Is the obesity epidemic caused by too much exercise?

by Monte Ladner, MD

The August 17, 2009 issue of TIME magazine ran a cover story entitled “The Myth About Exercise” with a subtitle claiming it won’t make you lose weight. The author of the article cherry picked bits of data from several scientific studies to make the case that exercise won’t help with weight loss, and might even lead to weight gain by causing people to eat more.

The TIME article cites a single set of figures from the Minnesota Heart Survey (MHS) suggesting that more people in Minnesota are exercising today than in 1980. He then conflates this statistic with the observation that the rates of obesity nation-wide are increasing and concludes that this must mean exercise makes us fat.

In the MHS the amount of leisure time physical activity reported by study subjects depended on how the survey question was asked. The MHS also uncovered that the percentage of people who spend more than half their day sitting at work increased from 57% in 1980 to 71% in 2000. The TIME article fails to mention that in the MHS over the 20 years between 1980 – 2000 the people who did the most exercise gained the least weight. The Minnesota Heart Survey concludes that increased physical activity needs to be part of the solution to the obesity epidemic

The TIME article also draws heavily on research by Dr. Timothy Church of the Pennington Biomedical Research Center in Louisiana. Dr. Church has reported data on over 400 previously sedentary women who were put through supervised exercise programs for six months. The women were divided into 4 groups, a control group and 3 exercise groups. Each of the exercise groups did a very specific amount of exercise under direct supervision. Over the 6-month training period the 3 study groups did 72, 136, and 194 minutes of exercise per week. Exercise sessions alternated between treadmill walking and stationary cycling. The exercise intensity was carefully controlled at 50% of each participant’s measured maximum oxygen uptake. The subjects were told that this was not a weight loss study and they should not change their diet.

Over the 6-month trial all 3 study groups lost weight associated with their exercise. The 2 lower duration exercise groups both lost exactly as much weight as the researchers calculated they should lose based on the number of calories they were burning. The highest duration exercise group only lost half as much weight as expected based on calories burned while exercising. Dr. Church labeled these people “compensators” and speculates that they may have started eating more and this reduced the magnitude of their weight loss.

Dr. Church, in an interview with me, is quick to point out that his study was not designed to investigate the phenomenon of compensation and therefore he can only guess as to why the women in the third group didn’t lose as much weight as expected. However, they did lose weight and this contradicts the theme of the TIME article that compensatory eating causes exercisers to gain weight. Dr. Church believes that compensation is probably related to people rewarding themselves with food and overestimating how many calories they burn during exercise and underestimating how many calories they eat.

The National Weight Control Registry has followed a group of 5,000 people who lost an average of 66 pounds and sustained their weight loss for an average of 5.5 years. Ninety percent of these people exercise for at least an hour per day.

Exercise is clearly important for health and weight control. I don’t know whether the TIME article was just poorly researched and fact-checked or whether it is intentionally misleading to create controversy where none exists.

The bottom line is that this article misleads the public about the value of exercise.

Monte Ladner is an anesthesiologist who blogs at FitnessRocks.org.

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