Nov. 1, 2006

It's the size of the meal, not the size of the person, that determines how people underestimate calories, Cornell study finds

Researchers have observed that overweight people underestimate how much they eat by twice as much as normal-weight people do. So are heavier people's underestimates intentional?

No, reports a new Cornell study published in the October issue of the Annals of Internal Medicine. Rather, it is because everyone consistently underestimates size as things get larger -- distance, weight, heights of buildings and loudness. And that applies to estimating how many calories we eat.

"Scientists, physicians and counselors have often blamed overweight people as trying to fool others -- or themselves -- about how much they are eating," said Brian Wansink, the John S. Dyson Professor of Marketing and of Applied Economics at Cornell, and author of "Mindless Eating: Why We Eat More Than We Think." He noted that studies have reported that overweight people underestimate their calorie intake by 40 percent, compared with normal-weight people who underestimate their calorie intake by an average of 20 percent.

"This study, however, shows that it is meal size, not people size, that determines how accurate we will be at estimating the number of calories we have eaten," Wansink said. "That popsicle-stick-skinny person eating a 2,000-calorie Thanksgiving dinner will underestimate how much he has eaten by just as much as the heavy person eating a 2,000-calorie pizza dinner.

"The trouble is that the heavy person tends to eat a whole lot more of these big meals."

Wansink, who is director of the Cornell Food and Brand Lab, which explores the psychology behind what people eat and how often they eat it, conducted two studies with Pierre Chandon, a marketing professor at INSEAD, an international business school based in France, to explore the popular notion that overweight people consistently underestimate how much they eat much more than normal-weight people do.

In the first study, trained interviewers stationed at fast-food restaurants asked 105 normal- and overweight persons to estimate how many calories they had just consumed.

"We found that the more people had eaten, the less accurate they were," said Wansink. "It did not matter whether the person was skinny or huge, male or female -- the bigger the meal, the less they thought they ate."

In the second analysis, 40 undergraduate students estimated the calories of 15 fast-food meals presented on white plates on separate tables.

"Once again, the smaller the meal, the more accurate people were at estimating its calorie-level," said Wansink. "The larger the meal, the fewer calories they thought it contained. There were no differences in the estimates of the skinniest people or of the largest people."

However, because overweight people tend to eat more large meals, they underestimate their food intake more frequently.

Better understanding that overweight people aren't "lying" or are "in denial" about how much they really eat, said Wansink, is important for those helping the overweight change their eating habits.