How good are we at estimating the number of calories in the foods we eat? It turns out, if the meal is small, we are pretty good. But if the meal is large, we grossly underestimate the number of calories it contains.

Brian Wansink and Pierre Chandon, Ph.Ds from New York and France, respectively, designed two studies to look at how accurately people estimate the number of calories in fast food meals. They were also interested in whether overweight individuals differed from normal weight individuals in their ability to estimate calories in meals. The results are reported in an article, “Meal Size, Not Body Size, Explains Errors in Estimating the Calorie Content of Meals,” that appears in the September 2006 issue of the Annals of Internal Medicine.


Here are the studies

In Study 1, the researchers sent trained interviewers to local fast-food restaurants. They asked 150 men and women who had just finished eating to estimate the number of calories in their meals and to provide their heights and weights. A hundred and five (105) people agreed to participate. A little more than half of the participants (59%) were normal weight (BMI less than 25). The rest were overweight. Men and women’s estimates of meal calories were similar as were estimates of normal and overweight individuals. What was significantly different was the calorie underestimation of different size meals. People underestimated small meals by an average of about 3%. However, they underestimated big meals by 38%.

In the second Study, the researchers asked 40 undergraduate students to estimate the number of calories in 15 different meals purchased from a local fast food restaurant. Each meal consisted of three items (chicken nuggets, French fries, and a regular cola drink). What differed was the portion size of each item. The smallest meal consisted of 3 nuggets, a small (1.45 oz) order of fries and a 10 oz cola. The largest meal was 12 nuggets, 5.8 oz of fries, and a 40 oz cola. The rest of the meals were different combinations of the different portion sizes of the same items. The meals ranged from 445 calories to a whopping 1780 calories.

Similar to Study 1, they found that the undergrads were very good at estimating calories of the small meals, but they underestimated the 7 largest meals by a mean of 23%. Again, there was no difference between normal and overweight people in their ability to estimate calories.


What did they find?

Now here is the punch line: Although normal and overweight individuals were similar in their ability to estimate the number of calories in food, the overweight individuals in Study 1, who had eaten a fast food meal, ate larger meals. Because everyone significantly underestimates the number of calories in large meals, these overweight people actually ate many more calories than they thought they had.

An analysis of the data in these two studies showed a meal that increased in calories by 100%, which was only thought, by the participants, to have increased by 50%. Whew, that’s a big mistake that can really impact the bottom line a.k.a. the “waistline”.


What can be done?

The authors had several suggestions on ways to improve an individual’s ability to estimate calories. One was to have fast food restaurants clearly post calorie content on the meal servings themselves, as opposed to just being available somewhere in the restaurant or on the company’s website. Another was to provide people with an idea of what a portion size should be (e.g., 3 chicken nuggets or an amount of fries equal to the size of your fist).

An interesting option they propose takes advantage of what they learned from the data in this study. Since people can accurately estimate the calories in small amounts of food, they should estimate the amount of calories in a large meal by estimating the calories in each item (chicken nuggets, fries, cola) and then adding them up to get the total. Whether this works or not, of course, will have to be tested. But if it does, it will be one more tool that we can use to help us ensure that our daily calorie intake stays in a range that allows us to achieve and maintain a healthy weight.