our culture is that if you exercise, you will lose weight. But I exercise all the time, and since I ended that relationship and cut most of those desserts, my weight has returned to the same 163 lb. it has been most of my adult life. I still have gut fat that hangs over my belt when I sit. Why isn't all the exercise wiping it out?

    It's a question many of us could ask. More than 45 million Americans now belong to a health club, up from 23 million in 1993. We spend some $19 billion a year on gym memberships. Of course, some people join and never go. Still, as one major study - the Minnesota Heart Survey - found, more of us at least say we exercise regularly. The survey ran from 1980, when only 47% of respondents said they engaged in regular exercise, to 2000, when the figure had grown to 57%.

    And yet obesity figures have risen dramatically in the same period: a third of Americans are obese, and another third count as overweight by the Federal Government's definition. Yes, it's entirely possible that those of us who regularly go to the gym would weigh even more if we exercised less. But like many other people, I get hungry after I exercise, so I often eat more on the days I work out than on the days I don't. Could exercise actually be keeping me from losing weight?

    The conventional wisdom that exercise is essential for shedding pounds is actually fairly new. As recently as the 1960s, doctors routinely advised against rigorous exercise, particularly for older adults who could injure themselves. Today doctors encourage even their oldest patients to exercise, which is sound advice for many reasons: People who regularly exercise are at significantly lower risk for all manner of diseases - those of the heart in particular. They less often develop cancer, diabetes and many other illnesses. But the past few years of obesity research show that the role of exercise in weight loss has been wildly overstated.

    "In general, for weight loss, exercise is pretty useless," says Eric Ravussin, chair in diabetes and metabolism at Louisiana State University and a prominent exercise researcher. Many recent studies have found that exercise isn't as important in helping people lose weight as you hear so regularly in gym advertisements or on shows like The Biggest Loser - or, for that matter, from magazines like this one.

    The basic problem is that while it's true that exercise burns calories and that you must burn calories to lose weight, exercise has another effect: it can stimulate hunger. That causes us to eat more, which in turn can negate the weight-loss benefits we just accrued. Exercise, in other words, isn't necessarily helping us lose weight. It may even be making it harder.

    The Compensation Problem

    Earlier this year, the peer-reviewed journal PLoS ONE - PLoS is the nonprofit Public Library of Science - published a remarkable study supervised by a colleague of Ravussin's, Dr. Timothy Church, who holds the rather grand title of chair in health wisdom at LSU. Church's team randomly assigned into four groups 464 overweight women who didn't regularly exercise. Women in three of the groups were asked to work out with a personal trainer for 72 min., 136 min., and 194 min. per week, respectively, for six months. Women in the fourth cluster, the control group, were told to maintain their usual physical-activity routines. All the women were asked not to change their dietary habits and to fill out monthly medical-symptom questionnaires.

    The findings were surprising. On average, the women in all the groups, even the control group, lost weight, but the women who exercised - sweating it out with a trainer several days a week for six months - did not lose significantly more weight than the control subjects did. （The control-group women may have lost weight because they were filling out those regular health forms, which may have prompted them to consume fewer doughnuts.） Some of the women in each of the four groups actually gained weight, some more than 10 lb. each.

    What's going on here? Church calls it compensation, but you and I might know it as the lip-licking anticipation of perfectly salted, golden-brown French fries after a hard trip to the gym. Whether because exercise made them hungry or because they wanted to reward themselves （or both）, most of the women who exercised ate more than they did before they started the experiment. Or they compensated in another way, by moving around a lot less than usual after they got home.

    The findings are important because the government and various medical organizations routinely prescribe more and more exercise for those who want to lose weight. In 2007 the American College of Sports Medicine and the American Heart Association issued new guidelines stating that "to lose weight … 60 to 90 minutes of physical activity may be necessary." That's 60 to 90 minutes on most days of the week, a level that not only is unrealistic for those of us trying to keep or find a job but also could easily produce, on the basis of Church's data, ravenous compensatory eating.

    It's true that after six months of working out, most of the exercisers in Church's study were able to trim their waistlines slightly - by about an inch. Even so, they lost no more overall body fat than the control group did. Why not?

    Church, who is 41 and has lived in Baton Rouge for nearly three years, has a theory. "I see this anecdotally amongst, like, my wife's friends," he says. "They're like, 'Ah, I'm running an hour a day, and I'm not losing any weight.'" He asks them, "What are you doing after you run?" It turns out one group of friends was stopping at Starbucks for muffins afterward. Says Church: "I don't think most people would appreciate that, wow, you only burned 200 or 300 calories, which you're going to neutralize with just half that muffin."

    You might think half a muffin over an entire day wouldn't matter much, particularly if you exercise regularly. After all, doesn't exercise turn fat to muscle, and doesn't muscle process excess calories more efficiently than fat does?

    Yes, although the muscle-fat relationship is often misunderstood. According to calculations published in the journal Obesity Research by a Columbia University team in 2001, a pound of muscle burns approximately six calories a day in a resting body, compared with the two calories that a pound of fat burns. Which means that after you work out hard enough to convert, say, 10 lb. of fat to muscle - a major achievement - you would be able to eat only an extra 40 calories per day, about the amount in a teaspoon of butter, before beginning to gain weight. Good luck with that.

    Fundamentally, humans are not a species that evolved to dispose of many extra calories beyond what we need to live. Rats, among other species, have a far greater capacity to cope with excess calories than we do because they have more of a dark-colored tissue called brown fat. Brown fat helps produce a protein that switches off little cellular units called mitochondria, which are the cells' power plants: they help turn nutrients into energy. When they're switched off, animals don't get an energy boost. Instead, the animals literally get warmer. And as their temperature rises, calories burn effortlessly.

    許多人剛從健身房出來會吃得更多，而且是像炸面包圈這樣的垃圾食品。這或許是因為鍛煉讓我們感覺饑餓，也可能是我們想要犒賞自己。

    當我寫這篇文章的時候，第二天是星期二，計劃是鍛煉心肺功能。我會花5分鐘在心肺訓練器上進行熱身，這種立式器材需要你的胳膊和腿協調運動。然后我會在模擬爬樓的器械練30分鐘。星期三會有一個私人教練在一個小時里把我當做農場的動物一樣折騰，有時會讓我感到眩暈，并且奢侈地花掉了和我一個禮拜購物一樣多的錢。星期四是"楔身"課，是另外一種鍛煉花樣，我得花一個小時用一種難受的方式把自己推進一個大泡沫楔形器械中。星期五跑5.5英里，那多出的半英里是對本周自己放縱飲食的贖罪。

    我已經這樣著迷地、嚴格地鍛煉了多年，而最近我開始奇怪：為什么我要做這些？除了一段不幸愛情結束后的兩年時間里靠著意大利美食治愈傷痛以外，我從來沒有超重過。在我們的文化傳統中，有一個被廣泛接受并不斷重復的理論：（鍛煉）能讓人體重下降。然而我不停地鍛煉，并且是在結束那段感情和停止美食治療之后，我的體重回到了163磅，和我成年期絕大部分時候的體重一樣，但當我坐下來時仍然有腰腹脂肪掛在腰部。為何所有的鍛煉沒能趕走它們？

    這是一個我們許多人都會問的問題。現在參加健康俱樂部的美國人已經從1993年的2千3百萬增加到了4千5百萬，我們每年花在健身協會上的錢差不多有190億。當然，有些人加入了但從沒去過（健身房）.盡管如此，正如一個主要研究機構-明尼蘇達心臟調查機構所發現的，越來越多的人會說至少我們有規律地進行了鍛煉。這項調查開始于1980年，當時只有 47%的被調查者說他們進行了有規律的鍛煉，到2000年，這個數字已經上升到了57%.

    然而戲劇性的是，同一時期的肥胖癥患者也增加了：按照聯邦政府的定義，三分之一的美國人肥胖，另三分之一的美國人超重。是的，這很有可能是因為我們之中某些人到健身房進行了有規律的鍛煉，但一旦減少鍛煉會增重更多。但是和另外許多人一樣，我會在鍛煉后感到饑餓，所以我在鍛煉后吃的比不鍛煉時多。難道鍛煉實際上是在保持我的體重不會減少嗎？

    鍛煉對減肥來說很重要，這個傳統觀點實際上是相當新的。就在二十世紀60年代，醫生們還一貫反對高強度鍛煉，特別是對于可能讓自己受傷的老年人。今天的醫生們則會鼓勵病人甚至是最年老的病人也去鍛煉。這個建議很合理，理由有很多：經常鍛煉的人患各種疾病的風險大大降低，特別是心臟方面，他們得患癌癥、糖尿病和其他病癥的幾率較低，不過過去幾年對肥胖癥的研究顯示：鍛煉對于減肥的作用被過分夸大了。

    "一般來說，鍛煉對減肥的影響不大，"艾瑞克。拉弗森說。他是路易斯安那州大學的糖尿病和新陳代謝專家，也是一名優秀的運動生理學者。近期許多研究發現：鍛煉對幫助人們減肥的重要性并不如健身房廣告或電影"最大的窩囊廢"或是像雜志里聽到的那樣。

    有個簡單的問題：如果鍛煉能消耗熱量并且消耗了熱量就可以減肥的話，鍛煉也會產生另一個效果：刺激食欲。這會讓我們吃的更多，到頭來我們剛剛得到的減肥成果白費。換句話說，鍛煉對于幫助減肥并非必要條件，甚至會讓減肥更困難。

    補償問題

    今年年初，標準期刊"PloS ONE"（"PloS"是一家非盈利科學圖書館）發布了一個由拉弗森的同事-迪蒙森。徹奇博士主持的著名研究成果，徹奇博士在路易斯安那州立大學健康學方面享有盛譽。徹奇的團隊將464位不常鍛煉的女性隨機分為4組。其中三組被要求分別和私人教練每周鍛煉72分鐘、136分鐘和194分鐘，持續6個月。第四組也就是控制組的女性被告知維持她們原有的身體活動規律。所有的女性被要求不改變其飲食習慣并填寫每月的病征問卷。

    試驗的發現令人驚訝。平均來說每個組甚至是控制組的女性體重都下降了，但是那些跟著教練一周數天連續6個月揮汗如雨的人并沒有比控制組的人明顯減少了更多的重量。（控制組的女性也許因為填寫健康問卷的緣故，促使她們少吃了些炸面包圈，從而體重有所下降）.每個組中都有一些人確實增加了體重，有些人每人增加了10磅。

    怎么回事呢？雖然徹奇稱其為補償，但你我都知道那就是在一場辛苦的鍛煉后走出健身房時，人們砸吧著嘴巴期待著的一頓咸淡合適顏色焦黃的法式炸雞。不管是因為鍛煉之后的饑餓還是因為她們想犒勞自己（或兩者皆是）,多數進行鍛煉的女性會比她們開展試驗前吃的更多，或者以回家后大幅減少活動量的方式獲得了補償。

    這個發現很重要，因為政府和各種醫療組織對于想要減肥的人們開出了增加再增加鍛煉的處方。2007年美國運動醫學院和美國心臟醫學聯會發布的新指導方針這樣說："要降低體重……可能需要60到90分鐘的身體運動。"對于我們這些正在想要保住工作或找工作的人來說，占用一周幾乎每天60到90分鐘（進行鍛煉）不僅是不現實的，而且根據徹奇的基礎數據，還會很容易導致大量的補償性飲食。

    在經過6個月的鍛煉后，徹奇的研究項目中的多數鍛煉者確實能夠稍微減少她們的腰圍-大約一英寸。盡管如此，與控制組的人相比，她們并沒有減掉更多的全身脂肪，為什么呢？

    現年41歲并曾在巴頓魯治居住過近3年時間的徹奇有一個理論。"我恰巧發現這個情況在我妻子的朋友們中存在"他說。"比如她們說'啊，我現在每天跑一個小時，但我體重一點沒減'".徹奇問她們，"那么跑步之后你們做了什么？"結果是一群朋友隨后停在辛巴克門口吃松餅。徹奇說："我想多數人不會重視這個問題。哎，你們剛剛才消耗了200或300卡路里，就馬上用那半塊松餅找回了平衡".

    你們也許會認為一整天才吃半塊松餅不會有什么關系，特別是經常鍛煉。究竟鍛煉能不能將脂肪轉化成肌肉？難道肌肉也不能比脂肪更有效率地消耗多余的熱量嗎？

    盡管肌肉和脂肪的關系常常被誤解，但答案是肯定的。根據2001年哥倫比亞大學團隊所出版的過度肥胖研究期刊計算，一磅肌肉在身體休息時月消耗6卡路里的能量，而相比之下一磅脂肪只能消耗2卡路里。這說明當你艱苦鍛煉轉換了10磅脂肪為肌肉后-經過粗略計算-你每天只能多攝入約40卡路里熱量才能保證不增加，這大約是一勺黃油的熱量，還得上帝保佑。

    基本上，人類可不是那種不考慮生存需要而隨意拋棄額外熱量的種族。對于其他種族的老鼠來說，它有一個比人類大得多的可容納脂肪的空間，因為其有較多的一種被稱為棕色脂肪的組織。棕色脂肪有助于產生一種蛋白質，這種蛋白質將一種被稱作線粒體的小細胞單元關閉，而線粒體是細胞的能量工廠：它們幫助將營養轉化為能量。當它們被關閉時，動物的能量不會增加。相反，動物才會越來越溫暖。當它們的溫度上升時，燃燒卡路里的效率則會下降。