A Comparison of Body Composition Models

Abstract

There has been ample interest in body composition of special populations such as children and older adults. A significant problem occurs when applying the conventional two-component body composition model to these special groups. This two-component model assumes body composition is composed of fat and fat-free components and the composition of the fat-free component is constant across ages, sexes, and races. The fat-free component of the human body can be chemically separated into water, mineral, and protein. In a multicomponent model, the fat-free component varies by age, sex, and race as each subcomponent in the fat-free mass changes according to biological conditions. Changes in water content are important because body water is a major and variable fraction of the fat-free component. Infants and juveniles tend to have a larger water fraction than men. Consequently, in a two-component model, FFM is underestimated in the young. The assumptions in a multi-component model are biologically true but limited in accuracy because measurement errors for each component may accumulate and inflate the errors in the body composition estimates. An alternative is to use dual-energy X-ray absorptiometry (DXA) to obtain body composition measures. Software to allow soft tissue measurements from DXA has become recently available. The equipment is simple to use but the theory behind this soft tissue measurements is not well-defined.