Body Mass Index (BMI) is a number calculated from a person’s weight and height. BMI is a fairly reliable indicator of body fatness for most people. BMI does not measure body fat directly, but research has shown that BMI correlates to direct measures of body fat, such as underwater weighing and dual energy x-ray absorptiometry (DXA).1, 2 BMI can be considered an alternative for direct measures of body fat. Additionally, BMI is an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems.

How is BMI used?

BMI is used as a screening tool to identify possible weight problems for adults. However, BMI is not a diagnostic tool. For example, a person may have a high BMI. However, to determine if excess weight is a health risk, a healthcare provider would need to perform further assessments. These assessments might include skinfold thickness measurements, evaluations of diet, physical activity, family history, and other appropriate health screenings.

Why does CDC use BMI to measure overweight and obesity?

Calculating BMI is one of the best methods for population assessment of overweight and obesity. Because calculation requires only height and weight, it is inexpensive and easy to use for clinicians and for the general public. The use of BMI allows people to compare their own weight status to that of the general population.

Water is a good conductor of electricity, and most body water is found in the lean mass. Fat, which has almost no water in it, is such a poor conductor of electricity that it actually impedes the electrical flow. BIA equipment comes in two basic forms. In one form, the subject lies down and the right wrist and right ankle are fitted with electrodes, which produce an electrical current that runs from the wrist to the ankle. In another form, the subject stands on a platform with bare feet, and an electrical current runs from the right foot, up the right leg, down the left leg, and out the left foot. Regardless of the BIA equipment used, the principle behind the technique is the same.

Although BIA has an excellent theoretical basis for making good body composition predictions, several important protocols must be followed for the results to be accurate and repeatable. Since the technique is dependent on electrical conductivity through the lean mass, the hydration state of the subject can alter the results. If someone having a BIA test is not well hydrated, the electrical current will not be conducted through the lean mass as well, so the subject will appear to have more fat mass than they actually do.

Dual-energy X-ray absorptiometry (DEXA) is the latest, most accurate, and most expensive means of determining body composition, and it is generally considered the current gold standard for this purpose. The information you can derive from a full-body scan on an athlete is invaluable, including bone density; body fat percentage; lean body mass; fat mass; and the distribution of fat and lean tissue in the arms, trunk, and legs. DEXA output even provides the differences in lean mass and fat mass between the left and right sides.
DEXA works by passing two X-ray beams through the subject and measuring the amount of X ray absorbed by the tissue it has passed through. One beam is high intensity and one is low intensity, so the relative absorbance of each beam is an indication of the density of the tissue it has passed through. The higher the tissue density, the greater the reduction in X-ray intensity. If you can find a lab with DEXA, the usual cost for a full-body scan is somewhere between $100 and $250.

The BOD POD Express Body Composition Tracking System uses patented Air Displacement Plethysmography for determining percent fat and fat-free mass in adults and children. The simple, 5-minute test consists of measuring the subject’s mass (weight) using a very accurate electronic scale, and volume, which is determined by sitting inside the BOD POD chamber. From these two measurements, the subject’s body composition is calculated.

The skinfold estimation methods are based on a skinfold test, whereby a pinch of skin is precisely measured by calipers at several standardized points on the body to determine the subcutaneous fat layer thickness. These measurements are converted to an estimated body fat percentage by an equation. Some formulas require as few as three measurements, others as many as seven. The accuracy of these estimates is more dependent on a person’s unique body fat distribution than on the number of sites measured. As well, it is of utmost importance to test in a precise location with a fixed pressure. Although it may not give an accurate reading of real body fat percentage, it is a reliable measure of body composition change over a period of time, provided the test is carried out by the same person with the same technique.

Skinfold-based body fat estimation is sensitive to the type of caliper used, and technique. This method also only measures one type of fat: subcutaneous adipose tissue (fat under the skin). Two individuals might have nearly identical measurements at all of the skin fold sites, yet differ greatly in their body fat levels due to differences in other body fat deposits such as visceral adipose tissue: fat in the abdominal cavity. Some models partially address this problem by including age as a variable in the statistics and the resulting formula. Older individuals are found to have a lower body density for the same skinfold measurements, which is assumed to signify a higher body fat percentage. However, older, highly athletic individuals might not fit this assumption, causing the formulas to underestimate their body density.