Elements are distributed into molecular components which can be grouped into the five main categories of lipids, proteins, glycogen, water and minerals (Figure 6.3) (1). The main atomic level elements are organized into molecular level components.
The molecular level is often divided, for practical purposes, into two main components, fat and fat-free mass (FFM). Fat-free mass is usually considered to be the metabolically active portion of body mass at the molecular body composition level.
Another molecular level model is based on three components: fat, lean soft tissue and bone mineral (3) (Figure 6.3). This model is appropriate for use with the dual-energy X-ray absorptiometry methods described in later sections.
Three-, four-, and even six-component molecular
^ Energy ^ losses
Figure 6.1 Interrelations between energy intake, output and stores
Figure 6.2 Atomic level of body composition. O, oxygen, C, carbon; H, hydrogen; N, nitrogen
level models are applied with body volume measurements as a core and other additional measurements added for quantifying more components.
Molecular level body composition components form the basis of functioning cells, and the cellular level of body composition is usually described as three components: cells, extracellular fluid and extracellular solids (Figure 6.4). Cell mass, for measurement purposes, is further divided into fat (a molecular level component) and a metabolically active portion referred to as body cell mass (1,4). The resulting four components are fat, body cell mass, extracellular fluid and extracellular solids.
The cellular level components form the tissues and organs of the body such as adipose tissue, skeletal muscle, bone, skin, heart, and other visceral organs (Figure 6.5). These organs and tissues comprise the human body and ultimately complete the link in the five levels of body composition, atomic, molecular, cellular, tissue-system and whole-body.
Body weight, according to this model, represents a complex interplay of energy-nutrient exchange associated with ~ 40 components at the first four levels of body composition. Methods for evaluating adiposity are summarized in Table 6.1. The following section presents a summary of selected methods based on their clinical and research applicability.
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