I came across a very interesting article in the Ottawa Citizen this weekend, unpleasantly titled “For Canada’s obese, exercise alone isn’t going to cut it“. The crux of the article is this – exercise will not help you lose weight. Every few months it seems that this issue pops up, including a cover article in TIME magazine last year, which Peter has previously dissected. This is a complicated issue, and given the sensational title, I wasn’t expecting much from the Citizen article. But the article is actually very well written, and includes interviews with a number of excellent researchers (including Bob Ross, who supervised my MSc, and Tim Church, who has co-authored papers with both Peter and I), as well as physician Yoni Freedhoff of Weighty Matters. Since this issue comes up so frequently, and because of its public health importance, I thought this would be an excellent opportunity to “weigh in” with my opinion.
So, does exercise reduce body weight? To be completely honest, it depends on the situation.
Efficacy vs Effectiveness
In the Citizen article, they explain that some studies have found that exercise dramatically reduces body weight, while other studies have found it to have little to no impact. Why the discrepancy? It really boils down to study design. Some studies examine what exercise can do in an ideal situation. This is known as an efficacy study, and is the type of research that has been performed by Dr Bob Ross, with whom Peter is currently completing his PhD. For example, in one well-known study Dr Ross had a group of obese men walk or jog on a treadmill everyday for 3 months. The daily exercise session burned 700 calories, and diet was strictly controlled so that caloric intake neither went up nor down during the intervention. What happened? At the end of the 3 months, the participants lost an average of 7.5 kg (~16.5 lbs), through exercise alone!
This and other similar studies in women and the elderly have clearly shown that exercise alone can reduce body weight, especially when caloric intake is held constant. But it should be noted that participants in these studies are closely followed, with participants taking detailed diet records to make sure that caloric intake neither increases nor decreases, while every exercise session is strictly monitored by study staff. In other words, these participants represent a best case scenario, and a lot of time and effort goes into ensuring that they strictly follow both their nutritional and exercise plans.
Now of course the vast majority of people don’t have trained staff following their every move. This is where effectiveness studies come into play. Instead of looking at the ideal situation, effectiveness studies look at what happens in everyday life. And in these studies, exercise often has a much more modest impact on body weight. For example, a very large study performed by Dr Tim Church and colleagues examined the impact of 6 months of aerobic exercise in obese, post-menopausal women. The women were randomized to three separate conditions – one group expended 4 calories/kg/week, one group expended 8calories/kg/week, and the final group expended 12 calories/kg/week. None of the groups were told to change their diet in any way. What happened? Surprisingly, all three groups lost about the same amount of weight – roughly 5 lbs. What’s more, the weight loss was no different from control subjects who didn’t exercise at all! How did this happen? Dr Church offers his best guess in the Citizen article:
“I think it was because they were rewarding themselves,” says lead author Dr. Timothy Church, a medical doctor and professor of preventive medicine at Pennington Biomedical Research Center at Louisiana State University. “That’s what we heard anecdotally: ‘Man, I’ve made all my sessions this week. The wheels are off, I’m going to have a big dinner.”
This is more like the situation that people commonly experience. If they are exercising without worrying about their diet, they may lose a few pounds, but probably not as much as they’d like. But is that really the end of the world?
The most interesting findings of both of the studies that I’ve discussed so far today is what happens in response to long-term exercise in the absence of weight loss. For example, in the above-mentioned study by Dr Ross, he had another group of men expend 700 calories per day through aerobic exercise, but he had those same men increase their caloric intake by 700 kcal/day, so that they experienced no change in body weight. Even though body weight remained unchanged, these men experienced significant reductions in visceral fat, which is a very strong predictor of diabetes, cardiovascular disease, and even mortality. Similarly, even though Dr Church reported no change in body weight following 6 months of aerobic exercise, the women still showed significant improvements in aerobic fitness, waist circumference, and fasting glucose.
And this is why this whole issue of exercise and weight loss is a bit of a distraction. Research has consistently shown that physical activity is associated with a lower risk of mortality, regardless of body weight. And both of the studies that I have discussed today have shown that exercise can result in important health benefits even when there is little or no weight loss taking place. When weight loss (or weight maintenance) is the goal, exercise still has an important role, but obviously so does diet. And even without weight loss, exercise is associated with tremendous health benefits. Can we all accept these simple facts, and avoid the sensational headlines for a few months?
UPDATE: An excellent letter to the editor from Dr Mark Tremblay explaining why neither diet nor exercise trumps the other when it comes to health, and why we should move past our fixation with body weight.
Church, T., Earnest, C., Skinner, J., & Blair, S. (2007). Effects of Different Doses of Physical Activity on Cardiorespiratory Fitness Among Sedentary, Overweight or Obese Postmenopausal Women With Elevated Blood Pressure: A Randomized Controlled Trial JAMA: The Journal of the American Medical Association, 297 (19), 2081-2091 DOI: 10.1001/jama.297.19.2081
Ross R, Dagnone D, Jones PJ, Smith H, Paddags A, Hudson R, & Janssen I (2000). Reduction in obesity and related comorbid conditions after diet-induced weight loss or exercise-induced weight loss in men. A randomized, controlled trial. Annals of internal medicine, 133 (2), 92-103 PMID: 10896648