BMI & Body Composition

Body mass index, or BMI, is one tool physicians and fitness professionals use to determine whether a person is overweight or obese. BMI is a ratio of weight to height, and it does not attempt to assess your percentage of lean tissue and fat. In general, people with a high BMI tend to have high levels of body fat, but this is not always true. If you want a more accurate picture of what your body is made up of, you can do body composition screening.

BMI

According to the American Council on Exercise, or ACE, BMI tends to predict a person's level of body fat fairly reliably. To find your BMI, divide your weight in kilograms by height in meters squared. A person with a BMI of less than 18.5 would be considered underweight, 18.5 to 24.9 is considered normal weight, 25 to 29.9 is overweight, and more than 30 is considered obese. This simple formula is easy to print on posters and pin to a wall in the doctor's office, gym or break room. It's also a simple widget to add to your web browser home page or your personal gadgets.

However, BMI has its shortcomings. Nutritionist Katherine Zeratsky of the Mayo Clinic notes very fit people can have a high BMI that on paper makes them appear to be overweight. On the other hand, people with too high a percentage of body fat and too little muscle could appear to be in the normal range.

Body Composition

Your body weight -- and BMI -- may change throughout the day depending on food intake and food waste, how much water you just finished drinking and other factors. Body composition, the percentage of fat and lean tissue, tends to be very stable. The average body fat percentage for women is 25 to 31 percent, and 18 to 24 percent for men.

There are several tests that can estimate your body composition. Some are more accurate than others, but none of them is perfect. Many are good in theory, but the accuracy of results varies greatly depending on the tester.

Assessments

One category of body composition tests is called anthropomorphic testing. If you measure your chest, waist and hips, you have just done an anthropomorphic test. Circumference measurements are simple and inexpensive --- all you need is a tape measure --- and they are great for assessing your personal progress over time.

Skin fold testing --- in which a tester uses calipers to measure your loose skin in specific areas like the side of your belly and back of the arm --- can be very accurate if done correctly, but the results can vary from one tester to another.

Applications

The purpose of body composition screening or BMI is to put together a picture of your overall health. Carrying excess body fat puts you at risk for heart disease, diabetes, certain cancers, high blood pressure and a variety of other physical and emotional ailments. Your family health history, personal habits and lifestyle choices also make up a big part of that picture.