By Gretchen Reynolds

For some time, researchers have been finding that people who exercise don’t necessarily lose weight. A study published online in September in The British Journal of Sports Medicine was the latest to report apparently disappointing slimming results. In the study, 58 obese people completed 12 weeks of supervised aerobic training without changing their diets. The group lost an average of a little more than seven pounds, and many lost barely half that.

How can that be? Exercise, it seems, should make you thin. Activity burns calories. No one doubts that.

“Walking, even at a very easy pace, you’ll probably burn three or four calories a minute,” beyond what you would use quietly sitting in a chair, said Dan Carey, Ph.D., an assistant professor of exercise physiology at the University of St. Thomas in Minnesota, who studies exercise and metabolism.

But few people, an overwhelming body of research shows, achieve significant weight loss with exercise alone, not without changing their eating habits. A new study from scientists at the University of Colorado School of Medicine in Denver offers some reasons why. For the study, the researchers recruited several groups of people. Some were lean endurance athletes; some sedentary and lean; some sedentary and obese. Each of the subjects agreed to spend, over the course of the experiment, several 24-hour periods in a special laboratory room (a walk-in calorimeter) that measures the number of calories a person burns. Using various calculations, the researchers could also tell whether the calories expended were in the form of fat or carbohydrates, the body’s two main fuel sources. Burning more fat than carbohydrates is obviously desirable for weight loss, since the fat being burned comes primarily from body fat stores, and we all, even the leanest among us, have plenty of those.

The Denver researchers were especially interested in how the athletes’ bodies would apportion and use calories. It has been well documented that regular endurance training increases the ability of the body to use fat as a fuel during exercise. They wondered, though, if the athletes — or any of the other subjects — would burn extra fat calories after exercising, a phenomenon that some exercisers (and even more diet and fitness books) call “afterburn.”

“Many people believe that you rev up” your metabolism after an exercise session “so that you burn additional body fat throughout the day,” said Edward Melanson, Ph.D., an associate professor in the division of endocrinology at the School of Medicine and the lead author of the study. If afterburn were found to exist, it would suggest that even if you replaced the calories you used during an exercise session, you should lose weight, without gaining weight — the proverbial free lunch.

Each of Melanson’s subjects spent 24 quiet hours in the calorimeter, followed later by another 24 hours that included an hourlong bout of stationary bicycling. The cycling was deliberately performed at a relatively easy intensity (about 55 percent of each person’s predetermined aerobic capacity). It is well known physiologically that, while high-intensity exercise demands mostly carbohydrate calories (since carbohydrates can quickly reach the bloodstream and, from there, laboring muscles), low-intensity exercise prompts the body to burn at least some stored fat. All of the subjects ate three meals a day.

To their surprise, the researchers found that none of the groups, including the athletes, experienced “afterburn.” They did not use additional body fat on the day when they exercised. In fact, most of the subjects burned slightly less fat over the 24-hour study period when they exercised than when they did not.

“The message of our work is really simple,” although not agreeable to hear, Melanson said. “It all comes down to energy balance,” or, as you might have guessed, calories in and calories out. People “are only burning 200 or 300 calories” in a typical 30-minute exercise session, Melanson points out. “You replace that with one bottle of Gatorade.”

This does not mean that exercise has no impact on body weight, or that you can’t calibrate your workouts to maximize the amount of body fat that you burn, if that’s your goal.

“If you work out at an easy intensity, you will burn a higher percentage of fat calories” than if you work out a higher intensity, Carey says, so you should draw down some of the padding you’ve accumulated on the hips or elsewhere — if you don’t replace all of the calories afterward. To help those hoping to reduce their body fat, he published formulas in The Journal of Strength and Conditioning Research last month that detailed the heart rates at which a person could maximize fat burning. “Heart rates of between 105 and 134” beats per minute, Carey said, represent the fat-burning zone. “It’s probably best to work out near the top of that zone,” he says, “so that you burn more calories over all” than at the extremely leisurely lower end.

Perhaps just as important, bear in mind that exercise has benefits beyond weight reduction. In the study of obese people who took up exercise, most became notably healthier, increasing their aerobic capacity, decreasing their blood pressure and resting heart rates, and, the authors write, achieving “an acute exercise-induced increase in positive mood,” leading the authors to conclude that, “significant and meaningful health benefits can be achieved even in the presence of lower than expected exercise-induced weight loss.”

Finally and thankfully, exercise seems to aid, physiologically, in the battle to keep off body fat once it has been, through resolute calorie reduction, chiseled away. In other work by Melanson’s group, published in September, laboratory rats that had been overfed and then slimmed through calorie reduction were able to “defend” their lower weight more effectively if they ran on a treadmill and ate at will than if they had no access to a treadmill. The exercise seemed to reset certain metabolic pathways within the rats, Melanson says, that blunted their body’s drive to replace the lost fat. Similar mechanisms, he adds, probably operate within the bodies of humans, providing scientific justification for signing up for that Thanksgiving Day 5K.

http://well.blogs.nytimes.com/2009/...to-weight-loss/

OK, so no afterburn in this one. Who cares though because weight was lost anyway, right? But then, the weight's going to come right back when we stop the exercise so what's the point. Ah, the point is to keep eating the same thing. Well, that appears to be impossible no matter how we look at it. Incidentally, cutting carbs will produce a much greater weight loss, even without exercise, than exercise alone, or even exercise combined with simple caloric reduction. 12 weeks, 7 lbs? Not even close.

How is this the message from this study?



But doesn't that contradict this statement: "In fact, most of the subjects burned slightly less fat over the 24-hour study period when they exercised than when they did not."

The body is compensating for the extra expenditure of energy. And, as Martin points out, you'll be hungrier to boot.



This is true - at least for me. I very quickly - in a matter of weeks - reduced my resting heart rate from about 93 to about 75 by walking a half hour most days.

Which means many lots quite a bit more. Looking at the data it appears that there are responders, and non-responders with very few getting "average" results which really makes the average a meaningless statistic. The real question is why do some lose via exercise while others do not.

I've heard of similar results with other studies. Sometimes the average loss is about zero, with some losing and some gaining.

I've also heard that people who are already close to their normal weights are the ones most likely to lose. The obese are the one who tend not to lose with exercise. [Don't remember where I heard this, so can't cite the source.]

BTW, kdill, I'm not the source of the quote you've cited. That comes from the original article.

Nothing new, Taubes wrote this in 2007 in the NYT and received a lot of flack for it.

Didn't their study just prove that it really doesn't?

This has been my experience (overall health indicators significantly improved from exercise and I am much more happy and active and alive), but if you want to lose significant weight, cut the carbs, exercise is not necessary.

Maybe it does, or maybe it doesn't. But, coming from someone whose never been a normal weight, exercise is enabling me to get to know how my body moves at a normal weight and like the above poster said, it makes me feel alive and well.

I'm not out to become a star athlete. It just makes me feel good.

Just sloppy cutting and pasting on my part. Mea Culpa.

These are great research opportunities that are lost or discarded. The possibility of "responders and non-responders" being in the raw data, but discarded from lack of ........?

In the famous Ancel Keys "Seven Nations Study" in most of the countries they studied, they gathered data from two locations in that country. There were as much as fivefold differences between the local regions, with little change noted in diet and whatever they considered as other variables. They indcated that this intra-country variation was interesting and would soon be reconciled -- but it never was reconciled.

They took this highly variable data for a country and merged it with (Variable) data from other countries an produced the famous "Cholesterol/Fat is the cause" conclusion.

The failure to reveal why seemingly similar parts of the same country could have as high as a fivefold variation in CHD is one of the great lost opportunities of diet science.

Some have argued that the purpose of the study was to verify the "diet Heart" hypothesis, and they were going to darn well do that somehow. And, they did.

Arguably, bodybuilders as a group know more about gaining and losing weight than any other group combined.

The prevailing wisdom in bodybuilding is that diet controls mass in a general sense, while exercise or more specifically lifting heavy objects stimulates the growth and retention of lean tissue, first during the grow phase, and then during the cut phase. In other words, bodybuilders don't use exercise to lose weight, they use it to retain mass during the caloric deficit phase, phase which is intended to reduce mass.

This implies that they understand that without the exercise, they'd lose lean and fat mass in a fixed proportion, i.e. they'd just end up a smaller version of themselves, and not a cut version of their bigger self. On the other hand, this implies that they don't actually understand what in the diet makes them grow bigger or smaller, as the case may be. But more specifically, they don't understand what makes their fat tissue bigger or smaller. Because, obviously, they know that to grow lean tissue, they need a boatload of protein. But I digress.

The point is that exercise is not used to lose fat mass but instead to grow and retain lean mass. By all accounts, for that purpose, exercise works.

There's a simple explanation. In Ancel Keys' paper, the data was willfully incomplete. He omitted the data that refuted his hypothesis. He lied:
http://high-fat-nutrition.blogspot....een%20countries

Actually, given the fact that he discarded all data that didn't fit his hypothesis (first hint of this is he gathered data from 22 countries, not seven), I'd say that I'd say that's a pretty compelling argument. The only thing that surprises me is now that this is common knowledge that people still think the lipid hypothesis is valid. People have built their entire careers on this being true, so I don't see views actually changing for a long time yet

'science advances one funeral at a time', old cynical adage. The hard sciences have got so used to the accelerated change in understanding that it is no longer true for them. Nutrition at the level we use it is not yet a science. There are scientists working at it, and they are doing some great things, but at the level which interfaces with us it is all by guess and by golly. Medicine itself is only a few steps in the direction of being a hard science.

Except astronomy. Then again that is such an indirect science, maybe that's why.

What I see is that any 'science' which has attempted to structure its study via 'psychology' has failed abysmally (including a great deal of 'psychology' itself). The science of nutrition was doing ok when it was a subset of endocrinology led by the germans. When the americans took it up, ignoring history, starting over, and started relegating it to the armchair instead of the test tube, is when things really went poorly. There ought to be a lesson there somewhere.