Tuesday, January 11, 2005


A new study on the efficacy of diets has come out. The finding:
U.S. researchers Tuesday said a review of popular diets finds that, except for Weight Watchers, there is little evidence to support claims they work.

The University of Pennsylvania study, published in the Annals of Internal Medicine, looked at Weight Watchers, Jenny Craig, LA Weight Loss, Health Management Resources, OPTIFAST, eDiets.com, Overeaters Anonymous and Take Off Pounds Sensibly.

Researchers reviewed clinical trials and Web sites, talked with representatives and searched medical databases for information.

In looking at clinical trials, Weight Watchers had the strongest studies to support its claims, and one Weight Watchers diet was most successful of all diets studied -- with an average loss of 3.2 percent of initial weight -- or about 5 pounds -- at two years into the program.
The study's release led to predictable headlines: "This Just In: Most Diets Don't Work," says one Washington Post story.

The abstract from the Annals of Internal Medicine is here. The relevant portion says this:
Study Selection: Randomized trials at least 12 weeks in duration that enrolled only adults and assessed interventions as they are usually provided to the public, or case series that met these criteria, stated the number of enrollees, and included a follow-up evaluation that lasted 1 year or longer.

Data Extraction: Data were extracted on study design, attrition, weight loss, duration of follow-up, and maintenance of weight loss.

Data Synthesis: We found studies of eDiets.com, Health Management Resources, Take Off Pounds Sensibly, OPTIFAST, and Weight Watchers. Of 3 randomized, controlled trials of Weight Watchers, the largest reported a loss of 3.2% of initial weight at 2 years. One randomized trial and several case series of medically supervised very-low-calorie diet programs found that patients who completed treatment lost approximately 15% to 25% of initial weight. These programs were associated with high costs, high attrition rates, and a high probability of regaining 50% or more of lost weight in 1 to 2 years. Commercial interventions available over the Internet and organized self-help programs produced minimal weight loss.
It's difficult to know whether this study examined people who stuck to a diet over the entire time period, versus people who tried a diet for a period of time and then quit. I bet the study is talking about the latter category. And that's an utterly crucial difference, right?

It would be one thing if the study found something like this: "People who stuck to the 'No Sugar Diet' initially lost an average of 20 pounds but then gained it back even though they still maintained the same diet/exercise patterns that enabled them to lose the weight in the first place." That would indeed be a startling finding. But I suspect the study found something more like this: "People who started out on the 'No Sugar Diet' initially lost an average of 20 pounds, but then gained most of it back when they quit dieting 6 months into the program."

If the study really means that people tend to regain weight when they quit dieting, why is that reported as a failure of the diets themselves rather than as a failure of people's own individual willpower? Would a study of exercise be reported the same way? Imagine a study that was reported as finding the following: "Jogging doesn't work, because people who jog regularly for 6 months but then quit jogging don't have any remaining health benefits after 2 years." Well, that's not surprising, but the problem isn't with "jogging." The problem is with people's inability to keep jogging regularly, year in and year out.

Now some diets are inherently easier to maintain over the long-term than others. A "diet" consisting of grapefruit juice and cabbage leaves might enable some people to lose weight over the first month, but no one could keep up that diet for the rest of their lives. So if you're considering whether to begin the "grapefruit and cabbage" diet, you should probably think to yourself, "There's no way I can keep up this diet, and it won't produce any permanent gain."

But for a more normal diet that consists mostly of reducing sugars and processed carbohydrates, eating more whole grains/fruits/vegetables, eating more lean meat, avoiding fast food -- well, there is no reason whatsoever that people would fall off the wagon after a month. If they do, the real reason is their own lack of willpower.

Stuart Buck


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