When you buy a car with a six-cylinder engine, you expect to get six cylinders. When you buy a dress in a size 10, you expect a size 10. And when you buy a burger at a fast-food joint that's listed on the menu as containing 500 calories, you jolly well expect 500. But you may be getting a lot more than that. The same may true of the omelet and the pasta you get at a sit-down restaurant—and of the frozen dinner with the label you read so carefully before you tossed it in your supermarket basket and took it home.

According to a new study published in the Journal of the American Dietetic Association, prepared foods may contain an average of 8 percent more calories than their package labels own up to and restaurant meals may contain a whopping 18 percent more. Worse still, as far as Food and Drug Administration regulations are concerned, that's perfectly OK.

The findings are the result of work conducted by Susan Roberts, professor of nutrition at Tufts University, and Jean Mayer, of Tufts' USDA Human Nutrition Research Center on Aging. It was Roberts who initiated the study, and it was her own struggles with weight that got her started. Author of the book "The Instant Diet," she was working on new recipes for the paperback version and, as was her practice, used herself as a guinea pig. As a rule, she lost weight on the menu plans she recommended to readers, but when she redeveloped some of the meals using what were supposed to be calorically equivalent supermarket or restaurant foods, the pounds stopped dropping off. Just as suspiciously, she always felt full.

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