The FDA is making a big change to nutrition labels. And it’s probably a big mistake.

The good news is that the U.S. Food and Drug Administration is poised to make the Nutrition Facts label (pdf) on many packaged foods significantly more honest. The bad news is that this well-intentioned fix could seriously backfire.

As part of a label overhaul, which the FDA announced last year, the agency is planning to update the serving sizes to better reflect the amount of food people actually consume. The proposed tweak, which is almost through its months-long comment period and is expected to begin to take effect next year, would affect packaged foods for which serving sizes are seen as too low (just under 20 percent of all packaged foods), including popular items like ice cream, potato chips and soda.

The thinking behind the portion adjustment is fairly simple. It’s meant to correct for the fact that recent studies show that Americans tend to eat a good deal more in one sitting than is indicated on current labels — which were last adjusted  20 years ago, based on data from a survey conducted from 1977 to 1988.  Changing the serving size for  ice cream, for example — from the current half cup to a more realistic full cup — means the amount of calories, fat and sugar per serving will double. And that would theoretically convince people to eat less ice cream.

It’s also in accordance with the Nutrition Labeling and Education Act, which was passed in 1990, and, in part, requires that serving sizes accurately reflect consumption patterns.

But what sounds good in theory doesn’t always pan out in practice. Harvard’s Behavioral Science and Regulation Group, citing a widespread misunderstanding of serving sizes, warned  that consumers would read the new label as “endorsing” larger portion sizes. And a  new study, published last week  in the journal Appetite, added new evidence that the new labels will have the opposite of their intended effect: People will perceive that the new portion size is normal, and they’ll eat more ice cream — or other fattening foods — instead of less.

“We found that people misinterpret serving size information, with the vast majority of consumers incorrectly believing that the serving size refers to how much can/should be consumed,” the researchers wrote. And they conclude: “The proposed Nutrition Facts label is intended to help consumers make healthier consumption decisions, but the current research suggests that it may backfire, leading consumers to serve more to themselves and others.”

For the study, researchers at New York University’s Stern School of Business tested the changes on a group of  more than 50 adults. In one experiment, half of the participants were shown the current nutrition label for mini chocolate chip cookies, and the other half saw the proposed label. Then they were told they would be offered some, and asked how many they expected to eat. Those who had seen the new label, which used six cookies as a serving size, said they expected to eat more than those who saw the current one, which used three cookies as a serving size.

In another experiment, participants were shown a family-size package of lasagna. Half of them saw the current label, which says the package contains six servings, while the other half saw an updated one, which says the package contains only three servings. All were then asked to estimate how many lasagnas it would take to feed a party of 20 people. And the outcome was similarly telling: Those who saw the new label said they would need to buy roughly two more lasagnas (the average was 7.17) than those who saw the traditional one (about five).

The reason larger serving sizes might only lead Americans to pack more food into their pantries and, eventually, their stomachs, is likely tied to an inherent misunderstanding of what the term “serving sizes” actually means. “The serving sizes listed on the Nutrition Facts label are not recommended serving sizes,” the FDA reminds on its Web site. And yet the majority of consumers believe that the metric is a recommended amount rather than a customary amount.

People view serving sizes as guidelines, and, therefore, tend to eat more as the serving sizes grow. The label on a bag of chips, which often communicates the serving size in a corresponding number of chips, can have a profound effect on how many chips a person feels comfortable eating.

One way to address the misconception about what serving sizes mean would be to simply get rid of the word “serving,” which implies a meal or individual amount. Another, proposed by the researchers, is to make clear on the label that the serving size “does not refer to how much of the product can be healthily consumed in one sitting.”

For more than two decades, the Nutrition Facts labels have used the same portion measurements to communicate nutrition facts. Adjusting for modern day eating habits without inadvertently leading to more gluttony might be a little harder than we think.

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