Body Composition

Introduction

Mankind has long been fascinated with the composition of the human body. Centuries ago, the Greeks dissected human cadavers to obtain an insight into the structure and build of the human body, and draw-ings from the Middle Ages of gross muscle structures grace the walls of many famous art galleries. They are prized not only for their artistic merit, but also for what they reveal of the work of the dissectionists of that era. With progress in the development of analytical chemical methods in the twentieth century, these studies of body composition were applied to body tissues, fetuses, and cadavers of newborns. Scientists such as Mitchell, Widdowson, and Forbes performed the most important work of chemical analyses in adult cadavers during the 1940s and 1950s. Today, neutron activation analysis allows the chemical com-position of the human body to be studied in vivo. These early chemical analyses of the body gave insights into the changes occurring during growth and development. They also form the basis for a number of methods now widely used to assess body composition in vivo.

Today, it is known that many diseases and disorders are related to abnormal body composition or to changes in body composition. The most common of these conditions is obesity, in which the amount of body fat is excessively high, leading to abnormalities in lipid and carbohydrate metabolism, high blood pressure, and adult-onset diabetes. At the other end of the nutritional spectrum, energy and protein malnutrition results in a decrease in the amount of fat and protein stores in the body, and many diseases are related to abnormalities in total body water or to the distribution of body water across the intracellular and extracellular spaces.

Because of the high variability between subjects in chemical body composition, mainly due to the high variation in body fat stores, the concept of fat-free mass (FFM) was introduced at the end of the nine-teenth century. If body composition data are expressed as a proportion of the FFM, data become much more consistent between individuals. For example, the fraction of water in the FFM (0.73 ± 0.02) is very consistent across individuals, whereas the between-subject variation is two to three times higher if expressed per kilogram of body weight. This high variability in body components led to the definition of a “reference man,” an imaginary person with a given body composition.

In this a (global) description of the composition of the healthy human body is given and discussed at the following levels:

●atomic

●molecular

●cellular

●tissue

●whole body.

Of the many methods available to measure body composition, a few are highlighted and a short description of each is given. For more detailed infor-mation, the books by Forbes (1987) and Heymsfield et al. (2005) on human body composition are recom-mended for further reading.