FYI on BMI: How and why to calculate your body mass index

The number on the scale isn't the only thing to take into consideration when determining weight and health risk. Doctors, trainers and dietitians also look at the body mass index, or BMI.

Kate Fairbairn, certified nutrition specialist and lifestyle strategies/wellness coach, ACE personal trainer at Fitness Pointe in Munster, says the BMI is a simple mathematical computation of the ratio of weight to height, and is another tool to help determine health risk for disease. "The higher the BMI number, the more risk for disease such as hypertension, diabetes, stroke, osteoarthritis, gallbladder disease, and coronary heart disease," she says. "Easy-to-use charts are available in doctors' offices, health clubs or online."

Dr. Holly Lucille, a nationally recognized licensed naturopathic physician, author and educator, says the number is reached by using weight plus height. So if you are 5 foot, 8 inches and weigh 160 pounds, your BMI is 24.3, the high end of normal.

Mortality rate decreases as the BMI decreases. The lowest mortality range for females is between 19 and 21 BMI and for males 20 to 22 BMI. Normal weight is 18.5-24.9 BMI, overweight 24.9-29.9, obese 30 plus, and over 40 is considered morbidly obese. The BMI does not distinguish between female and male in regards to weight ratings. "Although the BMI is an estimate, it does give us a good idea of the amount of fat versus lean muscle tissue on an individual," Dr. Lucille says.

It's important to note that the BMI does not measure body fat. "Individuals who are muscular, and have a large skeletal mass and therefore higher scale weight will have a higher BMI, but won't necessarily be considered overweight or have increased disease risk," Fairbairn says.

Fairbairn says the waist circumference factor is a useful tool to include with the BMI, since the higher the waist measurement the higher the disease risk. "Risk factors increase regardless of an individual's BMI when a women is over 35 inches around the waist and a man is over 40 inches," she says. "So essentially the risk factor is higher when body fat accumulates in the abdominal area versus hips and buttocks."

Dr. Lucille adds that this doesn't give pear-shaped and hourglass-shaped individuals a free pass. "Even if the weight is on the thighs and hips and not around the waist, the heart still has to work harder to pump nutrients to that extra weight, so the health risk factors are still there. However, try not to get hung up on the pieces of the puzzle. It's good to pay attention to the BMI but not to obsess about it."