Scientists gave two groups of people popcorn, but told one group that the snack was “healthy” and the other that it was “unhealthy.” The people who thought they were eating diet popcorn chowed down on twice as much—2.33 cups compared to 1 cup. MORE: The Better Man Project: 2,000+ Small Steps That Will Transform Your Body—and Your Life The reason: Your brain tricks you into feeling less full when you eat a food that you think is healthy, says study author Jacob Suher, a PhD student at The University of Texas at Austin. This phenomenon was confirmed in another study: The scientists gave people either a “healthy” or “unhealthy” cookie (it was the exact same cookie). The subjects who ate the supposedly virtuous treats reported that they were hungrier afterward. You may unconsciously think that healthy foods are light—as in, not filling, says Suher. That belief is drilled into your brain every time you eat a quarter pounder with cheese and feel stuffed afterward, or opt for a garden salad and feel famished 30 minutes later. Over time, your body becomes conditioned to feel dissatisfied when you eat things you consider “healthy”—even if the food contains plenty of calories, protein, fat, and fiber that should fill you up. MORE: The 6 Sinister Reasons You Overeat It’s like Pavlov’s dog, but instead of salivating when you hear a bell ring, you feel hungry after you eat healthy. Of course, it’s fine to gorge on something like fresh vegetables. But be careful with your portion sizes when eating high-calorie foods that are advertised as good for you, Suher says. Think: “All-natural” burritos, gluten-free pizza, or cookies in wholesome-looking packaging. These foods still contain lots of calories, and overeating them can spell bad news for your belly. Use the standby tips for managing portions: Use smaller plates and pre-measure servings rather than eating out of the container. (Follow the label guidelines or use these 12 Easy Ways to Estimate Serving Sizes.) The article “How Eating “Healthy” Makes You Fat” originally ran on MensHealth.com.