Body composition typically describes the molecular components that form the human body. Experts divide body composition into two separate components: the fat component and the fat-free component.

The fat component is made up of two types of fat:

• Essential fat. Essential fat is necessary for normal functioning in the body, including providing important metabolic fuel for energy production. Essential fat includes fat in the heart, intestines, kidneys, liver, lungs, marrow of the bones, muscles, spleen and lipid-rich tissues found throughout the central nervous system (brain and spinal cord).
  
  
• Storage fat. Includes the fat that accumulates in adipose tissue to both protect the body and to help conserve body heat. It is located around internal organs (internal storage fat) and just beneath the skin (subcutaneous storage fat).

The fat-free component includes everything in the lean body mass, which is the total weight of water, muscles, bones, ligaments, tendons and internal organs (and excluding the essential fat found in the marrow of bones and internal organs).

Research indicates that fat found in certain areas of the body presents greater risks than fat found in other parts of the body. For example, fat that accumulates around the waist creating an “apple-shaped” body (android obesity) tends to present greater health risks than fat that accumulates around the thighs and buttocks creating a “pear-shaped” body (gynoid obesity).

Being too thin (underweight) also poses potential health risks. People who do not receive enough calories may not have the energy they need to maintain good health and may be at risk for conditions such as infertility, increased vulnerability to infections and osteoporosis.

Body composition typically describes the molecular components that make up the entire  the human body. Water makes up the vast majority of body composition (more than 60 percent of males and more than 50 percent of females). Fat (which varies between men and women) and protein (15 percent) make up the next great largest share, followed by minerals (5 percent). Traces of more than 100,000 chemical compounds comprise the remainder of the body’s composition.

Experts divide body composition into two separate components: the fat component and the fat-free component. The fat component is made up of 80 percent adipose tissue and 20 percent water. The amount of fat varies significantly among individuals. In obese people, fat is sometimes the largest component of the body.

The fat component is made up of two types of fat:

• Essential fat. Essential fat is necessary for normal functioning in the body. For instance, fat provides important metabolic fuel for energy production. Essential fat includes fat in the heart, intestines, kidneys, liver, lungs, marrow of the bones, muscles, spleen and lipid-rich tissues found throughout the central nervous system (the brain and spinal cord). Essential fat is higher in women than men due to gender-characteristic fat needed for child-bearing. This includes higher levels of fat in the mammary glands and pelvic region.
  
  
• Storage fat. Includes the fat that accumulates in adipose tissue to both protect the body from injury and to help conserve body heat. It is located around internal organs (internal storage fat) and right beneath the skin (subcutaneous storage fat). The amount of storage fat a person has may fluctuate over the course of a lifetime. Storage fat is different from blood lipids, which include cholesterol and triglycerides and which are necessary for transporting vitamins.
  

  

The fat-free component includes everything in the lean body mass (the total weight of water, muscles, bones, ligaments, tendons and internal organs) excluding the essential fat found in the marrow of bones and internal organs. When determining a person’s fat-free mass, the amount of essential fat found in the constituents of the lean mass (e.g., bone marrow) is estimated and then subtracted (along with storage fat) from the lean body mass total.

A person’s body composition should contain the proper balance between the fat-free component and the fat component. Excessive body fat can present serious health risks, including cardiovascular disease, diabetes, elevated levels of blood lipids (fats and cholesterol), gallbladder disease, high blood pressure (hypertension), joint disease and respiratory dysfunction.

Research indicates that fat found in certain areas of the body presents greater risks than fat found in other parts of the body. For example, fat that accumulates around the waist creating an “apple-shaped” body (android obesity) tends to present greater health risks than fat that accumulates around the thighs and buttocks creating a “pear-shaped” body (gynoid obesity). Men are more likely to have the former shape, while women are more likely to have the latter shape. This tends to put men at greater risk from increased fat stores, post-menopausal women are often at higher risk for developing abdominal fat than younger women. Health problems associated with excess levels of abdominal fat include heart disease, diabetes, hypertension, some types of cancer and stroke.

Being too thin, or underweight, also poses potential health risks. People who do not consume enough calories may be malnourished. There are numerous causes of malnutrition, including eating disorders (e.g., anorexia nervosa, bulimia) and illnesses (e.g., cancer). Underweight women may develop menstrual problems or become infertile. In addition, underweight people may be less able to preserve underlying tissue when battling wasting diseases such as cancer.

Normal-weight body fat composition falls between 13 percent and 21 percent for men, and 23 percent and 31 percent for women. Health problems are more likely when body fat exceeds 22 percent in young men and 25 percent in men over 40. Health problems for women are more likely when body fat exceeds 32 percent in young women and 35 percent in women over 40. Normal body functions may be interrupted if body fat falls below 5 percent for men, or 15 percent for women.

Athletes tend to have lower body fat composition than these averages. For example, male athletes may have body fat of between 5 percent and 10 percent while female athletes may have body fat levels between 15 percent and 20 percent. However, athletes must be careful not to let their percentage of body fat drop too low. This can pose certain health risks, especially for women who may be at increased risk for cessation of menstruation (amenorrhea) and thinning of the bones (osteoporosis). Other problems associated with excessively low levels of fat include muscle loss and delayed growth in children and adolescents.

In contrast, some people need higher levels of body fat. For example, a woman who is pregnant may need more body fat to help ensure the healthy growth of the fetus. Meanwhile, a person who works outdoors in a cold-weather climate also may benefit from slightly higher levels of body fat.

After age 20, people tend to gain between 1 percent and 3 percent body fat per decade up to the age of 60. Bone mass also may decrease by about 3 percent per decade after about age 35. For this reason, people who weigh the same amount at age 60 as they did at age 20 may in fact have much higher stores of body fat than they did earlier in life.

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.

Preparing questions in advance can help patients to have more meaningful discussions with health professionals regarding their conditions. Patients may wish to ask their doctor or registered dietitian the following questions related to body composition:

1. Should I have my body composition measured?
   
   
2. What are the pros and cons of the various tests?
   
   
3. What type of test do you recommend?
   
   
4. Where should I go for this test?
   
   
5. Will I have to pay for this test?
   
   
6. Should I have more than one test for increased accuracy?
   
   
7. Once my body composition has been measured, what steps will follow?
   
   
8. I’m involved in athletics. How low can I safely allow my body fat percentage to sink?
   
   
9. Where can I find more information about diet and exercise that can reduce my percentage of body fat?
   
   
10. I’m trying to become pregnant. Should I increase my percentage of body fat?