I'm 16 and I recently got a scale that measures body fat, but it doesn't say what body fat percentage teens should have. I'm exercising a lot to lose weight, but I've noticed that the more I exercise, the more I gain on the BMI scale. How could I possibly be exercising more but getting fatter?
- Tania*

You may not be getting fatter at all. Actually, it could be the opposite: You may be losing fat but gaining muscle. This can affect your weight because muscle weighs more than the same-size amount of fat.

BMI stands for body mass index, which is a calculation that shows the relationship between a person's height and weight.

BMI can give you an idea of whether you have too much or too little body fat. But it's by no means perfect. For example, someone can have a high BMI number from having a large frame or a lot of muscle, without having too much fat. People usually gain muscle mass and lose fat when they exercise more. Some very muscular athletes may have a higher body weight and a higher BMI than people the same height and age who don't exercise and who have more fat and less muscle. So the fact that you're getting more exercise may be one reason your BMI scale shows an increase — you may be building more muscle.

The concept of BMI is confusing. Because it can be hard to understand what a BMI number means, your best bet is to work with your doctor or another health professional — such as a school nurse — to figure out what it means for you. A doctor can help you decide whether you are at a healthy weight or if it's something to be concerned about.

Reviewed by: Mary L. Gavin, MD
Date reviewed: January 2013

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*Names have been changed to protect user privacy.