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In physical fitness, body composition is used to describe the percentages of fat, bone and muscle in human bodies. The percentages of fat (body fat percentage) is of most interest because it can be very helpful in judging health in addition to body weight.

Because muscular tissue takes up less space in our body than fat tissue, our body composition, as well as our weight, determines how lean we appear. Two people at the same height and same body weight may look completely different from each other because they have a different body composition.

The National Institute of Health ^[1] recommends that a healthy adult male's body should have between 13 and 17 percent fat. A healthy female's should be composed of between 20 and 25 percent fat. Levels significantly above these amounts may indicate excess body fat. Athletes, leaner individuals, and more muscular individuals will have a body fat percentage lower than these levels. In general, most athletes experience greater performance benefits ^[2] at body fat percentages between 7 and 19 percent for men, and 10 and 25 percent for women, depending on the sport^[3].

Body composition (particularly body fat percentage) can be measured in several ways. The most common method is by using a set of measurement calipers to measure the thickness of subcutaneous fat in multiple places on the body. This includes the abdominal area, the subscapular region, arms, buttocks and thighs. These measurements are then used to estimate total body fat with a margin of error of approximately four percentage points.

Another method is bioelectrical impedance analysis (BIA), which uses the resistance of electrical flow through the body to estimate body fat.

A more accurate but less convenient method is using a large tank of water to measure body buoyancy. Increased body fat will result in greater buoyancy, while greater muscle mass will result in a tendency to sink.

A technique for measuring body composition has been developed using the same principles as under water weighing. The technique uses air, as opposed to water, and is known as air displacement plethysmography (ADP). Subjects enter a sealed chamber that measures their body volume through the displacement of air in the chamber. Body volume is combined with body weight (mass) in order to determine body density. The technique then estimates the percentage of body fat and lean body mass (LBM) through known equations (for the density of fat and fat free mass).

Body Composition is also estimated using cross-sectional imaging methods like magnetic resonance imaging (MRI) and computed tomography (CT). Since MRI and CT give the most precise body composition measures to-date, many pharmaceutical companies are very interested in this new procedure to estimate body composition measures before and after drug therapy especially in drugs that might change body composition. Image Reading Center at the New York obesity research center in Manhattan is the only expert facility that utilizes DEXA, MRI and CT in Body Composition estimation for large research studies and pharmaceutical clinical trials.