Not so fast, you low-fat-frozen-yogurt fiend! Labels are deceiving: people often eat more calories of low-fat foods than of regular, because they eat a lot more of them, period.

In this study, families visiting a university open house were offered either “regular” or “low-fat” M&Ms. Those given low-fat M&Ms took 28 percent more, especially when they were overweight, and most people so over-served themselves that they took more calories of the low-fat candy than the full fat candy. And they didn’t know it: All participants underestimated the number of calories in their bowls, especially if they took low-fat M&Ms.

Next, participants were shown a measuring cup each of granola and M&Ms, labeled “regular” or “low-fat.” As in the first experiment, those with the low-fat cups thought the appropriate serving size was 25 percent bigger than those with the regular cups, and believed that the low-fat foods were much lower in calories than they really were. They also thought they’d feel less guilty eating the low-fat foods.

Finally, participants watched a movie while munching on a bag of granola. Those with low-fat granola ate 50 percent more! Participants also ate much less if the bag indicated that it contained two servings rather than one, even though the actual amount was the same. And you guessed it…participants underestimated the number of calories they’d eaten, especially with the low-fat granola.

The bottom line: Beware the low-fat label, and heed the serving size!