I’ve written about various aspects of exercise for this blog over the past several years, including what to eat after exercise, getting back into an exercise habit after a break, and how to just get started moving around more. One of the exercise topics that I find the most interesting, however, revolves around the tendency that we have to consider our workouts to be more strenuous (and therefore more intensely calorie-burning) than they truly are—and how that impacts our food intake. I have written about how not to out-eat your workout previously (what might be called compensatory eating), but this time I’m going to focus on the science behind why we shouldn’t rely on exercise for weight loss.
Before I get into the bulk of this post, I do want to say that exercise is so valuable to health on a number of levels physically and mentally, that we absolutely should get regular exercise. This is not a post that discounts the value of exercise to humans—it’s certainly important. Even without weight loss, exercise improves health. Exercise can impact body composition (proportions of fat and lean tissues). My goal with this post is to show that an emphasis on exercise as a means to weight loss is misplaced.
Although it flies in the face of a commonly-recommended weight-loss strategy, combining exercise with eating less food isn’t necessarily more effective for losing weight than calorie restriction alone. It’s kind of shocking, really, as it makes logical sense that it would be easier to take in fewer calories if you could work some of them off and then just trim your intake a little. At least, it would seem easier. But, studies show that adding exercise to caloric restriction doesn’t make much difference in weight lost. Why might that be? Here are some reasons to consider…
Exercise only accounts for a small amount of daily energy use.
Our bodies use energy from our food in several ways; primarily for doing the body’s general work to keep us alive (basal metabolism), to support physical activity, and to break down the food we eat. The biggest component of our energy use is basal metabolism, at roughly 60-70%, and the smallest is the digestion of food at around 10%. That leaves 20-30% to cover physical activity—and that includes all types of activity during the day—not just actual exercise time (one study estimates exercise accounts for 7%-9% of total energy use). Given this, it’s not hard to see that even a daily 1-hour workout isn’t a major factor in the body’s energy expenditure. Interestingly, research shows that the relationship between exercise and energy expenditure is most significant at the lower end of physical activity levels. Those in the upper range of physical activity experience a plateau in energy expenditure. In other words, there appears to be an upper limit on the amount of energy the body will allow for exercise—the body tapers back on the amount of energy it expends when you reach a high level of exercise. The hypothesis is that the body is trying to conserve its resources. More research on this effect is needed, but if nothing else, it adds to the very complicated picture of the relationship between activity, metabolism and body weight.
Weight loss isn’t just a matter of calories in, calories out.
While the traditional “calories in, calories out” model of energy balance is easy to understand, it’s not all that accurate. On a basic level, it still holds—if you take in more energy (calories through food) than your body needs for weight maintenance, you will gain weight/add fat to your body. However, “Wishnofky’s rule” from 1958—that the equivalent of 1 pound of weight gained or lost is equal to 3,500 calories, is now recognized as too simplistic. Energy balance is a dynamic system that involves more than just adding or subtracting calories like a bank. The body adapts in several ways as weight loss progresses, so estimating weight loss using this simple equation doesn’t hold up as time goes by. For a more accurate way to predict weight loss, check out the NIH’s Body Weight Planner tool.
It takes a lot of exercise to make a real caloric impact (and your tracker isn’t accurate).
Adding exercise sessions to your life is good for you, no doubt, but the fact is that exercising doesn’t burn all that may calories—at least the way most people do it (for an hour or less each session, and not at all-out intensity levels). For a 155-pound person, 30 minutes of moderate biking on a stationary bicycle burns 260 calories. It’s really not that much when you consider that you may only be doing it a few times a week. Oh, and by the way, don’t rely on your Fitbit to know how many calories you’ve burned during your workout. Research shows that the wrist-worn trackers (several brands) were all at least 20% off in estimating calories burned, and gym equipment is notoriously inaccurate as well, for a variety of reasons.
We compensate for exercise in ways that can negate its calorie-burning contribution.
There are aspects of both human nature and physiology that end up working against any impact that exercise might make on weight. One of those, metabolic adaptation, was touched on above, but other compensatory measures include eating more after exercise in response to hunger (especially after vigorous workouts) and decreasing our energy output post exercise by being more sedentary. These adjustments are largely responses to physical cues and bodily processes. However, the mental aspect of compensatory eating can’t be ignored. Research shows that when people believe they’ve expended a higher amount of calories through a workout vs. a lower amount, they let themselves eat more afterward. It’s also been shown that when a workout is labeled as “fat burning,” exercisers allow themselves more food post workout than when the session is labeled as an “endurance” workout. Between all of these types of compensations, it’s not surprising that any progress made toward a caloric deficit is fairly easily negated later.
What’s the bottom line?
Exercise is good for our health, and it’s wise to include regular exercise in your life. Just don’t tie it in with your weight. Expecting your workouts to impact your weight in a significant way is misguided. Instead, try to enjoy and embrace exercise for its own sake, and for what it does to help your body function well and feel good.