Calorie counting is not as easy as you think

Published: Nov 9, 2010 12:01 a.m. ET




CHICAGO (MarketWatch) — The U.S. obesity epidemic may have less to do with fast food and more to do with being bad at math. A recent study from Northwestern University shows that many people simply can’t add correctly when it comes to calories.

Which has fewer calories: a bowl of chili and a salad, or just the chili? On average, the people surveyed guessed a lower calorie count for the chili and salad together than for the chili alone. This illogical thinking is the result of a “negative calorie illusion,” said Alexander Chernev, author of the study and a professor of marketing at Northwestern.

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“People think if you add something healthy to something unhealthy, then somehow the calories will magically disappear,” Chernev said. “Or that somehow it will reduce the calories of an unhealthy item.”

Half of the nearly 1,000 survey respondents were asked to estimate the calorie content of a relatively unhealthy food, like a hamburger or a bowl of chili. The other half were shown the same foods but with a healthier item added, like an apple, celery sticks or a salad. The average estimate for the two-item meal was 43 calories lower than the average for the unhealthy food alone.

A possible explanation for this effect is that advertising and popular diets have conditioned us to think of foods as inherently good or inherently bad, Chernev said. Instead of thinking about the sum of a meal’s components, we average them. When you add a “good” food to the plate, the meal overall seems healthier. And in our thin-obsessed culture, healthier usually means fewer calories.

The ‘dieter’s paradox’

The study, which will be published in the April 2011 issue of the Journal of Consumer Psychology, had another surprising finding. Respondents who described themselves as weight-conscious were more likely to underestimate the calories in healthier foods than those who said they weren’t concerned about their weight. Chernev calls this finding the “dieter’s paradox.”

“Even though they’re more motivated, they are more likely to make a mistake when estimating calories,” Chernev said. “The effect for them is actually more pronounced.” It’s possible that weight-conscious people are more exposed to advertising or diets that encourage thinking about foods as either good or bad, he said.

Mixed health messages

The subjects in Chernev’s study likely saw the fruit or vegetable and immediately assigned healthy attributes to the entire meal, despite the fact that the portion of hamburger or chili remained the same, said Marci Campbell, professor of nutrition at the University of North Carolina.

“There’s a sense that you can counteract something unhealthy with something healthier, and that that somehow balances out in your body,” said Campbell, who was not involved in the Northwestern study. “What you really want is for people to eat the celery or the salad and then take a smaller portion of the chili or the macaroni and cheese.”

Campbell studies strategies for communicating health and nutrition information. She said people know they are supposed to eat more fruits and vegetables, but they often add them to a meal as extra items rather than using them to replace portions of unhealthy foods.

“We have a lot of information when trying to select food,” Campbell said. “Sometimes it’s just too much information. There’s a lot of confusion about low-fat, low-calorie and what that means.”

Product labeling can be deceptive, she said. Often a food that is advertised as reduced fat will have only a small difference in calories from the full-fat version. Bargain shopping can also sidetrack attempts to eat healthier.

“People are constantly trying get a good value, and that may sometimes lead to buying large quantities or buying a less healthy product,” Campbell said. “You get more value for your money if you buy a big bag of chips. But when people buy a big serving, it’s really hard for them to restrain themselves or to estimate how much is OK to eat.”

Teaching consumers to estimate calorie content more accurately is just one piece of the puzzle, she said. Consumers also have trouble choosing appropriate portion sizes and selecting balanced meals.

Not a new phenomenon

Misconceptions about food are nothing new. Back in 1996, researchers at the University of Pennsylvania found that Americans had some fairly irrational ideas about nutrition. Respondents to their survey said that even a trace amount of salt or fat in a diet is unhealthy, that a single ounce of chocolate has more calories than five ounces of bread, and that a teaspoon of ice cream has more calories than a pint of cottage cheese.

Paul Rozin, lead author of the 1996 study, said the huge amount of nutrition information available causes many people to resort to using easy rules of thumb, like thinking of foods as either inherently good or inherently bad, instead of making nuanced judgments. The message that “a diet high in salt is bad” is simplified to the idea that “all salt is bad.”

“It’s easier to just use good or bad, and it works much of the time,” said Rozin, professor of psychology at the University of Pennsylvania. “It’s not bad to have these rules of thumb. It makes your life easier. But every once in a while, you pay a price for it.”

The problem is when the rules of thumb stop working. In Chernev’s study, subjects were using the wrong arithmetic and averaging the two foods, rather than adding them together. Scientists need to identify which heuristics are the most harmful and then educate consumers on how to make better nutrition decisions, Rozin said.

“People know a lot of things about food. For example, they know that you need it. They know that you shouldn’t drink sewer water,” Rozin said. “They even know that the more you eat, the fatter you get. They’re just not adding up what they know.”

Rose Pastore is a freelance writer in Chicago.

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