Body composition tests estimate the proportions of fat and lean (fat-free) mass in the body. Although there are many types of body composition tests, the simplest—and by far the most common—are the body mass index (BMI) and the waist-to-hip ratio.
A body composition test is most often performed to determine whether an individual is of a healthy weight, underweight, overweight, or obese. Obesity affects millions of American children, adolescents, and adults, posing significant health risks. A body composition test that indicates excessive accumulation of fat is associated with a higher risk for obesityrelated conditions, disability, and death. Obesityrelated conditions include high blood cholesterol, high blood triglycerides, high blood pressure, and chronic illnesses such as heart disease and type 2 diabetes. A body composition test that indicates underweight may be a sign of other health problems or poor nutrition. Body composition tests are particularly useful for tracking changes that occur over time with weight gain or loss or as a result of diseases, such as HIV/AIDS or cancer, or their treatments.
BMI is usually used to screen for and diagnose weight problems. Because BMI depends only on weight and height, it is inexpensive and easily used by both healthcare providers and the general public. It is also the best method for assessing the overall incidence of overweight and obesity within populations and communities and it enables individuals to compare their own weight with the general population. Unlike some other body composition tests, such as underwater weighing or dual-energy x-ray absorptiometry (DXA or DEXA), BMI does not directly measure body fat. However, research has shown that BMI correlates well with other, more direct body composition tests. The U.S. Department of Agriculture's individualized food plan, ChooseMyPlate.gov , makes diet and exercise recommendations based on an individual's BMI.
Easy-to-use adult BMI calculators in pounds, feet, and inches are available at http://nhlbisupport.com/bmi and http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html . BMI can be estimated from one's weight and height using a BMI index chart available at http://www.win.niddk.nih.gov/publications/better_health/bmi.jpg .
BMIs for children and teens aged two through 19 are calculated in the same way as for adults, but are compared to the BMIs of other children of the same age and gender and assigned to a percentile. This is because the amount of body fat changes with a child's age and differs between male and female children. Furthermore, the normal proportion of body fat is slightly higher in children and teens than in mature adults. Therefore BMI for children and adolescents is sometimes referred to as BMI-for-age. The U.S. Centers for Disease Control and Prevention (CDC) use BMI-for-age growth charts that take these age and gender differences into account. In the United States, the size and growth patterns of children are generally assessed using percentiles, so this is also a convenient way to assess BMI. The BMI percentile indicates the relative position of a child's weight compared to other children of the same age and sex. For example, a girl in the 75th percentile for her age group weighs more than 74 of every 100 girls her age and less than 25 of every 100 girls her age. A BMI percentile calculator for children and teens, in fractions of pounds, feet, and inches, is available at http://apps.nccd.cdc.gov/dnpabmi . BMI is not appropriate for children under age two.
Waist-to-hip ratio is a simple method of measuring abdominal fat. Excessive abdominal fat can be a risk factor for obesity-related conditions, even in people of otherwise normal weight. Waist circumference is determined with a tape measure around one's bare abdomen, at the narrowed point just above the hip bone. The tape should be snug, but should not compress the skin, and should be parallel with the floor. The measurement is taken with the individual relaxed and exhaling. The hip measurement should be taken at the widest point of the hip. The waist measurement is divided by the hip measurement to obtain the waist-to-hip ratio.
Anthropometric or body measurement methods, other than the BMI and waist-to-hip ratio, are performed by a trained technician. Sometimes a tape is used to measure not just the waist and hips, but other key regions, such as the biceps and thighs.
Skin-fold thickness is a more direct measurement of body fat. A metal tool called calipers is used to pinch body tissues in different areas, such as the back of the upper arm, to determine thickness and distinguish between muscle and fat tissue. The measurements are compared with standards.
Other body composition tests require complex and/or expensive equipment and highly skilled professionals. They can be difficult to standardize because of differences among operators and machines, and are thus less useful for comparing different people or for tracking individual changes in body composition over time. Underwater weighing, isotope dilution studies, air-displacement plethysmography (ADP), and imaging techniques are rarely used to determine body composition except for research studies.
Underwater or hydrostatic weighing, also called hydrodensitometry, is considered the most accurate method of estimating body fat. The patient is weighed, exhales as completely as possible, and is immersed in a tank of water. The body volume corresponds to the volume of water displaced and body density is determined from the weight and volume. Since fat tissue is less dense than bone and muscle, the proportion of body fat can be calculated.
Bioelectrical impedance analysis (BIA) uses a weak electrical current to measure the percentage of body fat. Electrodes are attached to the skin on various parts of the body and an electrical current—which is too weak to be felt—is circulated through the prone patient. The electrodes measure impedance or resistance to the signal at various points. Water decreases the resistance. Since fat is about 10%–20% water, and fat-free body mass—muscle, bone, and water outside of the muscles—averages 70%–75% water, the resistance indicates the proportion of fat to muscle in various parts of the body. Body composition is analyzed by a computer, based on the resistance readings and the patient's physical characteristics, including weight, height, age, gender, body type, level of physical activity, and ethnicity.
Dual-energy x-ray absorptiometry (DXA) scanning provides a very accurate—but very expensive—body composition profile. X-rays are used to determine the amounts of fat and fat-free (lean) mass and bone mineral content and density, since these different types of body tissue absorb different amounts of x-ray energy.
Computed tomography (CT or CAT) scans or magnetic resonance imaging (MRI) can also be used to determine body composition. CT scans visualize slices through the body with x-rays and can be used to calculate the ratio of abdominal fat to fat under the skin. MRI uses magnets to image the body and determine the amounts and locations of fat.
The BMI is not a direct measure of body composition. Although it is very useful as a screening tool, it is not in itself diagnostic of overweight or obesity or of overall health. Most significantly, the BMI does not distinguish between muscle and fat. Thus, the BMI may overestimate the proportion of body fat in athletes and others with muscular builds, who may weigh far more than sedentary people of similar height while having significantly less body fat. Furthermore, the BMI can underestimate body fat in older people, chronic dieters, and others who have lost muscle mass and therefore weigh less and may appear thin although they have a high proportion of body fat. Although the correlation between BMI and direct body fat measurements, such as underwater weighing or DXA, is fairly good, the correlation does vary with gender, age, and race. For example, women tend to have more body fat than men with the same BMI. Older people also tend to have more body fat than younger adults with the same BMI. The BMI is usually more reliable at indicating obesity than it is at indicating overweight.
Other body composition tests have limitations as well. For example, in contrast to the BMI, underwater weighing can underestimate the percentage of body fat in athletes and overestimate body fat in the elderly.
No specific preparations are necessary for most body composition tests. It is recommended that beginning at age two, children and teens have their BMI calculated at each routine physical exam.
Although obesity is generally diagnosed from the BMI, a body composition test should never be the only consideration in determining health or disease risk. In addition to BMI, a patient's waist circumference or waist-to-hip ratio should be considered, since abdominal fat is a predictor of obesity-related disease risk. Physicians also observe how the patient's body fat is carried: “apple-shaped” patients who carry most of their weight around their abdomen and waist are at a higher risk for diabetes, heart disease, stroke, and cancer than “pear-shaped” patients who carry excess pounds primarily in their hips and thighs. Other risk factors for obesity-related conditions, such as high blood pressure and physical inactivity, should also be considered.
There are usually no complications from body composition tests. However, it is possible to over-interpret the results.
In general, a healthy body composition is considered to be about 15% body fat for adult males and 20–25% for adult females. In developed countries, BMI generally increases with age, as does the proportion of intra-abdominal fat.
Research has shown that adults with BMIs within the normal range generally live the longest and enjoy the best health. For most adults, BMIs are interpreted as follows:
The BMI is considered a reliable indicator of body fat in most children and teens. The BMI-for-age categories for children are:
Although children in the highest 15 percentiles are considered to be at risk for developing health problems due to their weight, the CDC does not use the term “obese” for children and teens. This is because the proportion of body fat fluctuates significantly during growth and development.
In general, waist circumferences of more than 40 in. (102 cm) for adult men and more than 35 in. (88 cm) for nonpregnant adult women indicate an increased risk for cardiovascular disease, high blood pressure, and type 2 diabetes. This is true even for people with BMIs within the healthy range. A healthy waist-to-hip ratio is considered to be below 0.9 for men and below 0.8 for women. However, this assumption may not hold for people with fat accumulation around the waist who have diseases such as HIV/AIDS.
Using hydrostatic weighing, obesity is generally diagnosed in women with body fat exceeding 30%–32% of total body mass and in men with body fat exceeding 25%–27% of total body mass. With other types of body composition tests, the results depend on a variety of factors. For example, BIA analysis depends on the patient's age and other factors.
See also Age and exercise ; Body mass index ; Cardiovascular disease ; Cholesterol ; Obesity ; Waist circumference .
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Margaret Alic, PhD