In the study, participants were shown a meal and asked to estimate the total number of calories. One group was shown unhealthy meals, such as a cheeseburger or a meatball pepperoni cheesesteak. The other group was shown the same foods, but paired with healthier side items, such as a small salad or celery sticks.
Those who saw the unhealthy foods alone rated them as having more calories, but the second group tended to deduct calories due to the healthy side items. For example, the first group rated a bowl of chili with cheese as having 699 calories. The second group, who saw the chili paired with a salad, rated the entire meal as having only 656 calories. In the eyes of the participants, adding a salad to the meal lowered the caloric content of the meal by 43 calories — the negative-calorie illusion. This same tendency occurred with all four meals shown.
"Because people believe that adding a healthy option can lower a meal's caloric content, the negative-calorie illusion can lead to over-consumption, thus contributing to the obesity trend," said Chernev.
Also surprising was the fact that the individuals who were more weight-conscious were even more likely to succumb to the negative-calorie illusion. Although dieters would be presumably more educated about and familiar with the caloric content in different foods, they tended to underestimate the number of calories by more the double the rate of individuals who classified themselves as less concerned about their weight.
Chernev suggests that the public's thinking needs to shift away from "good" and "bad" foods and focus instead of the quality of foods consumed. "Motivating people to lose weight without educating them on how to monitor their caloric intake might not be enough to combat obesity," he says. "As the dieter's paradox shows, motivation without knowledge can be counterproductive."