Body mass index, or BMI, might be a term you hear your doctor use or you read about in articles. Mathematically, it's a measure of your weight in kilograms divided by the square of your height in meters. In practical terms, it helps evaluate whether or not you're carrying too much body fat. BMI isn't a perfect measurement, but it does offer a simple, noninvasive way to determine body fat percentages in the general population.
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BMI Uses and Calculations
A normal BMI is between 18.5 and 24.9. When your BMI measures 25 to 29.9, you're considered overweight, and obese if it measures above 30. Having too high of a BMI often indicates you're carrying too much body fat and at risk for chronic disease. This high BMI warrants further investigation from your doctor, possibly including a full blood workup, assessment of your family medical history and evaluation of your diet and physical activity level.
A BMI below 18.5 indicates that you're underweight and may need to gain weight to reach a healthy size that supports quality nutritional intake, healthy immunity and optimal energy.
There are many online calculators for BMI -- find one on a reputable health site. To do the calculation by hand, divide your weight in pounds by your height in inches squared. Then multiply by 703 to get your BMI.
The formula reads: BMI = weight in pounds / (height in inches x height in inches) x 703.
Limitations to BMI for Athletes
BMI isn't a perfect measure, as it can overestimate fatness in some people. Body builders and other highly muscled athletes may register as having a BMI that puts them in the category of being overweight or obese because muscle is more dense and makes them seemingly weigh too much for their height. In reality, though, they have very low body fat percentages and aren't at any immediate health risk. Usually, a medical professional can determine visually whether a high BMI calculation is due to body fat or an abundance of lean muscle.
BMI Misses Normal Weight Obesity
BMI can also miss a condition known as "normal weight obesity." When a man has a body fat percentage of above 20 percent, or a woman above 30 percent, they carry too much adipose tissue, even if their weight and BMI fall into a healthy range.
Having this high amount of body fat makes you vulnerable to the typical health complications associated with obesity, including type 2 diabetes, high blood pressure and cardiovascular disease. A study in a 2008 issue of the "International Journal of Obesity" noted that the ability of BMI to determine excessive fat decreased as a person's age increased. Many older adults are vulnerable to this condition, because they're likely to have lost muscle mass and have lighter bones.
BMI for Children and Teens Versus Adults
A different interpretation of the BMI formula is used for children and adolescents. Kids' and teens' weights and heights are specific to their age; while age is less of a factor when you're a fully grown adult of 20 or older. The amount of body fat a child carries is much higher as a toddler or young child, and then slowly diminishes as they enter into the teen years.
A high BMI in a child or teen may catch a doctor's attention so he monitors the individual's weight. It's common to see major fluctuations in weight and height during periods of fast growth, such as puberty, however.