Test Your Knowledge

Direct measurements of body composition are not possible on a living person because fat surrounds many types of body tissues. Although people usually weigh themselves to determine whether or not their fat level increased, this technique actually reveals nothing about changes in body composition. People can gain weight due to factors such as increased muscle mass or increased bone density. Therefore, people whose weight has increased may believe they have gotten fatter, when in fact they have merely added muscle mass.

Health care professionals often use one or more of three basic measurements when monitoring a patient’s weight and body composition. None of these tests offer precise measures of body composition, but they are often used for basic testing because they are easy to perform and inexpensive.

Commonly available tests include:

• Body Mass Index (BMI). BMI is a number that shows body weight adjusted for height. It is a metric measure used to estimate the amount of body fat the person carries. It is arrived at by dividing weight by height. Using standard measurement, divide weight (in pounds) by height (in inches squared) and multiply by 705. Using the metric system, divide weight (in kilograms) by height (in meters squared). In general, healthy weight is defined as a BMI of between 18.5 and 24.9. Meanwhile, a BMI below 18.5 is considered underweight while a BMI over 24.9 is considered overweight. Statistics show that more than half of adults in the United States have a BMI greater than 24.9. BMI is an effective technique for measuring body composition because it typically correlates highly with a person’s percentage of body fat. However, BMI may be misleadingly high in well-trained athletes, others with dense muscle mass and people with low muscle mass, such as the elderly. BMI measures also may be less accurate when applied to people in certain ethnic groups. For example, some African Americans tend to have greater bone density and muscle mass than some other racial groups. Therefore, BMI measures may not be as accurate for this group.  In addition, BMI should not be used on its own to determine a person’s health or risk for certain illnesses. Instead, it should be considered along with other factors, including personal and family medical history and lifestyle factors such as history of smoking.
  
  
• Skinfold measurements. A practical and economical method of measuring body composition, it involves measuring the thickness of a skinfold at specific sites on the body. Typically, measurements must be taken in at least three locations. An instrument called a caliper is used to help obtain the measurements. The accuracy of the skinfold method depends highly on the skill and experience of the person performing the test. Skinfold tests can be obtained in many different places, including schools, local fitness clubs and hospital-based wellness programs.
  
  
• Waist circumference. A tape measure is used to gauge the abdominal fat around a person’s waist. This test is the most practical indicator of a patient’s level of abdominal fat. It typically is performed in the morning before a person’s first meal but after the bladder is emptied. Men with a waist circumference of greater than 40 inches (102 centimeters) and women with a waist circumference of greater than 35 inches (88 centimeters) have a greater risk of health-related problems.
  
  
• Waist-to-hip ratio. This ratio is determined by measuring the circumference of the waist and dividing it by the circumference of the hips. If the stomach measurement is 1.5 times larger than the hip measurement, indicating abdominal or visceral fat, the risk of various diseases (e.g., diabetes, cardiovascular disease) is increased. The waist-to-hip ratio is considered to be less accurate than some other measures. For this reason, organizations such as the American Heart Association (AHA) no longer recommend using it as a measure of body composition.

Experts also use more sophisticated techniques to indirectly measure body composition. They are all based on the assumption that body weight equals fat added to lean tissue (including water). Body composition is measured by using one or more of several tests based on the two-component model of body composition that divides the body into fat and fat-free components.

Tests of body composition that are not commonly available and that are usually performed in special facilities include:

• Air displacement plethysmography. Estimates body composition by having a person sit inside a chamber in which sensors calculate the amount of air displaced by the person’s body. This test typically is available only at special facilities such as research institutions and colleges and universities.
  
  
• Bioelectrical impedance analysis (BIA). During this test, a person lies on a table or on the floor with electrodes attached to the hands and feet. BIA uses safe, low-level electrical current that flows from the electrodes and through intracellular and extracellular fluids. A person’s body water makes up the largest percentage of their fat-free body weight. This makes fat-free body weight a much better conductor than body fat, which contains little water. Fat-free body mass and body fat is then estimated from this measure of total body water. Bioelectrical impedance may be available at schools, health and fitness clubs, and health fairs.
  
  
• Bodpod. A test that uses a computerized, oval chamber to measure a person’s volume and mass. The person’s whole-body composition can then be calculated, including fat and muscle percentages.
  
  
• Dual energy x-ray absorptiometry (DEXA). Uses two low-dose x-rays to make distinctions among fat-free soft tissue, fat tissue and bone tissue. This provides an accurate measurement of total fat and its distribution in most people. This test typically is available only at special facilities such as research institutions and colleges and universities.
  
  
• Hydrostatic weighing. Technique considered the “gold standard” for determining a person’s body composition, it measures  body volume and body density by immersing the individual in water. As the person is submerged, air is completely exhaled from the lungs except for a small amount known as the residual lung volume. Once the body weight, underwater weight and residual lung volume are known, total body density can be calculated. The percent of body fat is then calculated from the total body density using the person’s age, height and equations specific to gender. This test typically is available only at special facilities such as research institutions and colleges and universities.
  
  
• Near infrared interactance. Uses the principles of light absorption, reflectance and near infrared spectroscopy to estimate body composition. A probe is placed on a part of the body and sends an infrared light through both fat and muscle that is reflected back to the probe. Some studies have found near infrared interactance to be less accurate than tests such as the skinfold method. This test may be available at health clubs and weight-loss centers.