Which Works Better: Choosing a Diet or Being Assigned One?

Say you love ice cream, but you’d really like to drop a few pounds. If someone came up with a diet that would let you eat two scoops a day, as long as the rest of your menu was low-cal, would you:

A. Happily stick with that diet for years, watching the pounds melt along with the mint chocolate chip

B. Find it tough to stop at two small scoops, falling off the diet quicker than you can "butter brickle, butter brickle, butter brickle"

A new study in the Annals of Internal Medicine suggests that for many folks, the answer would be “B,” despite the conventional wisdom that including favorite foods in a weight-loss diet can help you stick to it.

“Intuitively, people think, ‘If we help people choose a diet that fits with their tastes, they’re more likely to adhere to it,’” says Dr. William Yancy Jr., one of the study's authors, who is a general internist at Duke University Medical Center and the Veterans Affairs Medical Center in Durham, N.C. That’s why dietitians try to work favorite foods into clients’ weight-loss plans, Yancy says.

But Yancy and his collaborators concluded that if you let people eat the foods they like, they’re likely to overeat. “Their food preferences might be the reason they’re in the state they are,” he says.

For their study, the researchers recruited 207 volunteers (average age 55), served by an outpatient clinic at the Durham VA Medical Center. About three-quarters were men and all of them were obese, with a body mass index or BMI of at least 30. (If you’re 5-foot-8 and weigh 197 pounds, your BMI is 30.)

Everyone completed a questionnaire about how much they liked 72 foods in the typical U.S. diet. They were then randomly divided into two groups: One got to choose whether to go on a low-fat or a low-carb diet, while the other was randomly assigned to one of the two diets.

Members of the choice group received a summary of their food preference questionnaire results that noted which diet more closely aligned with their favorite foods. They learned about the diets and were told to go home and think about which they’d prefer. About six out of 10 picked the low-carb diet, including half of the people whose food preferences better fit the low-fat diet. Meanwhile, people in the no-choice group learned about their assigned diet. Everyone in the study then met regularly in group sessions with a dietitian.

By the end of 48 weeks, participants assigned a diet actually lost slightly more than those who picked one, and the two groups were equally likely to have stayed on their diets.

Peter Clifton, a nutrition researcher at the University of South Australia in Adelaide who published a similar study, says he’s not surprised by Yancy’s findings, since most of the participants were men. (Clifton was not involved in Yancy’s study.)

In his study published last year, Clifton and his coauthors reported that overall, people assigned a diet were as likely to stick to it and lose a similar amount of weight as people who got to pick a diet. But his team found that men did better when assigned a diet, while women did better when they picked one.

Clifton’s explanation for the gender differences? When it comes to losing weight, men like being told what to do.