Exercise boosts the metabolism? It seems the reverse might be true

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I want to preface this post by saying I am a huge advocate of exercise, preferably outside if weather conditions allow. I believe there are physical and psychological benefits to being active, and I even ‘walk the talk’: although I do not run any more, I am a notorious walker and swim regularly. I also do a quick resistance-based exercise regime at home (or in a hotel room, say) most days.

But while I strongly advocate activity and exercise, I have more than once written about the limited role that exercise, as is commonly advised, has in weight control. Those seeking to attain or maintain a healthy weight are often advised to walk, jog or cycle regularly. The idea here is that burning extra calories through exercise will assist weight loss, and the theory certainly seems to make sense. The problem is, when researchers have studied the impact of regular exercise on weight loss, the results have been pretty dismal. When added to dietary change over the medium term (e.g. few months), regular exercise boosts weight loss by about 2 lbs on average. In other words, if someone were to lose 20 lbs over 4 months through dietary change, adding regular aerobic exercise to this would, generally speaking, lead to a loss of 22 lbs. (Remember, though, there are other benefits to be had from exercise).

The idea that activities such as running and cycling (aerobic exercise) are not particularly effective for weight loss is counter-intuitive. Some people imagine that individuals must be losing fat and gaining muscle. But aerobic activity will build minimal muscle, if any at all. So, what rational explanations exist for the observation that aerobic exercise does not translate into significant weight loss for many?

Well, the first thing is that exercise does not burn much in the way of calories unless we’re doing it in very significant quantities. Let’s say you jog for half an hour and burn about 200 calories more than you would have burned sitting down. That’s obviously better than nothing, but this is not a ton of calories, and as there are about 3,500 calories in a pound of fat, theoretically you’d have to do 17 or 18 of those half-hour runs before you’d lose a pound of fat from your body. That, for many, would not seem like a particularly worthwhile return on investment.

But another problem with exercise is its tendency to stimulate the appetite. And even if it does not do that, some may ‘reward’ themselves with food or drink (e.g. alcohol) after exercise. And it doesn’t usually take too many additional calories to undo the calorie deficit induced by exercise.

I’ve written about these factors before, and write about them again here as a prelude to writing about something I learned when I was on the ‘low-carb cruise’ in May. One of the other speakers on the programme was diet and exercise researcher Dr Jeff Volek from the Neag School of Education at the University of Connecticut. Jeff’s presentation on the cruise included details on how a low-carbohydrate diet can stimulate fat-burning during exercise (more about that in another post, perhaps). During his presentation, he remarked that (as we know), exercise is not a very powerful weight loss tool.

However, he went on to talk about a mechanisms here that came as quite a surprise to the audience, I think: aerobic exercise can suppress the metabolic rate. We’re often told that exercise not only increases calorie burn while we’re exercising, and also for some time after. It turns out, that may well not be the case for many people. In fact, according to research, the opposite is quite likely to be the case.

Jeff has written a book with his colleague Dr Stephen Phinney called the Art and Science of Low Carbohydrate Performance which contains this passage:

There are 4 well-controlled, inpatient, metabolic ward studies (the gold standard for human research) published from 1982 thru 1997 that showed statistically significant reductions in resting metabolic rate when overweight subjects performed 300-600 Calories per day of endurance exercise for weeks at a time [1-4]. There are no equally rigorous human studies showing the opposite. There are animal (rat) studies that show the opposite, and there are human studies done under less controlled conditions that show the opposite. However there are also similarly less rigorous studies that agree with the above four gold-standard studies. When the quality/rigor of the studies is taken into account, the weight of the evidence supports two main conclusions:

1. Humans vary one-from-another in how their metabolism responds to endurance exercise, and much of this inter-individual variation is inherited (genetic). Given this wide individual variance, studies involving small numbers of subjects could get differing results based on random chance.
2. Although genetically lean people as a group may respond differently, when overweight humans do more than one hour of endurance exercise daily, resting metabolism on average declines between 5 and 15%.

The fascinating question is, if our interpretation of this published literature turns out to be correct, then how come most doctors, dietitians, and sports scientists think the opposite? Part of the answer is that there is a lot of simple logic suggesting that exercise speeds resting metabolism. First, exercise builds muscle, and muscle burns energy even at rest. Second, there are a lot of skinny athletes out there who think they are skinny because they train hard (as opposed to being able to train hard because they are skinny). Third, it is a common observation that heavy people tend not to exercise much, so it is easy to blame their weight problem on a lack of exercise. And finally, everyone loves a ‘2-for-the-price-of-one’ sale. It’s just way too tempting to think that you could burn 600 Calories during a 1-hour run and then, as a result, burn another 600 Calories over the course of the next day?

They go on to say that:

We are not saying that exercise isn’t good for people. Both of us are personally committed to leading vigorous lives, and encouraging others to consider doing the same. What we object to, however, is mis-informing the public as to what and how much benefit they can expect from exercise, particularly as it pertains to weight loss. From our perspective, telling heavy people to exercise because it speeds resting metabolism (and thus markedly increasing one’s rate of weight loss) is about as credible as selling them the Brooklyn Bridge.

The idea that the body would down-regulate the metabolism in response to exercise makes, I think, intuitive sense. We know, for example, that when people consciously cut calories to lose weight, it very often puts a sizeable dent in the metabolism. This is probably part of a survival mechanism (the body doesn’t know we’re not going to starve ourselves to death, and will put into play mechanisms which help the body preserve its weight and fat stores). It’s not too difficult to imagine that the body would have a similar response to increased calorie expenditure (through exercise).

None of this should put you off taking exercise if that’s what you like to do and are physically able. However, these observations may go some way to explain why all the effort you may be putting in pounding the streets or exercising on a treadmill or cross-trainer are not causing the pounds to melt away. My experience tells me that most bang for the buck for weight loss is had by getting the diet right. For me, that means a diet based on real food that it generally higher in fat and lower in carbohydrate than the diet we are traditionally advised to eat. The scientific rationale for such a diet is explained in my book Escape the Diet Trap.


1. Bouchard C, et al. The response to exercise with constant energy intake in identical twins. Obes Res 1994, 2(5):400-410.

2. Heymsfield SB, et al. Rate of weight loss during underfeeding: relation to level of physical activity. Metabolism 1989, 38(3):215-223.

3. Phinney SD, et al. Effects of aerobic exercise on energy expenditure and nitrogen balance during very low calorie dieting. Metabolism 1988, 37(8):758-765.

4. Woo R, et al. Voluntary food intake during prolonged exercise in obese women. Am J Clin Nutr 1982, 36(3):478-484.

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Walking versus running

I recently read an interesting editorial in the Journal of American College of Cardiology about the relative benefits of walking and running [1]. The editorial

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