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In physical fitness, body composition is used to describe the percentages of fat, bone, water and muscle in human bodies. Because muscular tissue takes up less space in our body than fat tissue, our body composition, as well as our weight, determines leanness. Two people of same sex and body weight may look completely different from each other because they have a different body composition. == Body composition from body density== The most accurate estimation of body composition is derived from body density by means of the equation of fractional densities which states that the overall density of a mixture containing more than one substance (each with a different density) can be calculated if the proportion and density of each component substance is known. For determination of body composition the body is often assumed to be composed of 4 basic substances ("four compartment model") with the general form of the equation as follows: 1/Db = w/Dw + f/Df + p/Dp + m/Dm Where: Db = overall body density, w = proportion of water, f = proportion of fat, p = proportion of protein, m = proportion of mineral, Dw = density of water, Df = density of fat, Dp = density of protein, Dm = density of mineral In a research laboratory setting the overall density of the body (Db) is calculated from its mass and volume (Db = mass/volume). The mass of the body is found by simply weighing a person on a scale. The volume of the body is most easily and accurately determined by completely immersing a person in water and calculating the volume of water from the weight of water that is displaced (via "underwater weighing"). The proportions of water, protein and mineral in the body are found by various chemical and radiometric tests. The densities of water, fat, protein and mineral are either measured or estimated. The equation is then rearranged to solve for the proportion of fat (f) from the other quantities. A reasonably accurate estimation of body fat can be obtained by means of a "two compartment model" of the human body which is based upon two simplifying assumptions: 1. Human fat has a density of 0.9 grams/ml, and 2. The lean (non-fat) components of the human body have an overall density of 1.1 grams/ml. The two compartment model allows for an estimation of the proportions of fat and lean components of the body to be made from the overall body density. And, as explained above, the overall body density (Db) is easily calculated from body mass and body volume. Rearranging the equation of fractional densities to solve for the proportion of fat (f) will yield the following equation: f = 495/Db - 450 The density of human fat is remarkably constant across all sub-groups studied but variations have been reported in the density of the fat free mass (i.e. lean or non-fat components). To address these differences unique equations for the two compartment model have been proposed for specific populations 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Body composition」の詳細全文を読む スポンサード リンク
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