A new report in a medical journal looks at some common beliefs about obesity and weight loss, and finds them a little thin on proof. Among the obesity myths is the one about sex burning 100 to 300 calories. Since it lasts about 6 minutes, on average, that total is way too high, the authors say. They also say it's not true that small changes over time lead to a large weight loss. The body's energy needs will change, so you'll lose less than you expect. School gym classes also don't reduce obesity because they are too short or not intense enough. It's also a myth that losing weight slowly is better than losing a lot fast, the authors say. Research shows that people who take the slow approach often lose less in the long run. The New England Journal of Medicine published the study. The Associated Press (AP) wrote about it January 31. Experts interviewed by AP agreed with some of the report. But they noted that the authors have ties to makers of food, drinks and weight-loss products and programs. They said the paper might push people toward these products or surgery.

By Robert H. Shmerling, M.D.
Harvard Medical School

What Is the Doctor's Reaction?

There's little doubt that myth and misconceptions abound in matters of health and disease. Obesity is no exception.

A new report in the New England Journal of Medicine says that all of these common beliefs are myths:

Why are these myths? According to the authors, the best available research suggests they are simply false.

Some are based on assumptions that aren't true, the authors say. For example, the idea that giving up a few potato chips each day will lead to major weight loss over years ignores how your body works. As you change your diet (or increase exercise), your energy needs change. So small changes have less impact than commonly believed.

Similarly, the amount of energy burned during school gym classes tends to be too small to make a big impact on weight.

And what about sex? The calculated amount of energy burned is about the same as walking at a moderate pace. But the authors cite research showing that sex lasts an average of 6 minutes. So the calories burned fall well below the commonly quoted figure of 100 to 300. It's the equivalent of a saltine cracker or two, not an apple or a hamburger.

The new report also lists some widely held beliefs that haven't been studied well enough to consider true or false. These include:

• Eating breakfast regularly reduces the risk of obesity.
• Eating more fruits and vegetables will lead to weight loss whether or not you make other changes in your diet.
• Losing and gaining weight over and over again ("yo-yo" dieting) is risky.
• Snacking leads to obesity.
• The lack of neighborhood parks or sidewalks contributes to obesity.

These may turn out to be true, but we need more research to prove it.

This report may upset advocates of breastfeeding or gym classes, for example. But I think it's important to point out misconceptions. These beliefs might hold people back from making healthy changes. For example, if you are trying to lose weight slowly because you believe faster weight gain is risky, you may lose no weight at all.

It's important to point out that the authors don't say exercise (including gym classes) or vegetables aren't worthwhile. There may be benefits other than weight loss. But, when it comes to avoiding obesity, they may not be enough.

What Changes Can I Make Now?

I think a good dose of skepticism is helpful when it comes to common assumptions about health. We may not have convincing evidence to support all advice that doctors give. But when good research suggests a common belief is wrong, it's worth noting.

This new report also lists some facts that can be useful as you plan changes to maintain a healthy weight.

• Dieting can be quite effective. But dieting as the only method of weight loss usually does not succeed over the long term.
• If it's intense enough and lasts long enough, exercise improves health even if you don't lose weight. And if you do reach a healthy weight, exercise can help maintain it.
• Structured weight-loss programs that provide prepared meals may be more effective than other diets.
• Weight-loss medicines and/or surgery can be effective and may reduce the risk of health problems linked to excess weight.

If you are overweight or obese, talk to your doctor about a program to achieve a healthier weight. Even though mistaken beliefs are common, here are some reasonable measures you can take now:

• Increase your physical activity. Try to get in 45 minutes of moderate-intensity exercise most days of the week. More may be even better.
• Get an exercise partner. He or she can encourage you to exercise even when you're tempted to skip it.
• Keep track of your weight. If you notice it's creeping up (and it often does in middle age), make a change. Think about ways you can increase your activity.
• Improve your diet. Reducing portion size and "mindless eating" are a good start. Meeting with a nutritionist may help.
• Consider a structured weight-loss program, a prescription medicine or even surgery if other methods have not led to lasting weight loss. Your doctor can help you design a weight-loss program that's right for you.

What Can I Expect Looking to the Future?

Don't buy in to myths about obesity. Much is known about this common condition, and much of this knowledge flies in the face of common beliefs.

And remember that some of these "myths" may turn out to be true. After all, the difference between medical myth and medical fact sometimes depends only on a well-designed study or two.