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BODY COMPOSITION. The term body composition describes the various components that make up a person's body. The absolute and relative amounts and distribution of these components are relevant to diverse body functions and, thus, influence the state of health and various disease risks. A commonly used body composition model organizes the body to five levels of increasing complexity: from atomic to molecular, cellular, tissue-system, and whole body.

At the atomic level, the body is made up of chemical elements essential for life. Four major elements, oxygen, carbon, hydrogen, and nitrogen, collectively account for more than 96 percent of adult body weight. The remaining are minerals present in the form of salts. Calcium and phosphorus make up the major bulk of remaining minerals, found mostly in bone.

The four elements, oxygen, carbon, hydrogen, and nitrogen, are present at the molecular level in water and as organic compounds. Water serves as a solvent where chemical reactions take place. Protein and phospholipids serve as major structural components of the body. Proteins, glycogen, and lipids that include phospholipids and fats are all organic compounds. Protein, in the form of enzymes and hormones, performs important biochemical and physiological roles in the body. Glycogen reserves are small and used mainly as metabolic fuel. Fat serves as insulation and as an energy store. The two major organic compounds, protein and fat, plus water are usually grouped with the mineral component (osseous and non-osseous) to form the four-compartment model, a model used most often when considering the nutritional status of a person.

Two alternate groupings of these components used to describe body composition at the molecular level are the division of the body into a fluid and a dry component. The latter is comprised of proteins, minerals, and fat. The second alternative, also referred to as the classic two-compartment model, divides the body into fat and fat-free masses. Fat mass is the most variable as it is affected by energy balance. The fat-free mass is composed of water, proteins, and minerals. This term is used synonymously with lean body mass. This compartment also includes essential lipids. It is metabolically important and its chemical composition is assumed to be constant in a healthy adult.

At the cellular level, the chemical compounds are assembled into either the cellular component (the body's main functional components) or the extracellular supporting components; for example, extracellular fluid and solids, of which the skeleton makes up its major bulk. Because living cells consist of metabolically important structures and an inert fat component, the cellular component is further subdivided into a body cell mass and fat. This three-compartment model of body cell mass, fat, and extracellular components presents a physiological view of the body.

The tissue system level is also of structural and functional importance. Tissues contain cells that are mostly similar in appearance and function. Tissues and organs are categorized into adipose tissues, skeletal muscles, skeleton, blood, and a "residual" category that includes the skin and visceral organs. Adipose tissue includes fat cells, blood vessels, and structural elements. White adipose tissue is located mainly in the subcutaneous and visceral compartments. Subcutaneous fat provides insulation, and most visceral fat serves as an energy store. Brown fat is present in small quantities in discrete locations and plays an important role in heat production in neonates during cold exposure.

The whole body level of organization involves physical characteristics, such as body size and shape.

Normal Changes Throughout the Life Cycle

Growth. The growth process involves an increase in body size and compositional changes of tissues and organs, physiological changes during adolescence, and finally, chemical maturation of tissues and organs to reach a "stable" composition in adulthood (Table 1). Growth in height, weight, tissues and organs, and changes in chemical composition are not uniform. Thus, the relative proportions of various tissues and organs vary at different stages of growth (Table 2).

Length and weight increase rapidly during the second half of gestation and continue to change rapidly through the first year of postnatal life. There is a relative slowing in growth rates as gestation approaches term,

due to the physical constraints imposed on the fetus. Rapid growth in skeletal muscle and adipose tissue causes a concomitant surge in the relative protein and fat contents in the fetus and a decrease in its relative water content. At the same time, a progressive fluid shift occurs from the extracellular into the intracellular compartment. Progressive mineralization of the skeleton also occurs during this period.

After the first year, growth rate slows until the second major growth spurt at adolescence. Hormonal changes during adolescence cause major physiological differences between sexes. The "adolescent growth spurt" lasts about two to three years and begins earlier in females. In females, there is a larger accretion of body fat. In males, the increase in skeletal muscle mass is more intense and of longer duration. This sex difference in skeletal muscle and fat content persists throughout the adult years. "Chemical maturation" of the fat-free mass is completed during adolescence, when there is a relative decrease in water and a relative increase in fat, protein, and bone mineral mass.

Normally, adult height and weight are reached at about eighteen years of age by females, and twenty years of age by males. The height and weight is 170 cm and 70 kg respectively for the reference adult male, and 160 cm and 58 kg for the reference female. Relative weights of skeletal muscles and adipose tissue are higher, and that of viscera lower, in adults compared with infants. Although relative weight of the skeleton is similar between infants and adults, the adult skeleton has a higher mineral content.

Aging. The aging process produces a decline in height, lean weight, muscle mass, and skeletal size. Loss of skeletal muscle and bone mass is related to the age-associated decline in physical activity and to the decline in various hormonal secretions.

Changes in Body Composition under Different Conditions

Weight loss. Prolonged food deprivation causes growth faltering in children and weight loss in adults. Recurrent infections due to poor hygiene and health care may exacerbate food deprivation. Severe weight loss also occurs in diseases, such as malignant cancers, hepatic and renal diseases, and those involving the gastrointestinal tract. Loss in weight in severe undernutrition is due to loss in both body cell mass and fat mass.

Loss in body cell mass and preservation of the extracellular fluid results in an increase in water content in the fat-free mass and an increase in the ratio of extracellular fluid to intracellular volume.

Weight gain. Most weight gain involves a mixture of fat and lean tissues with their relative contribution depending on the initial body composition, physiological status, and physical activity. For example, an obese person gains a larger proportion of fat than lean tissues than does a lean person.

Overweight and obesity, and their associated health risks, are of increasing prevalence in affluent societies. In the United States, the incidence of obesity has increased from 12 percent in 1991 to 17.9 percent in 1998.

The significant consequences of increase in adiposity are not limited to net changes in body composition. Specific regional fat distributions appear to be associated with diverse levels and types of morbidity. Higher levels of upper-body obesity, especially of the visceral, is associated with abnormalities of fatty acid metabolism and is related to the higher risks of hypertension, premature coronary death, and type 2 diabetes mellitus.

Physical training. In general, physical training increases muscle and bone mass, and decreases fat mass. Gains in muscle mass and losses in fat mass vary with the intensity and duration of usual physical activity. Changes in body composition associated with physical activity are mediated by increases in the secretion of anabolic hormones. These increase lean body mass. Increases in catecholamines facilitate fat loss.

Immobilization. Reduction or loss of mobility increases the nutritional risk of obesity or sarcopenia, an abnormally low lean body mass. Prolonged immobilization causes loss of body nitrogen and calcium, hence a decrease in muscle mass accompanied by decreases in muscle strength and decreases in bone density.

Osteoporosis. The high prevalence of osteoporosis in the elderly, especially in females, is a major public health concern. Loss of bone mass and deterioration of bone tissue is a feature of the normal aging process that is attributable to an intrinsic deterioration of the ossification process. It leads to increased bone fragility and, consequently, increased risk of bone fracture. Although reduction in bone density affects every individual, in some, loss in bone mass is severe. The skeleton is not uniformly involved; the spine and other trabecular bone are more affected commonly and severely than is the cortical bone of the axial skeleton. The greater severity of osteoporosis in females is attributed to a lower peak bone mass achieved at puberty and estrogen withdrawal at menopause. Other factors causing loss of bone mass are a lack of physical activity and decreased calcium intake.

Hwai-Ping Sheng