Determination of Total Body Fat

Overweight is defined as a body mass index (BMI) of 25 to 29.9 kg/m². Obesity is defined as an excess of total body fat that is documented by a BMI of 30 kg/m². Several methods are available for determining or calculating total body fat: total body water, total body potassium, bioelectrical impedance, and dual-energy X-ray absorptiometry.

Rationale: Even though accurate methods to assess body fat exist, measuring body fat content by these techniques is often expensive and is not readily available clinically. Although bioelectrical impedance devices are becoming more readily available, they lose accuracy in severely obese persons and are of limited usefulness for tracking changes in total body fat in persons losing weight. Thus, bioelectrical impedance offers no significant advantage over BMI in the clinical management of patients. No trial data exist to indicate that one measure of fatness is better than any other for following overweight and obese patients during treatment. No studies have been published to compare the effectiveness of different measures for evaluating changes in body fat during weight reduction.

BMI provides a more accurate measure of total body fat than relying on weight alone. It has an advantage over percent above ideal weight (e.g., based on the Metropolitan Life Insurance Tables). Ideal body weight tables were developed primarily from white, higher socioeconomic status populations and have not been documented to accurately reflect body fat content in the public at large. In addition, separate tables are required for men and women. The weight tables also are based on mortality outcomes and do not necessarily predict morbidity.

BMI is recommended as a practical approach for the clinical setting. BMI provides an acceptable approximation for assessment of total body fat for the majority of patients (525-527).  However, simply measuring body weight is a practical approach to follow weight changes.

Rationale: The panel concentrated on tools available in the office, i.e., weight, height, and the BMI. BMI is a practical indicator of the severity of obesity, and it can be calculated from existing tables. BMI is a direct calculation based on height and weight, regardless of gender (528). The limitations of BMI as a measure of total body fat, nonetheless, must be recognized. For example, BMI overestimates body fat in persons who are very muscular and can underestimate body fat in persons who have lost muscle mass (e.g., the elderly).

The BMI is calculated as follows:
BMI=weight (kg)/height squared (m²)

To estimate BMI from pounds and inches use:
[weight (pounds)/height (inches)²] x 703=

(1 lb=0.4536 kg)
(1 in=2.54 cm=0.0254 m)

A simple BMI chart is provided below.

Body Mass Index