Think twice before trying the high-fat diet trend: Just five days of eating fattening foods can seriously screw up the way your body burns calories

By Charlotte Hilton Andersen
Corbis Images

High-fat diets may be trendy, but you might want to rethink all-cheese-all-the-time: A high-fat diet resets your metabolism, making it harder for your body to burn calories, reports a new study in Obesity. Even more shocking is the fact that it takes less than a week to undo all the hard work from your high-intensity interval classes.

"Most people think they can indulge in high-fat foods and get away with it, but all it takes is five days for your body's muscles to start to protest," said Matt Hulver, an associate professor at Virginia Tech and lead author, in a press release.

In the study, healthy students traded one meal a day for similar calories consisting of 55 percent fat-mainly the unhealthy saturated kinds. The results: Eating that much fat for a work week reduced people's muscles ability to oxidize sugar post-dining. Why that matters: Muscles are the primary way your body process sugar. If they can't do their job-in this case because fat is interfering-you can wind up with insulin resistance which can lead to diabetes, high blood pressure, and, of course, weight gain.

These findings are backed up by research published last year in Biological Psychiatry, which found that people's metabolisms slowed after just one high-fat meal. (Learn The 7 Biggest Myths About Metabolism-Busted.) To make sure this effect wasn't just from the unhealthy saturated fats people were given, Janice Kiecolt-Glaser, Ph.D., and lead author of that study repeated the experiment using healthy fats like polyunsaturated oils, found in canola and olive oil. Surprisingly, the results were the same-regardless of the type of fat!

Over time, Kiecolt-Glaser estimates that indulging once a week in a high-fat meal could add up to an 11-pound weight gain in one year. And that's just from one meal. Hulver points out that if you're consistently eating a high-fat diet, the effect could be much larger. Even more: Any time your metabolism is altered, it can have dire consequences not just for your weight but your whole body. But before you freak out counting the number of weekends you've succumbed to pizza binges, Hulver's team cautions that more studies need to be done, especially to look at the different unhealthy ingredients in foods. (In the meantime, snack on 15 Smart, Healthy Alternatives to Junk Food.)

And also remember that the effect can be reversed. We all have a powerful tool for increasing metabolism, says Keicolt-Glaser: exercise.

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