The Body Mass Index Mystery

From the red carpet in Hollywood to the runways of Milan, skinny is the rage.

But, in September, Madrid shocked the international fashion world by turning away models they considered too thin from its annual fashion week. Those who were banned had a body mass index, or BMI, below 18.

But many people are confused about what BMI means. The Centers for Disease Control and Prevention say it is a number calculated from a person's weight and height. It's commonly used to classify weight as healthy or unhealthy. The banned models in Madrid, who had a BMI of 18 or less were considered unhealthy.

The BMI is calculated using a ratio between weight and height. According to the BMI chart, anything between 19 and 25 is normal and 25 to 30 is overweight. Anything above 30 is obese. But experts say that BMI can be misleading.

"You can't take an NFL player, do a BMI on him, and say, 'You know what? Your BMI's at 28. You're overweight,' " Joe Reyes, a fitness trainer at The Sports Club LA in New York City told The Early Show correspondent Debbye Turner. "You know what, he is overweight because he's very, very muscular. He has to be that way."

It also doesn't take into consideration other factors such as the differences between men and women, or young and old. Plus BMI doesn't reflect health status like heart disease or high blood pressure.

"It should be used maybe as a base and then other tests should be done in order to confirm what that BMI is saying," Reyes said.

Suzanne Meth, an exercise physiologist at Equinox gyms in New York City, said BMI is a good way to talk about large populations, but isn't always the best way to determine an individual's weight issues.

"When you're looking at an individual's health, you want to lock at body composition, frame type, things like that," she told The Early Show co-anchor Julie Chen. "It's one indicator, but in my opinion, it's not the best … What really matters is composition so you want to have a high amount of lean muscle fat and low amount of body fat. That's the goal, much less the weight."

Meth said the best approach is to simply start exercising.

"You want to put on muscle mass and try to lower your body fat through cardio, resistance training and eating well," she said.

Meth said an easy way to gage physical fitness is divide the waist measurement in inches by the hip measurement in inches.

"You want a ratio to be .08 for men and below .01 for men," she said. "You should get your fat measured. You can go to a gym and get a trainer to assess you."