But before you get mad at the companies, consider this: According to the government, even a 20 percent margin of error is acceptable under current nutrition labeling guidelines. That means a 200-calorie frozen diet dinner, for example, could really be 240 calories — not a huge difference, but definitely something that could sabotage your attempts to lose weight or even maintain it if you're eating a frozen entree several times a week.
Hidden calories add up
Tufts nutrition professor Susan Roberts, lead investigator for the study, calls these "hidden calories" a real problem for those over 50 and watching their weight. "Eating 10 percent more calories than you think is enough to cause 10 or 20 pounds of weight gain a year."
In fact, Roberts got the idea for the study when she couldn't lose weight while researching her own weight-loss book, The Instinct Diet. The book's two-track menus allow dieters to either eat at home or eat restaurant or packaged food. Roberts lost weight on the home-cooked food, but when she was on the eat-out track, "I completely stopped losing weight," she says. Suspicious of the stated calorie counts on the prepared foods she was eating, she decided to test them in Tufts' Energy Metabolism Laboratory.
You can't count on the calorie counts
She and her researchers found about 18 percent more calories than the stated value in foods served at 29 quick-serve and sit-down restaurants, as well as an average of 8 percent more calories than stated in 10 frozen meals purchased from supermarkets. Some of the restaurant dishes had up to twice as many calories as reported — other foods had fewer calories than reported.
Many of the foods were marketed as diet meals. The researchers found that Lean Cuisine shrimp and angel hair pasta, for example, had 28 percent more calories than stated on the package, while Weight Watchers lemon herb chicken piccata had 21 percent more calories than listed. The biggest calorie bonanza: Denny's grits with butter packed a whopping 200 percent more calories than stated, the study reported.