The body is not of constant composition, and is an assembly of different organs of differing composition. Body composition is affected by nutritional status and may be assessed by clinical examination.
Conventional two-compartment models of body composition separate body weight into fat mass and fat-free or lean body mass.
The human body can be regarded as being in three compartments:
1. Cell mass (55% of the total weight) which is the active tissue, performing the work of the body
2. Extracellular support tissue (30% of the total weight) which supports the cell mass. This includes: (i) blood, plasma and lymph and extracellular fluid; and (ii) minerals and protein fibres in the skeleton and the connective tissue including collagen.
3. Energy stores (15% of the total body weight),are predominantly held in adipose tissue, subcutaneously and around organs and some in glycogen. Most of the protein but only 1 kg of fat are essential cell components, the residual fat is storage.
Body composition and nutritional stores
Lean body mass – water and protein content of the body
Fat – energy stores
Bone – calcium and minerals
The relative proportions will vary with a number of factors: age, sex, stage of development, physical fitness, hormonal status, pregnancy and even mood. In the embryo the proportion of water is higher than that in the mature animal:
• 28-week foetus 88% water
• new-born baby 75 % water
• 2-month baby 65 % water
• 4 months to adult 60% water
The proportion of fat increases somewhat with age. In the elderly the amount of water slowly declines by small amounts.
1. The body is not of constant composition, and is an assembly of different organs of differing composition. Body composition is affected by nutritional status. This may be assessed by clinical examination.
2. Conventional two-compartment models of body composition separate body weight into fat mass and fat-free or lean body mass, the latter including viscera, muscles, organs, blood and bones.
3. Measurements of body composition include the use of the dilutional principles for total, extracellular and cellular water, body fat and a range of measurements for skeletal mass. Height and weight, body shape, subcutaneous fat, anthropometric measures and muscle function tests are also utilized. Biochemical estimations include electrolytes in extracellular and intracellular compartments, plasma proteins and urinary nitrogen.
4. Lipid-free body mass can be calculated from isotope-labelled water, potassium, neutron activation analysis, bioelectrical impedance, imaging or nuclear magnetic resonance imaging (MRI).