A Kellogg economist proposes the Dieter’s Paradox. Put a healthy vegetable next to something fatty or fried and people think there are fewer calories in the two items combined versus when looking at the fatty/fried item alone.
Those who viewed the chili alone rated it as averaging 699 calories. By contrast, those who were shown the chili combined with the green salad estimated the meal to have only 656 calories. Thus, adding a green salad to the bowl of chili lowered the perceived caloric 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 caloric content, the negative-calorie illusion can lead to overconsumption, thus contributing to the obesity trend,” said Chernev.