Diets and Reality

Diets and Reality

Diets and Reality

Science, technology, and life.
July 17 2008 11:04 AM

Diets and Reality

There's quite a dustup over the diet study published today in the New England Journal of Medicine . The study reports that for two years, 322 "moderately obese" people were assigned "to one of three diets: low-fat, restricted-calorie; Mediterranean, restricted-calorie; or low-carbohydrate, non-restricted-calorie." On average, each participant lost 6 to 10 pounds.

In her New York Times blog, Tara Parker-Pope laments the study's findings. "All it really showed is that dieters can put forth tremendous effort and reap very little benefit," she writes. Her report is headlined, "More Evidence That Diets Don't Work."

William Saletan William Saletan

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.


Diet guru Dean Ornish disagrees. Writing in Newsweek , he calls the study " extremely flawed " because participants "on the 'low-fat' diet decreased their total fat intake from 31.4 percent to 30.0 percent, hardly at all." Their diet "was based on the American Heart Association (AHA) guidelines, which I have long criticized as not being enough of a change in diet to show much benefit."

Which writer is correct? Both, and neither. The study's lesson isn't that "diets don't work" or that the only diets worth studying are more radical. The lesson is that there's a tradeoff between results and compliance.

Look at the study's abstract on the NEJM Web site. It begins, "Background: Trials comparing the effectiveness and safety of weight-loss diets are frequently limited by short follow-up times and high dropout rates." Scroll down, and you'll see that the first finding reported isn't average weight loss. It's compliance. "Results: The rate of adherence to a study diet was 95.4% at 1 year and 84.6% at 2 years."

In other words, the study was designed in part to measure the cost of making diets easy enough to maintain. The lead author makes this clear in an interview with Parker-Pope:

In order to keep participants on the diet for long term as a way of life, we did not impose extreme diet protocols. More dramatic diet protocols could probably reduce more weight for the short term, but participants would have dropped out.

So Parker-Pope is right that the average weight loss was depressingly modest, and Ornish is right that more radical diets would probably have produced better results. But Ornish is wrong that this amounts to a damning flaw in the study, and Parker-Pope is wrong that it shows "diets don't work." The study set out to see what would happen if people were put on diets that the vast majority of them could psychologically sustain for two years. What happened was that by making the diets sustainable, the researchers made them less potent at reducing weight.

I've said this before, and I'll say it again: Compliance is part of a diet's effectiveness . Unless you plan on jailing people and sliding food under the door, their ability and willingness to adhere to the regimen are crucial factors in whether it works. If you want to complain about flaws in diet studies, complain about the studies with high dropout rates, which conveniently eliminate the real-world failure of diets that reduce weight if perfectly followed but are, for too many people, unsustainable.

This study isn't one of them. The news it brings is bad but important: We have to figure out how to design diets that are both potent and sustainable. We're not there yet.