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Although the majority of people underestimate the calories they consume, this calorie underestimation is especially extreme among overweight people. It has been argued that such underestimation is one of the main causes of rising obesity rates in the United States.
We tested 3 hypotheses: that every person's estimation of calories follows a predictable pattern of diminishing sensitivity to increases in meal sizes, that once meal size is controlled for, there are no differences in the estimation biases of overweight and normal–weight persons, and that the differences between overweight and normal–weight people are a result of overweight people choosing larger meals.
We conducted a field study in which we asked 105 lunchtime diners to estimate the number of calories in the meals they had ordered. In a second study of 40 undergraduate students, we asked both overweight and normal–weight students to estimate the number of calories in 15 specific meals so that we could look at body size independent of meal size. We found that although participants strongly underestimated the number of calories in larger meals, they almost perfectly estimated the number of calories in smaller meals. After we controlled for body weight–related differences in meal size, the calorie estimations of normal–weight and overweight participants were identical in both studies.
We conclude that while overweight people underestimate calories more so than normal–weight people, this difference is a consequence of their tendency to consumer larger meals. Calorie underestimation is related to meal size, but not body size.
Wansink, Brian and Pierre Chandon (2008). Meal Size, Not Body Size, Explains Food Intake Estimation Errors. Annals of Internal Medicine, 145(5), 326-332. doi:10.7326/0003-4819-145-5-200609050-00005