Active Voice: Understanding Physical Activity Guidelines and Weight Loss
Active Voice is an occasional column by ACSM experts. These comments do not necessarily reflect the views or positions of ACSM.
Barry Braun, Ph.D., FACSM, is Associate Professor in the Department of Kinesiology, University of Massachusetts at Amherst. His research focus includes interactive effects of exercise and diet on endocrine function and metabolism. See the January 2009 issue of ACSM's Exercise and Sports Sciences Reviews for a related research review he coauthored: "Physical activity and hormonal regulation of appetite: sex differences and weight control."
In late 2009, a cover story in TIME magazine surprised many of us with the provocative title "The Myth of Exercise." The central theme of this article was not only that exercise is ineffective as a weight loss tool, but that bouts of intense, structured exercise (e.g. what most people do at "a gym") are actually counterproductive because they increase appetite and lead to excess food intake that can cause weight gain. For those who read the article carefully, the utility of exercise as medicine to reduce risk for chronic diseases, strengthen bones, enhance mood and improve cognitive function was unchallenged. Rather, the author took great pains to argue that it would not "make you thin."
The idea that exercise alone, with no conscious change in diet, induces only modest weight loss is not new. Anecdotally, all of us have been cornered by people claiming to have spent hours each week walking, running, stair-stepping, etc., and are displeased with the results on the scale or in the mirror. At least in my experience, more of these stories are related by women than by men. Well-designed studies bear out the anecdotes. When individuals are placed in a supervised exercise program with no diet intervention, weight loss tends to be modest, particularly for women. These results are usually explained as “compensation,” either less physical activity outside the structured exercise program, increased energy intake, or both. Smaller, more controlled studies lend support to the idea that caloric intake rises in response to higher energy output. Upon initiating an exercise training program there are changes in appetite and some key energy-regulating hormones that are consistent with a rise in calorie intake; particularly in women.
The discordance between the potent benefits of physical activity for mental and physical health outcomes and its less impressive effects on weight loss is reflected in current physical activity guidelines. The most commonly cited recommendation, to engage in at least 30 accumulated minutes of moderate-vigorous activity most days of the week, is deliberately geared to promote health/wellness outcomes, not induce weight loss. Most health organizations note that the dose of exercise likely to cause sustained weight loss is considerably higher. For example, the 2007 ACSM/AHA guidelines state that, to lose weight, 60-90 minutes of daily exercise may be necessary. Similarly, the 2005 Dietary Guidelines, which have a well-written section on physical activity, also recommend 60-90 minutes of daily physical activity to sustain weight loss. The >3000 kcals of “extra” energy expenditure generated by 500+ minutes of moderate-vigorous activity per week seems sufficient to mitigate compensatory weight gain.
Leaving the controversial issue of whether 60-90 minutes of daily activity is a realistic goal to the scientists with expertise in behavior modification, does the possibility of “caloric compensation” that opposes weight loss warrant tweaking our recommendations for the general public? First, it might be useful to reinforce the message that the many benefits of 150-200 minutes of weekly physical activity are unlikely to include marked weight loss. Second, the important contribution of sedentary behavior to energy imbalance and weight gain during the many hours of the day when people are not engaged in moderate-vigorous physical activity/exercise needs to be made more prominent. Emerging evidence suggests that, unlike bouts of moderate-vigorous activity, low-intensity ambulation, standing, etc. may contribute to daily energy expenditure without triggering the caloric compensation effect. Replacing sedentary activity (i.e., sitting) with any movement at all could be an important weight loss/maintenance strategy that has received far less attention than exercise. Third, embedded in the small print of the Physical Activity Guidelines for Americans is a statement that “benefits of physical activity include weight loss when combined with reduced calorie intake,” which brings to mind another strategy. It is time to consider a unified set of dietary and physical activity guidelines that marry (or at least, create a civil union of) the energy intake and expenditure sides of the energy balance equation. Certainly, the public will appreciate that the importance of integrating physical activity and nutrition to benefit physical, mental and emotional health is no myth!