Introduction

Body composition analysis has evolved from the single unit of body weight, through the classical division into fat and fat-free mass and then onto three, four or more compartments. This chapter will describe each model in terms of the principle of the method, the compartments which are measured, the techniques required and the theoretical limitations or advantages that the model presents. A summary of each model is given in Table 13.1. Alternative approaches to the division of the body including elemental analysis, intra- and extracellular constituents and body segments will not be addressed further here.

Weight: a single-compartment model?

Although weight alone gives no indication of body composition, in adults it is not uncommon for judgements to be made on the basis of weight, particularly with reference to obesity. There is still a tendency to say that men greater than 100 kg or women greater than 80 kg are overweight with the implied assumption that they have excess body fat. The limitations of this approach are obvious.

Two-compartment models

In the two-compartment model the body is divided into fat and fat-free mass. At its simplest the measurement of weight and height gives some indication of body composition, by reference to tables of ideal body weight, although this again makes the assumption that the majority of excess weight is fat.

The two-compartment model is also the basis of most classical body composition techniques, notably the measurement of body density, total-body water and total-body potassium.

Density

Body density is calculated as the mass of the body in air divided by its volume.