The organization of human body composition with an emphasis on body adiposity. The human body can be organized into five levels of increasing complexity—atomic, molecular, cellular, tissue organ, and whole body. Obesity-related components are recognized at each of the five body composition levels.
The human body is composed of 11 elements that account for >99.5% of body weight.1 Three of these elements, carbon, hydrogen, and oxygen, are found in storage triglycerides or “fat.” The average proportions of triglyceride as carbon, hydrogen, and oxygen are considered stable at ~76.7%, 12.0%, and 11.3%, respectively. These stable elemental proportions allow the development of methods for deducing total body fat from total body carbon and other elements. Carbon is the characteristic component of adiposity at the atomic level. “Reference Man,” for example, contains 16 kg carbon, and 60% or 9.6 kg of carbon exits in adipose tissue.
- Major molecular-level body composition components include water, lipids, proteins, minerals, and glycogen. Each of the nonaqueous components represents many different but closely related chemical compounds.
- Lipid is the main molecular-level component of interest in the study of human obesity. The term “lipid” refers to all chemical compounds that are insoluble or weakly soluble in water but are soluble in organic solvents such as chloroform and diethyl ether.
- Lipids isolated from human tissues include triglycerides, sphingomyelin, phospholipids, steroids, fatty acids, and terpenes.
- Triglycerides, commonly referred to as “fats,” are the primary storage lipids in humans, and comprise the largest fraction of the total lipid component. The Reference Man has 13.5 kg of total lipid, of which 12.0 kg, or 89%, is fat.
- The term “ectopic fat” refers to the lipids frequently associated with metabolic derangements that are found within cells of the liver, pancreas, heart, and organs or tissues other than adipose tissue.
- The “stable” relationships between the various components form the basis of many widely used molecular-level body composition methods. The most important of these is the hydration of fat-free mass (FFM) (i.e., mean total body water [TBW]/FFM = ~0.73), which estimates fat mass (FM) from the difference between body mass and FFM, and the densities of FM (0.9007 g/cm3 ) and FFM (1.100 g/cm3 ).
- FM can be calculated as FM = body mass − TBW/0.73 and FM = (4.95/Db − 4.50) × body mass, where Db is body density measured by UWW or ADP.6–9 Since the assumption of a constant FFM density of 1.100 g/cm3 causes some model error, three-, four-, and five-component models were developed for measuring total body fat with improved accuracy.
The cellular level includes three main components: cell mass, extracellular fluid, and extracellular solids. Adipocytes or fat cells serve as the primary storage site for triglycerides. All cell mass can be separated into two components: one metabolically active “body cell mass” and the other triglycerides, or “fat.”
- The main components at this level are adipose tissue, skeletal muscle, bone, and visceral organs (e.g., brain, liver, kidneys, heart).
- Human adipose tissue is often assumed to have an average composition consisting of 80% lipid, 14% water, 5% protein, and <1% mineral, and a density of 0.92 g/cm3 at body temperature.
- Since level of adiposity, age, gender, and heredity all play an important role in determining fat content of adipose tissue, this average belies the large variation observed in adipose tissue composition.
- For example, for every 10% increase in relative adiposity, there is a corresponding increase in lipid fraction of 0.124.14 A notable feature of adipose tissue is the large extracellular fluid compartment relative to cell mass.
- Of the 14% of average adipose tissue samples as water, 11% is extracellular water. An important aspect of obesity research is an examination of regional adipose tissue biology (subcutaneous and internal).
- Adipose tissue occurs in abundance in subcutaneous sites as well as in and around female breasts. Subcutaneous adipose tissue (SAT) can be partitioned into superficial and deep SAT by fascial planes.
- The most important internal adipose tissue component in the obesity field is found in the visceral compartment (i.e., thoracic, abdominal, and pelvic visceral adipose tissue [VAT]). The word viscera is Latin for “organs in the cavities of the body.”
- However, the medical literature varies widely in defining VAT based on imaging methods Of the non-visceral internal adipose tissue, interstitial adipose tissue is interspersed within the tissue (e.g., skeletal muscle) so tightly that it may be included with the tissue/organ at dissection.
- Adipose tissue is almost absent in some anatomic sites such as the penis, scrotum, labia minora, nipple, nose, ear, eyelids, and brain. Some triglyceride exists within skeletal muscle, liver, and other nonadipose tissue cells and this component is classified as intracellular lipid. Each anatomic adipose tissue component has specific metabolic and functional characteristics.
Skinfolds, circumferences, lengths, linear dimensions, body surface area (BSA), and body volume are all measurements at the whole-body level. These measurements are often used with prediction equations to estimate components at the other four body composition levels.