Megan McArdle points to a Time Magazine piece on exercise. Exercise, it turns out, won’t make you thin.

I’ll try to limit how much I point out that the notion of losing weight simply by exercising had to be questionable to anyone with spreadsheet program and even a moderate desire to verify conventional wisdom. The numbers just don’t add up. The simulations just don’t bear any resemblance to reality.

What’s important is that this is entering the intellectual ether and has some chance of influencing policy and thought on the obesity epidemic. There isn’t a whole lot of evidence that being lazy makes you fat, though being fat might make you reluctant to engage in lots of activity.

The Time article still makes a couple of routine mistakes like conflating theories regarding how much people eat and the composition of the food we eat. Composition theories tend to work by assuming the quantity of eating is not a choice variable. That is, it may seem like you can cut calories by avoiding a second helping at dinner but you’ll just be hungrier the next morning. Eventually, the theory goes, everyone gives in to hunger.

They also, understandably, soft pedal criticism of the public health organizations.

Then how did the exercise-to-lose-weight mantra become so ingrained? Public-health officials have been reluctant to downplay exercise because those who are more physically active are, overall, healthier.

Really? Reluctant to downplay? I would go so far as to say they engaged in a campaign of misinformation bordering on outright deception. I think they did this with the absolute best of intentions. I also would be willing to concede that by convincing millions of Americans that the Stairmaster will help them loose weight, they have done a lot to improve cardiovascular health. However, the fact remains that the basic idea that exercise can help you loose weight was endorsed without any evidence to support it.

From the American Heart Association

Top 10 Tips for Starting a Physical Activity Program

The American Heart Association recommends that all adults get at least 30 minutes of physical activity every day, or at least more days than not. If you’re trying to lose weight or maintain weight loss, you should get at least 60 minutes each day

From the American Cancer Society

Maintain a healthy weight throughout life.

  • Balance calorie intake with physical activity.
  • Avoid excessive weight gain throughout life.
  • Achieve and maintain a healthy weight if currently overweight or obese.

“Balance calorie intake with physical activity.” Now it doesn’t actually say that exercise can make you loose weight. It effectively says: maintain caloric balance. Sort of like telling a sick person: maintain homeostasis. Clearly, however, the implication is that exercise will cause you to loose weight.

All in all, however, the best thing that we can take away is more skepticism about obesity theory. We just don’t know for sure why people get fat. We don’t even know if historical fatness is related to the current epidemic.

That is, for most of human history people could have been gaining weight for reason X. This is still the reason some people are gaining weight. However, reason Y has come out of the blue and is the reason lots of people are now gaining weight.