Not Tonight, Honey, I'm Trying to Lose Weight

Mary Duenwald writes editorials on energy, health care and science for Bloomberg View. She was deputy editor of the New York Times op-ed page and a senior editor at Harper’s Bazaar, Real Simple, the Sciences and Vogue.
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No matter what you may have heard, having sex doesn't help you lose weight. Six minutes of activity (a 1984 study found that to be the average) uses up only 14 more calories than does six minutes of watching TV.

This may be the fact people will most remember about the obesity study published in this week's New England Journal of Medicine. But the report highlights a dozen other myths and scientifically baseless presumptions about how people lose weight. This is all data that Americans -- more than a third of whom are obese -- ought to understand. Some of it could usefully inform public health policy.

We often hear it's smart to avoid setting too-ambitious goals for weight loss, that more modest ones are more effective. But the study authors, from the University of Alabama at Birmingham, found research showing that bigger goals lead to better results and, in the long term, crash diets lead to about the same amount of weight loss as slower, more moderate ones do.

Another myth: the notion that adding a little bit of exercise to one's daily routine or cutting out small snacks can lead to significant weight loss. Once a person loses a little weight, his caloric needs shift, so the net benefit ends up being modest.

On the bright side, there is no strong or consistent evidence that snacking contributes to obesity.

Eating a regular breakfast or increasing one's consumption of fruits and vegetables may be healthful in many ways, but these things on their own don't bring about weight loss, either. Ditto for exercise -- unless the exerciser also consumes fewer calories than he uses each day, and for a prolonged time (ideally, for a lifetime).

The study should affect how we think about public health as well as individual weight loss. It's wonderful to get fruit and vegetable stands into underserved neighborhoods, for example, or to make cities more walkable. But neither alone will solve the U.S. obesity problem.

Public health efforts need to come in many forms. And sometimes, by necessity, they need to be experimental. Requirements that restaurants or makers of packaged foods provide calorie counts, for instance, are not based on scientific evidence that such labeling helps Americans lose weight. But such initiatives are easily worth a try. Then someone can do a study to find out whether they work.

This column does not necessarily reflect the opinion of Bloomberg View's editorial board or Bloomberg LP, its owners and investors.

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Mary Duenwald at