Picture this: You’re in the snack aisle of the grocery store and as your hand is reaching for the Pepperidge Farm Milano Cookies you remember your upcoming tropical vacation, your new bikini and the fact that your sister-in-law (whose baby weight “just fell right off”) will also be there. So instead your hand makes a swift move to the right and grabs the fat-free chocolate cookies instead.

It sounds logical—you eat low-fat foods, you lose weight—but studies show that the opposite is more likely. That’s because sticking a low-fat label on food can trick you into believing that tasty bag of cookies is better for you than the full-fat version. “People automatically perceive low-fat and fat-free products as healthier or lower in calories, so they don’t worry as much about watching their portions,” explains Joy Bauer, R.D., nutrition and health expert for NBC’s “Today” show. “And food companies know this so they try to use this to their advantage when labeling products. But just because a product is low in fat doesn’t mean you have carte blanche to eat as much as you want…those calories still count!

”Reduced-fat foods are often perceived as having a health halo—that they’re so good for you, they reside in a calorie-free zone—so people tend to supersize their portions without being wracked with guilt. In fact, when people are given foods marked as low fat, they chow down 25 to 44 percent more calories than when foods are labeled as regular fat, according to Brian Wansink, Ph.D., author of “Mindless Eating: Why We Eat More Than We Think” and director of Cornell University’s Food and Brand Lab.

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They also underestimate the total number of calories they’re consuming. Most snack foods are lower in calories only by about 11 percent compared to their full-fat versions, according to Wansink, yet typically, people believe that low-fat foods are 44 percent lower in calories. What’s more, “they believe they are entitled to eat more because they sacrificed by eating a low-fat food,” says Wansink.Adds Bauer, “people feel virtuous for choosing low-fat ice cream over the full-fat regular kind so they might order a large instead of a small. But just because foods are low-fat or fat-free doesn’t mean they’re low in calories, and controlling calories—not fat—is most important when it comes to losing pounds.”

For example, reduced-fat peanut butter has almost as many calories as regular peanut butter, according to Bauer, and low-fat cookies have more carbs and sugar than regular cookies and about the same number of calories. While swapping out fat sounds like it can only be a good thing, experts say that’s not necessarily so. When food manufacturers cut the fat, they often replace it with extra sugar to make the food more palatable, particularly in low-fat cookies and other baked goods, according to Bauer. They also may add thickening agents, such as carrageenan and various gums, to mimic the texture of fat.

“Fat is a very addictive and satisfying component of food, and you’re replacing it with another very addictive food—sugar,” says Kristin Kirkpatrick, R.D., wellness manager for Cleveland Clinic’s Lifestyle 180 program and YouBeauty Nutrition Advisor. The trouble is, sugar doesn’t provide the same level of satiety as fat—that satisfaction you get after eating a meal—which can leave you searching for another fix and vulnerable to eating more later on. “We need fat in our diets,” says Kirkpatrick. “Fat is extremely satisfying and keeps us fuller for longer, while sugar tends to spike our blood sugar, followed by a very big drop, and causes us to feel hungrier—and look for more sugar.

”While that doesn’t mean you have free rein to down that double cheeseburger and chase it with a pint of Ben & Jerry’s, it does mean that there’s room for some fat in your diet—and that low-fat and fat-free foods aren’t always the healthiest option.

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To keep your portions and calories in check, follow these smart steps:* When your sweet tooth is demanding some serious attention, rather than inhaling half of a bag of fat-free cookies, go for a real chocolate chip cookie instead—just be sure to stop at one. “If you want to have a sweet every once in a while, don’t beat yourself up about it,” says Kirkpatrick.*

Never eat straight out of the package. Whether you’re indulging in low-fat or full-fat snack foods, check the serving size on the nutrition facts panel, take out one serving’s worth, put the package away and then enjoy your snack, suggests Bauer.* If you’re eating a low-fat food, such as a frozen yogurt, but aren’t sure how many calories it contains, give your best estimate—and then double it. “You’ll be more accurate,” says Wansink.

But not all low-fat and fat-free foods should be banished like a loser on “Survivor.” A good example is dairy. “Full-fat dairy products such as whole milk are high in saturated fat, the type of fat that raises LDL ‘bad’ cholesterol levels and promotes inflammation, so you definitely want to choose low-fat or fat-free dairy products to minimize the saturated fat in your diet,” says Bauer. Go for skim or one-percent milk, and nonfat or low-fat yogurt.

When it comes to cheese, choose reduced-fat cheeses made with two-percent milk. “Without some fat, the cheese has a chewy or rubbery rather than creamy texture and won’t melt well,” says Bauer. “But choosing cheeses made with two-percent milk instead of the typical whole milk will still save you a significant amount of calories and fat.”