You know the mantra: Exercise at least 30 minutes a day, three times a week and you'll lose weight. Hit the bike. Take a walk. Swim. Do something, anything, physical. That's the key that opens the weight-loss door.

At least that's what everyone thought.

According to six studies recently published by the Journal of Psychological Science, when it comes to taking off (or keeping off) the weight, exercise might not be as effective as watching what you eat.

The studies followed 1,200 people. Some thought exercise was the way to a leaner waistline. Others thought that eating a well-balanced, healthy diet would get them there quicker.

What the researchers found was that the latter group had the better idea; people who focused more on portion control and cutting carbs lost had lower body mass indexes than those who focused almost entirely on exercise.

Why? Well, the short answer, according to the researchers, is that it's much, much easier not to take in excess calories than it is to burn them once they have been digested.

The longer explanation has to do with how long it takes to actually burn off excess calories. According to the researchers, most people have an inflated sense of how many calories they're getting rid of while they're exercising—even if the stationary bike or elliptical has one of those "calorie counters."

Also, if you eat a few handfuls of potato chips, you're probably eating around 600 calories, whether it feels like it or not. For the average person, that means spending more than an hour on a stationary bike—and most likely two hours.

So the takeaway is this: when it comes to losing weight, focus on your diet. If you want to exercise, do it. It can't hurt. It can help. But it's not the most effective way to lose weight.