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.

    许多人刚从健身房出来会吃得更多，而且是像炸面包圈这样的垃圾食品。这或许是因为锻炼让我们感觉饥饿，也可能是我们想要犒赏自己。

    当我写这篇文章的时候，第二天是星期二，计划是锻炼心肺功能。我会花5分钟在心肺训练器上进行热身，这种立式器材需要你的胳膊和腿协调运动。然后我会在模拟爬楼的器械练30分钟。星期三会有一个私人教练在一个小时里把我当做农场的动物一样折腾，有时会让我感到眩晕，并且奢侈地花掉了和我一个礼拜购物一样多的钱。星期四是"楔身"课，是另外一种锻炼花样，我得花一个小时用一种难受的方式把自己推进一个大泡沫楔形器械中。星期五跑5.5英里，那多出的半英里是对本周自己放纵饮食的赎罪。

    我已经这样着迷地、严格地锻炼了多年，而最近我开始奇怪：为什么我要做这些？除了一段不幸爱情结束后的两年时间里靠着意大利美食治愈伤痛以外，我从来没有超重过。在我们的文化传统中，有一个被广泛接受并不断重复的理论：（锻炼）能让人体重下降。然而我不停地锻炼，并且是在结束那段感情和停止美食治疗之后，我的体重回到了163磅，和我成年期绝大部分时候的体重一样，但当我坐下来时仍然有腰腹脂肪挂在腰部。为何所有的锻炼没能赶走它们？

    这是一个我们许多人都会问的问题。现在参加健康俱乐部的美国人已经从1993年的2千3百万增加到了4千5百万，我们每年花在健身协会上的钱差不多有190亿。当然，有些人加入了但从没去过（健身房）.尽管如此，正如一个主要研究机构-明尼苏达心脏调查机构所发现的，越来越多的人会说至少我们有规律地进行了锻炼。这项调查开始于1980年，当时只有 47%的被调查者说他们进行了有规律的锻炼，到2000年，这个数字已经上升到了57%.

    然而戏剧性的是，同一时期的肥胖症患者也增加了：按照联邦政府的定义，三分之一的美国人肥胖，另三分之一的美国人超重。是的，这很有可能是因为我们之中某些人到健身房进行了有规律的锻炼，但一旦减少锻炼会增重更多。但是和另外许多人一样，我会在锻炼后感到饥饿，所以我在锻炼后吃的比不锻炼时多。难道锻炼实际上是在保持我的体重不会减少吗？

    锻炼对减肥来说很重要，这个传统观点实际上是相当新的。就在二十世纪60年代，医生们还一贯反对高强度锻炼，特别是对于可能让自己受伤的老年人。今天的医生们则会鼓励病人甚至是最年老的病人也去锻炼。这个建议很合理，理由有很多：经常锻炼的人患各种疾病的风险大大降低，特别是心脏方面，他们得患癌症、糖尿病和其他病症的几率较低，不过过去几年对肥胖症的研究显示：锻炼对于减肥的作用被过分夸大了。

    "一般来说，锻炼对减肥的影响不大，"艾瑞克。拉弗森说。他是路易斯安那州大学的糖尿病和新陈代谢专家，也是一名优秀的运动生理学者。近期许多研究发现：锻炼对帮助人们减肥的重要性并不如健身房广告或电影"最大的窝囊废"或是像杂志里听到的那样。

    有个简单的问题：如果锻炼能消耗热量并且消耗了热量就可以减肥的话，锻炼也会产生另一个效果：刺激食欲。这会让我们吃的更多，到头来我们刚刚得到的减肥成果白费。换句话说，锻炼对于帮助减肥并非必要条件，甚至会让减肥更困难。

    补偿问题

    今年年初，标准期刊"PloS ONE"（"PloS"是一家非盈利科学图书馆）发布了一个由拉弗森的同事-迪蒙森。彻奇博士主持的着名研究成果，彻奇博士在路易斯安那州立大学健康学方面享有盛誉。彻奇的团队将464位不常锻炼的女性随机分为4组。其中三组被要求分别和私人教练每周锻炼72分钟、136分钟和194分钟，持续6个月。第四组也就是控制组的女性被告知维持她们原有的身体活动规律。所有的女性被要求不改变其饮食习惯并填写每月的病征问卷。

    试验的发现令人惊讶。平均来说每个组甚至是控制组的女性体重都下降了，但是那些跟着教练一周数天连续6个月挥汗如雨的人并没有比控制组的人明显减少了更多的重量。（控制组的女性也许因为填写健康问卷的缘故，促使她们少吃了些炸面包圈，从而体重有所下降）.每个组中都有一些人确实增加了体重，有些人每人增加了10磅。

    怎么回事呢？虽然彻奇称其为补偿，但你我都知道那就是在一场辛苦的锻炼后走出健身房时，人们砸吧着嘴巴期待着的一顿咸淡合适颜色焦黄的法式炸鸡。不管是因为锻炼之后的饥饿还是因为她们想犒劳自己（或两者皆是）,多数进行锻炼的女性会比她们开展试验前吃的更多，或者以回家后大幅减少活动量的方式获得了补偿。

    这个发现很重要，因为政府和各种医疗组织对于想要减肥的人们开出了增加再增加锻炼的处方。2007年美国运动医学院和美国心脏医学联会发布的新指导方针这样说："要降低体重……可能需要60到90分钟的身体运动。"对于我们这些正在想要保住工作或找工作的人来说，占用一周几乎每天60到90分钟（进行锻炼）不仅是不现实的，而且根据彻奇的基础数据，还会很容易导致大量的补偿性饮食。

    在经过6个月的锻炼后，彻奇的研究项目中的多数锻炼者确实能够稍微减少她们的腰围-大约一英寸。尽管如此，与控制组的人相比，她们并没有减掉更多的全身脂肪，为什么呢？

    现年41岁并曾在巴顿鲁治居住过近3年时间的彻奇有一个理论。"我恰巧发现这个情况在我妻子的朋友们中存在"他说。"比如她们说'啊，我现在每天跑一个小时，但我体重一点没减'".彻奇问她们，"那么跑步之后你们做了什么？"结果是一群朋友随后停在辛巴克门口吃松饼。彻奇说："我想多数人不会重视这个问题。哎，你们刚刚才消耗了200或300卡路里，就马上用那半块松饼找回了平衡".

    你们也许会认为一整天才吃半块松饼不会有什么关系，特别是经常锻炼。究竟锻炼能不能将脂肪转化成肌肉？难道肌肉也不能比脂肪更有效率地消耗多余的热量吗？

    尽管肌肉和脂肪的关系常常被误解，但答案是肯定的。根据2001年哥伦比亚大学团队所出版的过度肥胖研究期刊计算，一磅肌肉在身体休息时月消耗6卡路里的能量，而相比之下一磅脂肪只能消耗2卡路里。这说明当你艰苦锻炼转换了10磅脂肪为肌肉后-经过粗略计算-你每天只能多摄入约40卡路里热量才能保证不增加，这大约是一勺黄油的热量，还得上帝保佑。

    基本上，人类可不是那种不考虑生存需要而随意抛弃额外热量的种族。对于其他种族的老鼠来说，它有一个比人类大得多的可容纳脂肪的空间，因为其有较多的一种被称为棕色脂肪的组织。棕色脂肪有助于产生一种蛋白质，这种蛋白质将一种被称作线粒体的小细胞单元关闭，而线粒体是细胞的能量工厂：它们帮助将营养转化为能量。当它们被关闭时，动物的能量不会增加。相反，动物才会越来越温暖。当它们的温度上升时，燃烧卡路里的效率则会下降。