Sex Doesn’t Burn Weight and 4 More Popular Myths About Dieting Debunked

Woman's feet on scale.

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I don’t cover a lot of dieting stories here on State of Health. I figure you get enough of that elsewhere. For example, here are 88 million places I found by Googling “How can I lose 10 pounds?”

But I love evidence-based medicine. So when a group of respected researchers shatter widely-held beliefs about weight loss, I’m there. In Thursday’s New England Journal of Medicine, a group of researchers does just that.

In the review, the researchers categorized as myths those “beliefs held to be true despite substantial refuting evidence.” In other words, people have been repeating these ideas for so long, everyone thinks they’re true. But they’re not.

So, here we go:

Myth #1: Small changes — eating less or exercising more — done over time will yield large weight loss.  This myth comes from the idea that a pound is equal to 3,500 calories. But the short-term studies that looked at burning 3,500 calories to lose one pound were done 50 years ago. More recent research shows that individuals will burn calories differently as they lose weight. So the 100 calories you’re burning in exercise today will affect your body differently than the 100 calories you burned, say 18 months ago, when you started these small changes. Note that it’s not to say that exercising more — or eating less — is pointless (you will see why later in this post).

Myth #2: If you lose a lot of weight really fast, you’ll just gain it back really fast; you’ll have better long-term results if you lose weight slowly. When researchers actually looked at the studies, they found “no significant difference” between the two approaches in relation to long-term weight loss.

Myth #3: Physical-education classes, in their current form, play an important role in reducing or preventing childhood obesity. As a parent, I found this to be somewhat depressing, although the writers did include a caveat: “There is almost certainly a level of physical activity … that would be effective in reducing or preventing obesity. Whether that level is plausibly achievable in conventional school settings is unknown, although the dose-response relationship between physical activity and weight warrants investigation in clinical trials.”

Myth #4: Breast-feeding protects against obesity. Say it ain’t so! No less than the esteemed World Health Organization reported that breast-feeding is protective against obesity. But our myth busters found evidence of “publication bias” in the WHO research. Furthermore, the myth busters report a study of 13,000 children that found “no compelling evidence of an effect of breast-feeding on obesity.” They do stress that breast-feeding has other benefits for both infant and mother “and should therefore be encouraged.”

Myth #5: You burn 100 to 300 calories by having sex. The myth busters start with a premise that some people may find troubling: “Given that the average bout of sexual activity lasts about six minutes,” the myth busters say, “a man in his early-to-mid-30s might expend approximately 21 (calories) during sexual intercourse.” But it gets even better: “Of course, he would have spent roughly one third that amount of energy just watching television.”

The myth busters also highlight presumptions, “widely accepted beliefs that have neither been proved nor disproved.” They call for gathering solid evidence so these presumptions can be proved or disproved.

At present, there is no evidence to support these ideas:

  • Regularly eating breakfast is protective against obesity.
  • Eating more fruits and vegetables will result in weight loss or less weight gain, regardless of whether any other changes to one’s behavior or environment are made.
  • Weight-cycling is associated with increased mortality. It’s likely that any observations that lead to this conclusion are confounded by a person’s health status.
  • Snacking adds to weight gain and obesity. Studies have not consistently found an association.

The myth busters insist they are not nihilistic, and their report includes these facts:

  • Exercise, exercise, exercise. “Regardless of body weight or weight loss, an increased level of exercise increases health,” the researchers write.
  • Exercise “in a sufficient dose” helps with long-term weight maintenance.
  • For overweight children, “parents and the home setting” should be included in any weight loss plan to improve success.
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