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This Diet Myth Often Fools Weight-Conscious People

Can adding a side salad to a cheeseburger help one lose weight? New research from the Kellogg School of Management suggests that people, particularly those who characterize themselves as weight conscious, tend to believe that adding a healthy option to an otherwise indulgent meal will lower the total calorie count.

"Despite the growing availability of healthier, diet-friendly foods, the proliferation of diets, and the increasing number of public policy initiatives aimed at obesity, the proportion of overweight individuals in the United States continues to increase," says Alexander Chernev, study author and associate professor of marketing at the Kellogg School Management at Northwestern University. "An important factor contributing to the obesity epidemic is the misguided belief about the relationship between a meal's healthiness and its impact on weight gain. People erroneously believe that eating healthy foods in addition to unhealthy ones can decrease a meal's calorie count."

In earlier research, Chernev identified consumers' tendency to underestimate their calorie intake. In his new study, Chernev builds on this concept, documenting a striking outcome: this propensity to misestimate calories is stronger among weight-conscious people, who would seem most likely to closely monitor their diets.

As a part of this study, 934 participants from a nationwide online research panel were asked to estimate the calorie content of several meals. Some were shown a series of relatively unhealthy meals, and others were shown the same meals combined with a healthy option. For example, some of the participants were shown a bowl of chili with cheese, whereas the others were shown the same bowl of chili paired with a small green side salad. The other food pairs included a cheeseburger, which for some of the participants was paired with three celery sticks; a bacon-and-cheese waffle sandwich (paired with a small organic apple); and a meatball pepperoni cheesesteak (paired with a celery-and-carrot side dish).

Results were astounding. Those who viewed the chili alone rated it as averaging 699 calories. In contrast, those who were shown the chili combined with the green salad estimated it to have only 656 calories. Thus, adding a green salad to the bowl of chili lowered the perceived calorie content of the entire meal by 43 calories—as if the green salad had negative calories. This negative calorie illusion was observed with all four meals tested, indicating the prevalence of the belief that one can consume fewer calories simply by adding a healthy item to a meal.

"Because people believe that adding a healthy option can lower a meal's calorie content, the negative calorie illusion can lead to overconsumption, thus contributing to the obesity trend," says Chernev.

What was even more surprising, however, was that dieters—who are presumably more involved in monitoring their calorie intake and more familiar with the calorie content of different meal options—were more likely to fall into the negative calorie illusion trap. In fact, the illusion was twice as strong for the weight-conscious groups (who on average underestimated the combined meal by 76 calories or 10.8%) than for those who were rather indifferent to their weight (who on average underestimated the combined meal by 33 calories or 4.8%).

To combat the "dieter's paradox," as Chernev has termed this phenomenon, he recommends shifting the focus of current public policy campaigns away from the stereotypes associated with "good" and "bad" foods. When product ads and public policy communications stereotype foods into virtues and vices, they tend to shift people's attention away from the quantity of food consumed. According to Chernev, this neglect of quantity might end up implicitly promoting the illusion of negative calories.

"The bottom line here is that motivating people to lose weight without educating them on how to monitor their calorie intake might not be enough to combat obesity. As the dieter's paradox shows, motivation without knowledge can be counterproductive," says Chernev. "Promoting the consumption of healthy foods without providing a complete picture of the factors influencing weight gain might paradoxically facilitate calorie overconsumption, leading to weight gain rather than weight loss."

The study, "The Dieter's Paradox," will appear in the April 2011 issue of the Journal of Consumer Psychology.

MORE INFORMATION:  To request the full study or to arrange an interview with Professor Alexander Chernev, contact Aaron Mays or Dawn McKenzie, APR at the contact information listed below.

For more information about the Kellogg School of Management at Northwestern University, visit


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