Levels Of Body Composition
The study of body composition historically has been driven by the availability of methods to measure or, more appropriately, to estimate it. Since the early 1980s, considerable progress has been made in the development and refinement of techniques to estimate the composition of the body, so that virtually all components of the body can now be measured. This progress has resulted in the modification of the models that provide the framework for studying body composition.
Body composition can be approached at a variety of levels. The five-level approach provides a sound guide: atomic, molecular, cellular, tissue, and whole body [1,2]. The multilevel view provides a framework within which the lure and difficulty inherent in the study of body composition can be appreciated.
Basic chemical elements compose the atomic level. There are 106 elements in nature. About 50 are found in the human body, and with more recent technologic advances, all 50 can be measured in vivo. Oxygen, carbon, hydrogen, and nitrogen account for greater than 95% of body mass, and the addition of seven other elements—sodium, potassium, phosphorus, chloride, calcium, magnesium, and sulfur—accounts for 99.5% of body mass [2].
The molecular level of body composition focuses on four of five components: water, lipid (fat), protein, minerals, and carbohydrate. The last component,
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sportsmed.theclinics.com carbohydrate, occurs in small amounts in the form of glycogen, largely in the liver and skeletal muscle, and is not usually considered in estimates of body composition. The following equation is used:
Most mineral is located in bone with a small fraction in other tissues. Historically, the relative contribution of each of the four components to body mass was derived from chemical analyses of human cadavers, although each can now be measured in vivo.
At the cellular level, body mass is viewed as composed of cells and substances outside of cells. The body cell mass (BCM) is defined by intracellular fluids and intracellular solids and is the metabolically active component of the body. Presently available methods do not permit measurement of cell solids in vivo. Extracellular fluids (ECF) and extracellular solids (ECS) compose the substances outside of the cells. The primary ECF are bone minerals and other components of connective tissues. Adipocytes are fat cells; they store lipids and comprise fat mass (FM). The equation is as follows:
At the tissue level, the study of body composition focuses on the contribution of specific tissues to body mass: skeletal muscle, adipose, bone, blood, viscera, and brain. Skeletal muscle, adipose, and bone tissues historically have been a primary focus in studies using traditional technologies, such as anthropometry and radiography. New technologies permit more refined assessment of these primary tissues (eg, the mineral content of bone tissue or subcutaneous versus internal adipose tissue).
The fifth level of body composition is the whole body, its size, shape, physique, and proportions. Anthropometry is the basic tool for estimating body size and configuration, although photographic techniques also have been used, especially for the study of shape and physique. The body mass index (BMI) (weight [kg]/height2 [m]) and skinfold thicknesses are perhaps the most widely used anthropometric indicators at this level of body composition. Two other properties of the whole body are crucial in the study of body composition—volume and density.
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