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.1,71 i C/ui Nuir 1992:56: 19-28. Printed in USA. 1992 American
Society for Clinical Nutrition 19
The five-level model: a new approach to
organizingbody-composition research12Zi-ifian Wang, Richard N
Pierson Jr. and Steven B Heyms/leld
ABSTRACT Body-composition research is a branch of hu-man biology
that has three interconnecting areas: body-com-position levels and
their organizational rules, measurementtechniques, and biological
factors that influence body compo-sition. In the first area, which
is inadequately formulated at pres-ent, five levels ofincreasing
complexity are proposed: I, atomic:II. molecular: III, cellular;
IV, tissue-system: and V. whole body.Although each level and its
multiple compartments are distinct,biochemical and physiological
connections exist such that themodel is consistent and functions as
a whole. The model alsoprovides the opportunity to clearly define
the concept ofa bodycomposition steady state in which quantitative
associations existover a specified time interval between
compartments at the sameor different levels. Finally, the
five-level model provides a matrixfor creating explicit
body-composition equations, reveals gapsin the study ofhuman body
composition, and suggests importantnew research areas. .1,n J C/in
Nutr 1992:56:19-28.
KEY WORDS Body composition, nutritional assessment.steady
state
Introduction
The study of human body composition spans > 100 y
andcontinues to be an active area of basic science and clinical
re-search. Nearly every aspect of clinical nutrition, selected
areaswithin many medical specialties, and components of
exercisescience are touched on by the study of body
composition.
Information related to body composition is accumulatingrapidly
and is extending our knowledge ofhuman biology. Mostofthis
information is now categorized as technical or biological.The
technical category includes the many classic and
continuallyemerging new body-composition methods. Although no
sys-tematic classification for body-composition methodology hasbeen
proposed. informal groupings are often published. such asdilution
techniques and neutron-activation analysis. which arebased on a
physical principle or other characteristics ofthe tech-niques
involved. The biological category includes informationon the study
ofhow growth, development, pregnancy. lactation,aging. exercise,
and disease influence body composition.
Although the technical and biological categories would appearto
encompass most body-composition information, a recentstudy ( 1 )
led us to appreciate a serious limitation of the field asit is now
organized. We recognized that not all of the rapidlyaccumulating
information emerging from body-compositionresearch could be
satisfactorily included into the technical and
biological categories. For example, there are many
mathematicalmodels that describe the relations between different
componentsin healthy subjects [eg, total body water (TBW)/fat-free
bodymass = 0.732] (2). This formulation indicates that some
quan-titative associations exist that describe the relationships
amongcompartments that are in equilibrium. Another example is
pro-vided by the reconstruction of human chemical compartmentsand
body weight (Bwt) from elements estimated in vivo by
neu-tron-activation analysis ( 1 ). This suggests that
relationships existnot only between individual components but
between differentlevels of body composition as well.
Another problem is that investigators are frequently con-fronted
with questions about terminology. For example: Arelipid-free body
mass, fat-free body mass, and lean body mass(LBM) the same or
different compartments? The lack of cleardefinitions for
body-composition components has a subtle butserious consequence:
many errors are evident in published body-composition equations and
models because ofoverlap or omis-sion ofcomponents. In fact, we
could find no clear approach todefining components and building
multicompartment body-composition models in extensive reviews of
previous literature.
Growing from these observations is the hypothesis that a
thirdcentral category of body-composition research exists that
untilnow has not been adequately formulated: the levels of
bodycomposition and their organizational rules. This report
presentsa comprehensive model ofhuman body composition consistingof
five distinct levels of increasing complexity in which eachlevel
has clearly defined components that comprise total Bwt.The five
levels are I. atomic: II, molecular: III, cellular: IV.
tissue-system: and V. whole body (Fig I).
The following section presents a detailed description of
eachlevel and its associated components. In the next section the
fea-tures or organizational rules of the model as a whole are
de-scribed. Important concepts related to development of
body-composition models and equations are presented in this
portionofthe paper, and the widely appreciated but never formally
de-fined concept ofa steady state ofbody composition is
introduced.
I From the Obesity Research Center. St Lukes-Roosevelt
Hospital,
Columbia University. College of Physicians and Surgeons. New
York,NY.
2 Address reprint requests to Z-M Wang, Weight Control Unit. 4 I
1West I 14th Street, New York, NY 10025.
Received September 17, 1991.Accepted for publication December
12. 1991.
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Le1I(Atoalc)
20 WANG ET AL
FIG 1. The five levels of human body composition. ECF and
ECS,extracellular fluid and solids, respectively.
TABLE 1Body composition on the atomic level (I) for the 70-kg
ReferenceMan
Element Amount Percent ofbody weight
kg %
Oxygen 43 61Carbon 16 23Hydrogen 7 10Nitrogen 1.8 2.6Calcium 1.0
1.4Phosphorus 0.58 0.83Sulfer 0. 14 0.20Potassium 0. 14 0.20Sodium
0.1 0.14Chlorine 0.095 0.14Magnesium 0.019 0.027
Total 69.874 99.537
* Information based on reference 2 (modified).
Five-level model
Atomic (I)
The fundamental building blocks ofthe human body are atomsor
elements. Ofthe 106 elements, 50 are found in the humanbody and
their distributions in the various tissues and organsare well
documented (2). Six elements (oxygen, carbon, hydro-gen, nitrogen,
calcium, and phosphorus) account for > 98% ofBwt, and one
element, oxygen, constitutes > 60% of total bodymass in the
Reference Man (Table 1) (2). The remaining 44elements make up <
2% of Bwt.
The equation for Bwt, as defined in the atomic level of
bodycomposition is
Bwt=O+C+H+N+Ca+P
+S+K+Na+Cl+Mg+R (1)
where R is the residual mass ofall elements present in
amounts< 0.2% percent of Bwt (1).
Elemental analysis of humans is traditionally carried out
incadavers or in biopsy specimens from selected tissues and
organs.In addition, the whole-body content of most major
elementscan now be measured directly in vivo: potassium by
whole-bodycounting: sodium, chlorine, and calcium by delayed--y
neutron-activation analysis (3); nitrogen by prompt-y
neutron-activationanalysis (1 , 3): and carbon by inelastic neutron
scattering (4).More than 98% of Bwt can now be reconstructed from
elementsthat can be estimated in vivo, largely by
neutron-activationtechniques. The atomic level is the foundation of
body-corn-position analysis and is the starting point for the five
levels wepropose.
P110/cell/ar (II)The 1 1 principal elements are incorporated
into molecules
that form > 100 000 chemical compounds found in the
humanbody. These molecules range in complexity and molecular
weightfrom water to deoxyribonucleic acid. It is neither useful
norpossible to measure all ofthese chemical compounds
individuallyin living humans. The alternative used in
body-composition re-search is to consider chemical compounds in
categories of closely
related molecular species. The major components in present
useare water, or aqueous (A): lipid (L); protein (Pro): mineral
(M):and glycogen (G) (Table 2). Because some confusion exists
inthese different categories. we now review the five chemical
corn-ponents in detail.
Water. The most abundant chemical compound in the humanbody is
water, which comprises 60% of Bwt in the ReferenceMan (2).
Protein. The term protein in body-composition research usu-ally
includes almost all compounds containing nitrogen, rangingfrom
simple amino acids to complex nucleoproteins. The mostwidely used
representative stoichiometry for protein isCH 59N26O32S07 . with an
average molecular weight of 2257.4and density of 1.34 g/cm3 at 37
#{176}C(1, 5).
G/icogen. The primary storage form of carbohydrate is gly-cogen.
which is found in the cytoplasm of most cells. The prim-cipal
distribution is in skeletal muscle and liver, which contain 1% and
2.2% of their respective wet weights in the form of
TABLE 2Body composition on the molecular level (II) for the
70-kg ReferenceMan*
Component Amount Percent of body weight
1cv %
WaterExtracellular 18 26Intracellular 24 34
LipidNonessential (fat) 12 17Essential 1.5 2.1
Protein 10.6 15Mineral 3.7 5.3
Total 69.8 99.4
* Glycogen. normally 400 g. is not included in the Reference
Man.
Information based on reference 2.
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FIVE-LEVEL BODY COMPOSITION MODEL 21
glycogen ( I. 2). The stoichiometry of glycogen is
(C6H10O5).with an average density of 1 .52 g/cm3 at 37 #{176}C( 1 ,
2).
.tIiThl(ll. The term mineral describes a category of
inorganiccompounds containing an abundance of metal elements
(eg.calcium. sodium. and potassium) and nonmetal elements
(eg.oxygen. phosphorus. and chlorine). Ash, a term similar to
mm-eral. is the residue ofa biological sample heated for a
prolongedperiod to > 500 #{176}C,and consists of the nonvolatile
portion ofmineral compounds. Total body ash is slightly lower in
weightthan mineral mass because of the loss of carbon dioxide
fromsome carbonate groups and the release of tightly bound
waterduring the heating period ( 1 . 2). Mineral is usually divided
intotwo subcategories: osseous and extraosseous. Osseous
mineral.the largest component of which is calcium
hydroxyapatite([Ca3(PO4)2]3 Ca(OH)2). contains > 99% of total
body calcium(TBCa) and 86% of total body phosphorus in the
ReferenceMan (2). Other elements, such as potassium, sodium. and
chlo-rime. are primarily found in extraosseous mineral.
Lipid. Among the five principal chemical components on
themolecular level, lipid is the most confusing because the
termslipid and fat are used interchangeably. even though
strictlyspeaking they refer to different compartments. The
traditionaldefinition oflipid refers to a group ofchemical
compounds thatare insoluble in water and very soluble in organic
solvents suchas diethyl ether, benzene, and chloroform (6, 7).
About 50 dif-ferent lipids are recognized in humans, and these are
divided byorganic chemists into five subcategories: 1) simple
lipids (in-cluding triglycerides and waxes): 2) compound lipids
(eg. phos-pholipids and sphingolipids): 3) steroids: 4) fatty
acids: and 5)terpenes (6).
The simple lipid, triglyceride, contains three fatty acids
ester-ifled to glycerol. The term fat is synonymous with
triglycerideand therefore fat is clearly a subcategory of total
lipid (6. 7). Acommon error is to confuse the terms fat and lipid,
which canlead to errors in constructing models of body composition.
Inthe adult. 90% oftotal body lipid is fat (2).
Lipids can also be classified physiologically into two
groups:essential (Le) and nonessential (Ln) (2). Essential lipids.
such assphingomyelin and phospholipids, serve important
functionssuch as forming cell membranes. The nonessential lipids,
largelyin the form of triglyceride, provide thermal insulation and
astorage depot ofmobilizable fuel. About 10% oftotal body lipidis
essential and 90% is nonessential in the Reference Man (2).
Although essential and nonessential lipids are structurally
andphysiologically different. their solubilities in organic
solvents aresimilar and it is difficult to clearly separate them
even in vitro(6. 7). An approximate separation can be accomplished
by carefulselection ofthe type oftissue analyzed, the extraction
time andtemperature, and particularly the type of solvent used (6).
Sol-vents such as petroleum or ethyl ether are usually used alone
toextract nonessential lipids, mainly the neutral fat or
triglyceride.The remaining lipids. which are primarily essential.
can be ex-tracted by using binary or ternary solvent mixtures such
as 45%chloroform. 10% methanol, and 45% heptane (8).
The fatty acid profile oftriglyceride varies with diet.
anatomicsite. and other factors. but the generally accepted
representativestoichiometry found in adult humans is C51H98O6, with
an av-erage molecular weight of 806 and a density of 0.900 g/cm3
at37#{176}C(2). The stoichiometry oftotal lipid in humans could
notbe found in a review ofprevious studies.
The equation for Bwt as defined by the molecular level ofboth
composition is
Bwt= L+A+Pro+M+G+R (2)
where R represents residual chemical compounds not includedin
the five main categories and that occur in quantities of < I
%oftotal Bwt (1).
On the molecular level, three related equations can also
bedefined as follows. Dry Bwt consists ofthe anhydrous
chemicalcomponents (Fig 2, left), and equation 2 can therefore be
re-written as
Bwt = A + dry Bwt (3)
Dry Bwt according to this equation is the sum of L + Pro + M+ G
+ R.
Bwt = L + lipid-free body mass (4)
In equation 4 (Fig 2, right). lipid-free body mass is the
materialremaining after extraction of a whole-body homogenate
withappropriate organic solvents and optimum conditions.
Thuslipid-free body mass can be expressed as the combined weightof
A + Pro + M + G + R.
As fat accounts almost entirely for total body
nonessentiallipid, then
Bwt = fat + FFM = Ln + FFM (5)
where FFM is fat-free body mass, which represents the
combinedweights of Le + A + Pro + M + G + R.
A similar term to fat-free body mass is LBM. The early
def-inition of lean body mass suggested included at least five
corn-ponents: water, protein. mineral. glycogen, and an
unspecifiedamount of essential lipid (9. 10). More recently, most
investi-gators have used the terms LBM and FFM interchangeably.
al-though some debate still prevails about whether or not these
arethe same or different compartments. Our suggestion is that
LBMand FFM henceforth be considered synonymous on the basis ofthe
following reasoning.
In equation 4 we clearly define two fractions ofBwt, lipid
andlipid-free body mass. The lipid fraction consists oftwo
portions.essential and nonessential or fat. Accordingly,
FIG 2. Body-composition model on the molecular level (II).
FFM.fat-free body mass: LFM. lipid-free body mass: and Le and L.
essentialand nonessential lipids. respectively.
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22 WANG ET AL
* Ln, nonessential lipid or fat; and Le, essential lipid.
Body weight Ln + LBM = fat + FFM
in which both LBM and FFM are the sum ofessential lipid
pluslipid-free body mass and the remaining Bwt is nonessential
lipidor fat. All ofthe terms ofthe molecular level are consistent
witheach other when defined according to these guidelines and
asshown in Table 3.
At present the main direct techniques available for
estimatingcomponents on the molecular level are for water and
mineral.TBW can be measured by several well-established
isotope-di-lution techniques ( 10. 1 1), and osseous mineral can be
estimatedby whole-body dual-photon absorptiometry (12). The
remainingcomponents ofthe model must be estimated indirectly by
usingmeasurements included in one of the other four levels. For
ex-ample, protein can be determined from total body nitrogen atthe
atomic level by making two assumptions: that all of bodynitrogen is
in protein and that 16% of protein is nitrogen (1).Another example,
total body fat can be calculated from bodydensity, which is a
dimension at the whole-body level, by as-suming that fat and FFM
have respective densities of0.900 and1.100 g/cm3 (9, 10).
The molecular level of body composition is the
conceptualfoundation for the higher levels that follow. Also, the
molecularlevel connects the study of body composition to other
researchareas, notably biochemistry.
Cellular (III)Although the human body can be divided into
different corn-
ponents at the molecular level, it is the assembly of these
corn-ponents into cells that creates the living organism. The
coordi-nated functions and interactions between cells are central
to thestudy of human physiology in health and disease. The
cellularlevel is therefore an important area of body-composition
re-search.
The human body is composed of three main compartmentson the
cellular level: cells, extracellular fluid, and
extracellularsolids. Each ofthese compartments is now described in
additionaldetail.
Cells. The cells possess the characteristics of life
includingmetabolism, growth, and reproduction. Although the l018
cellsof the adult human body share many properties in common,there
are great variations in size, shape, elemental and
molecularcomposition, metabolism, and distribution. Cells are
adapted tospecific functions, such as support, electrical
conduction, andcontraction. Based on these differences, four
categories of cellscan be defined: connective, epithelial, nervous,
and muscular (13).
(6) Connective cells include three groups: loose, dense. and
spe-cialized ( 1 3). Adipocytes. or fat cells, are a type of loose
con-nective cell in which fat is stored. Bone cells. the
osteoclasts andosteoblasts. and blood cells are representations of
specializedconnective cells.
Muscle cells include striated skeletal, smooth. and cardiac.The
striated skeletal muscle cells are the foundation of humanmovement
and account for a large fraction ofbody weight. Cellsconsist
offluid and solid components, the intracellular fluid
andsolids.
E.vtracel/u/ar fluid. The nonmetabolizing fluid surroundingcells
that provides a medium for gas exchange, transfer of nu-trients,
and excretion of metabolic end products is referred toas the
extracellular fluid.
Extracellular fluid, which is 94% water by volume, is
dis-tributed into two main compartments: plasma in the
intravas-cular space and interstitial fluid in the extravascular
space.Plasma and interstitial fluid account for 5% and 20% of Bwtin
the Reference Man (2), respectively.
Extrace/lular solids. Extracellular solids are also a
nonmetab-olizing portion of the human body that consists of organic
andinorganic chemical compounds. The organic extracellular
solidsinclude three types of fiber: collagen, reticular, and
elastic (13).Both collagen and reticular fibers are composed
ofcollagen pro-tein whereas elastic fibers are formed from the
protein elastin.
The inorganic extracellular solids represent 65% ofthe drybone
matrix in the Reference Man (2). Calcium. phosphorus,and oxygen in
bone are the main elements ofthe inorganic ex-tracellular solids
that are incorporated into calcium hydroxy-apatite ( 1 ). Other
inorganic components are also present in ex-tracellular solids.
including bicarbonate, citrate, magnesium, andsodium (1. 2).
From the previous discussion, the cellular level of body
corn-position can be accurately described by the equations
Bwt = CM + ECF + ECS
CM = muscle cells + connective cells
+ epithelial cells + nervous cells
ECF = plasma + 1SF
ECS = organic ECS + inorganic ECS
(7)
(8)
(9)
(10)
where CM is cell mass, ECF is extracellular fluid, ECS is
extra-cellular solids. and 1SF is interstitial fluid. However.
becausemost components in equations 7-10 cannot be measured in
TABLE 3Different body-composition terms on the molecular level
(II)
Lipi ds*
Ln Le Water Protein Mineral Glycogen
Bodyweight x X X X X xDry body weight x X x x xLipid-free body
mass x x x xFat-free body mass x x x x xLean body mass x x x x
x
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Inorganic Cell Residual
Inorganic ECI ResIdual
Inorganic ECS
Total
BodyMineral
FIG 4. Relationship between total body mineral and inorganic
solids.ECF. extracellular fluid: ECS, extracellular solids.
FIG 3. Relationship between body fluids. ECF. extracellular
fluid:ECW. extracellular water: ICF. intracellular fluid: ICW.
intracellular wa-ter: R, and R, . extracellular and intracellular
residual: and TBW. totalbody water.
FIVE-LEVEL BODY COMPOSITION MODEL 23
vivo at present. the following equation is suggested as a
practicalalternative for Bwt at the cellular level
Bwt = fat cells + BCM + ECF + ECS ( Ii)
where BCM is body cell mass. 13CM is a portion of total cellmass
that according to Moore et al ( 1 1 ) is the working.
energy-metabolizing portion ofthe human body in relation to its
sup-porting structure. Hence, BCM includes the protoplasm in
fatcells hut does not include the stored fat, which occupies 85%
to90 offat cell weight. Although no present method can
directlymeasure BCM. it is a widely used term and is assumed to
berepresented by exchangeable or total body potassium (TBK) ( I
1).A deviation must be noted in equation 1 1 in that BCM and
fatcells share in common the nonfat portion of adipoctes
andtherefore overlap by 1 kg in the Reference Man (2).
The fluid compartments at this level can also be related toTBW
as shown in Figure 3. According to this model. ECW andICW are
extracellular and intracellular water. and Re and Riare nonaqueous
residual extracellular and intracellular solids.
Another relation at the cellular level is between total
bodymineral and inorganic solids (Fig 4). Each ofthe three
compo-nents in equation 7 contribute to total mineral, inorganic
celland extracellular fluid residual, and the inorganic portion
ofextracellular solids.
Of the three primary compartments at the cellular level.
thevolume of extracellular fluid and its plasma subcompartmentcan
be quantified directly by dilution methods ( 10). In contrast.no
direct methods are yet available for estimating either cellmass or
extracellular solids. Indirect methods ofevaluating
somecompartments are available, such as extracellular solids
estimatedfrom TBCa measured by neutron-activation analysis (ECS=
TBCa/0. 1 77) ( 10). Another example is the calculation of BCMfrom
TBK [BCM (in kg) = 0.00833 X TBK (in mmol)] ( 1 1)
Because the cellular level is the first level at which
character-istics ofthe living organism appear. it occupies a
central positionin connecting the inanimate features of body
composition atthe lower levels with those of the animate features
of tissues.organs. and intact humans at the higher levels. Despite
its im-portance in the study of human body composition. very
littleresearch has been directed at this level, perhaps because of
thedifficulty in quantifying some ofthe compartments.
Tissue-System (I J )At the cellular level the human body is
composed of cells.
extracellular fluid. and extracellular solids. These three
corn-ponents are further organized into tissues. organs. and
systems-the fourth level of body composition.
iissl1Ls. Generally. tissues contain cells that are similar
inappearance. function. and embryonic origin. All ofthe
diversetissues ofthe body can be grouped into four categories:
muscular.connective. epithelial. and nervous ( 13).
Bwt at the tissue level ofhody composition is defined as
Bwt = muscular tissue + connective tissue
+ epithelial tissue + nervous tissue (/2)
Three specific tissues are particularly important in
body-corn-position research: hone. adipose. and muscular, which
togethercomprise 75 ofBwt in the Reference Man (2).
Bone is a specialized form of connective tissue that consistsof
bone cells surrounded by a matrix of fibers and ground sub-stance.
The distinguishing feature of bone is that the groundsubstance is
calcified and accounts for 65 ofdry bone weight(2). The calcified
ground substance is mainly hydroxyapatite([Ca3(P04)2]3Ca(OH),) and
a small amount of calcium car-bonate (14).
Adipose tissue is another type of connective tissue made upof
fat cells (adipocvtes) with collagenous and elastic fibers.
fi-broblasts. and capillaries. Adipose tissue can be divided
intofour types according to its distribution: subcutaneous.
visceral(ie. loosely surrounds organs and viscera). interstitial
(ie. inti-mately interspersed among the cells oforgans). and yellow
mar-row (2). Muscle tissue can be subdivided into striated
skeletal,smooth. and cardiac tissues (2).
Oq.aiis. The organs consist oftwo or more tissues combinedto
form large functional units such as skin, kidney, and
bloodvessels.
Siste,ns. Several organs whose functions are interrelated
con-stitute an organ system. For example. the digestive system
iscomposed of many organs. including the esophagus.
stomach,intestine. liver. and pancreas. Each organ. such as the
stomach.
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24 WANG ET AL
contains several kinds oftissue (muscular. connective.
epithelial.and nervous) and each tissue is made up of many cells
andextracellular material.
There are nine main systems in the human body. hence Bwtat the
system level of body composition can be defined as
Bwt = musculoskeletal + skin + nervous
+ circulatory + respiratory + digestive + urinary
+ endocrine + reproductive systems (13)
Although Bwt can be expressed accurately on the
tissue-systemlevel, most components in equations 12 and 13 cannot
be mea-sured in vivo at present. The following equation is
suggested asa practical alternative
Bwt = adipose tissue + skeletal muscle
+ bone + viscera + blood + R (14)
where the five components account for 85% and R accounts forthe
remaining 1 5% of Bwt in the Reference Man (Table 4) (2).
The tissue-system level is complex and interfaces with
severalbranches ofhuman biology, including histology and
histochem-istry at the tissue level and anatomy and physiology at
the organand system level. Physicians, nutritionists, and exercise
physi-ologists focus much oftheir interest in body composition at
thetissue-system level.
Although a great deal of information is available at this
level,most of it comes from cadaver studies or tissue biopsies.
Thereare only a few in vivo direct methods that can be used to
estimatethe major compartments at the tissue-system level. An
exampleis computerized axial tomography, which can directly
determinethe volume of subcutaneous and visceral adipose tissue
(15).Some indirect techniques are also available at this level.
such asestimation ofskeletal muscle mass from 24-h urinary
creatinineexcretion or from TBK and nitrogen content by
neutron-acti-vation analysis (16, 17).
i/i Ito/c body (I)Both humans and some primates have similar
body compo-
sitions at the atomic, molecular, cellular, and tissue-system
levels.It is at the whole-body level, however, with its complex
char-acteristics that distinguishes humans from all other primates.
Inaddition, many biological, genetic, and pathological
processeshave an impact not only at the first four levels but also
on thehuman body as a whole.
The whole-body level of body composition concerns bodysize,
shape, and exterior and physical characteristics. There are 10
suggested dimensions at the whole-body level (18).
I) Stature: This is a major indicator ofgeneral body size
andskeletal length.
2) Segment lengths: Many segment lengths are used in thestudy
ofbody composition, the most common ofwhich are lowerextremity
length, thigh length, calflength, shoulder-elbow length,and
elbow-wrist length.
3) Body breadths: Body breadths are a measure ofbody
shape,skeletal mass, and frame size. The sites most widely used
arethe wrist, elbow, ankle, knee, and biiliac.
4 ) Circumferences: The circumferences are useful indicatorsof
body density, FFM, adipose tissue mass, total body proteinmass. and
energy stores. The most widely used circumferencesare upper arm,
waist (abdominal), and thigh.
TABLE 4Body composition on the tissue-system leveand organs
ofthe 70-kg Reference Man*
I (IV) for principal tissues
Tissue or organ Amount Percent of body weight
&v %
Skeletal muscle 28Adipose tissue
Subcutaneous 7.5Visceral 5Interstitial 1Yellow marrow I .5
Bone 5Blood 5.5Skin 2.6Liver 1.8Central nervous system I
.4Gastrointestinaltract 1.2Lung I
40
1 17.11.42.17.17.93.72.621.71.4
* Information based on reference 2 (modified).
5) Skinfold thicknesses: Skinfolds represent a double layer
ofadipose tissue and skin at specific anatomic locations.
Triceps.subscapular, calf (medial), and abdominal are the most
corn-monly used sites. Skinfold thickness provides a simple
methodof estimating fatness and the distribution of subcutaneous
adi-pose tissue. Numerous equations for the prediction of body
fathave been developed that make use of skinfold thicknesses.
6) Body surface area (BSA): The total BSA is an
exteriorcharacteristic that is often used to estimate basal
metabolic rateand FFM.
7) Body volume: The total body volume is an important in-dicator
of body size and is used to calculate body density.
8) Bwt: One ofthe simplest and most important
morphologicindicators. Bwt is used in screening for growth rate,
obesity. andundernutrition. The Bwt equation that defines the
whole-bodylevel is
Bwt = head weight + neck weight + trunk weight
+ lower extremity weights + upper extremity weights (15)
9) Body mass index: Bwt and stature can be combined toform
indices that correlate with total body fat. The best knownof the
indices is body mass index (body weight/stature2. in kg!m2), which
is often used in obesity studies as a measure of fatness( 19).
However, more complex and population-specific indices,such as the
Fels index (Bwt2/stature33), often correlate betterwith total body
fat ( 18).
10) Body density: The density of the human body, derivedfrom Bwt
and volume, is widely used to indirectly estimate totalbody fat and
FFM (9, 10) and is defined at the molecular levelas
1/Db = fFat/DF + fFFM/DM (16)where Db. DF, and DFFM are the
densities (in g/cm3) ofthe totalbody, fat, and fat-free body,
respectively, and f represents thefractions of Bwt as fat and FFM,
respectively (20). Similar equa-tions for total body density based
on individual components atthe cellular, tissue-system, and
whole-body levels can also bewritten.
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FIVE-LEVEL BODY COMPOSITION MODEL 25
TABLE 5Some related but distinct components on different
levels
Atomic level Molecular level Cellular level
Tissue-syste m level
Tissue level Organ level
Total hod calciumand phosphorus
Mineral Extracellular solids Bone Skeleton
Total body carbon Lipid and fat Fat cells Adipose tissue
Skeletal muscle cells Skeletal muscle tissue Skeletal muscle
It is clear that any major changes in body composition on
theother four levels will manifest themselves on the
whole-bodylevel. Conversely. most differences at the whole-body
level arerelated to changes in composition on the other four
levels. Thislatter relation is the foundation for estimating the
componentsof the other four levels by using measurements at the
whole-body level. Most indicators at the whole-body level are
simplerand easier to perform than are measurements at the other
fourlevels. thus the techniques at this level are often well suited
forlarge-scale studies or for field work.
Features of the model
The five-level model provides a structural framework forstudying
human body composition that goes beyond an mdi-vidual compartment
or level. In this section we describe someof the features of the
five-level model as a whole.
Distinctions and connections bettieen diflirent levelsAn
essential aspect of the model is that the levels themselves
are distinct and have unique properties that should not be
con-fused with one another.
I) On the atomic level, there are no special elements or
anyfundamental differences between the human body and the
in-organic world. although the ratio ofelements to each other
varies.
2) On the molecular level, the human body is differentiatedfrom
the inorganic world because ofthe appearance of complexorganic
compounds such as lipid and protein.
3) On the cellular level, the human body is distinct from
thenonliving world because ofthe appearance ofcells that have
thecharacteristics of living organisms.
4) On the tissue-system level, the human body is differentfrom
the lower animal world because ofthe appearance of tissues,organs.
and systems having complex structures and functions.
5) On the whole-body level, the human body is differentiatedfrom
all other primates because ofthe presence ofdistinct mor-phological
features.
Although these distinct properties exist for each of the
fivelevels, linkages are also present that are clearly recognizable
inthe context ofthe five-level model. An example is that cells
thatappear first on the cellular level have many ofthe
characteristicsofliving organisms such as membrane transport,
energy metab-olism. and enzymatic processes. These characteristics
ofthe cellare still maintained at the tissue-system and whole-body
levels.Each higher level is thus unique but maintains some ofthe
char-acteristics of the level below it.
Recognition ofdistinct levels and their connections can
revealgaps in present body-composition information and suggest
a
direction for future research efforts. For example. it is
knownthat many biological factors including growth, development,
se-nescence. race, sex, nutritional status, exercise level, and
thepresence of disease all have important effects on body
compo-sition. However, most studies ofbody composition in these
areasare limited in scope, focusing on only a few components at
oneor two levels and thereby failing to appreciate the
connectionsbetween levels. For example, most previous obesity
studies werelimited to anthropometric changes (at the whole-body
level) andalterations in fat mass (at the molecular level). Very
few studieshave investigated how obesity influences the other
levels of bodycomposition or more importantly the coordinated
changes thatoccur throughout all five levels with increasing
Bwt.
Distinctions and connections between different componentsAn
important feature ofthe model is that every major corn-
ponent has a clear definition and can be included in one of
thefive levels. Each ofthese components has unique properties
andyet maintains relationships with other components at the sameand
different levels.
It was not unusual in earlier studies for related componentsto
be confused with each other, particularly ifthey were on dif-ferent
levels. An example of three sets of commonly confusedcomponents is
presented in Table 5. In the first set, TBCa andphosphorus.
mineral, extracellular solids, bone tissue, and skel-eton, are
related compartments but belong to different levelsand have
distinct differences from each other:
1) Calcium and phosphorus, and mineral: Most ofTBCa
andphosphorus exist in mineral although there is some phosphorusin
protein and lipid (eg, DNA, RNA, and phospholipid). On theother
hand, in addition to calcium and phosphorus, mineralcontains other
elements (eg, carbon, oxygen, hydrogen, mag-nesium, and
sodium).
2) Mineral and extracellular solids: Most oftotal body mineralis
in extracellular solids although there still is a small amountof
mineral in cells and extracellular fluid. On the other hand,in
addition to the mineral in the form of inorganic
material.extracellular solids contain organic solids such as
collagen, re-ticular fibers, and elastic fibers.
3) Extracellular solids and bone tissue: Most of total
bodyextracellular solids are in the form ofbone tissue although
therestill is a small amount ofextracellular solids in other
tissues (eg,in skeletal muscle). On the other hand, in addition to
extracellularsolids. bone tissue contains bone cells and
extracellular fluid.
4) Bone tissue and skeleton: Bone tissue constitutes the
ma-jority of the skeleton although the latter also includes
skeletalcartilage, periarticular tissue adhering tojoints, and red
and yel-low marrow.
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26 WANG ET AL
Another example of related but distinct components is totalbody
carbon (level I). lipid and fat (level II), fat cells (level
III),and adipose tissue (level IV) (Table 5). These terms are
oftenconfused with each other, a problem that the five-level
modelhelps to resolve.
The third and final example in the table is the distinctly
dif-ferent but related components skeletal muscle cells,
skeletalmuscle tissue, and intact whole skeletal muscles. The
modelthus demonstrates that differences and relations exist
betweencomponents on each of the five levels. It is therefore
advisableto develop equations for body weight, volume, or density
thatinclude components from the same level in order to avoid
overlapor omission of some components.
Steady-state OfbOdI compositionThe concept of a steady state is
important not only in bio-
chemistry, physiology, and other classic scientific disciplines
butalso in body-composition research. The meaning of a steadystate
of body composition can be defined in the context of thefive-level
model: A steady-state or dynamic homeostasis existsduring a
specified time period if Bwt and the mass of variouscomponents on
different levels is maintained relatively constant.
The important implication of a steady state is that there
arestable proportions among the different components on the
samelevel. For example, on the molecular level the average ratio
oftotal body water content to FFM is relatively constant in
healthysubjects (ie, total body water/FFM = 0.732) (2). On the
atomiclevel the correlation between TBK and TBCa is reproduciblefor
males [ie, TBK(g) 0. 1383 X TBCa(g) - 17. 1 ] (2 1). On
thewhole-body level the relation between BSA (BSA, in m2) andBwt
(kg) and stature (in rn) is also relatively constant such thatBSA =
0.007 184 x stature#{176}725 X body weight#{176}425(22).
There are also relatively constant proportions among the
rel-evant components on different levels when body compositionis in
a steady state. For example, total body protein/total body
nitrogen = 6.25 ( 1 ): BCM(kg)/TBK(mmol) = 0.00833 ( 1 1 ),
andfat(kg) = [(4.95/Db) - 4.50] x Bwt (20).
The steady state of body composition indicates that
althoughthere are so many components in the human body, and all
ofthese components differ from each other, they are well
organizedaccording to definable quantitative relations.
Quantitative hodi COlfl/)OsitiOfl relations
A primary aim of body-composition research is to estimatethe
size ofeach compartment. although there are numerous in-dividual
compartments ofclinical relevance that have not beenmeasured
directly. An alternative is to estimate the unknowncomponents by
establishing relationships to measurable corn-ponents. Body
composition is relatively stable in healthy adults.and it is this
property that enables investigators to establish thesereproducible
relations or rules. The five-level model of bodycomposition affords
a logical matrix within which to establishthe quantitative
steady-state relations between known measur-able components and
presently unmeasurable compartments.
Present research in developing body-composition
equationsprimarily involves estimating one unknown component from
ameasurable component. The five-level model suggests the
pos-sibility of reconstructing Bwt and volume by writing
simulta-neous equations that exploit steady-state relations between
sev-eral measurable and unknown components. An example is
thecalculation of the five major chemical components and Bwt atthe
molecular level from six elements (carbon, nitrogen,
sodium.potassium. chlorine, and calcium) measured by in vivo
neutron-activation analysis ( 1). Until recently the concept of
recon-structing whole levels ofbody composition from multiple
com-ponents was limited and the studies were fragmentary. The
five-level model defines explicitly the equations for Bwt at each
leveland presents the challenge of developing more complex
andcomprehensive body-composition equations.
TABLE 6The relation between direct and indirect body-composition
measurements organized by the five-level model
Direct
Indirect
Atomic level Molecular level Cellular level Tissue-system level
Whole-body level
Atomic level TBP = (0.456 X TBCa) Pro = 6.25 X TBN BCM = 0.00833
X TBK SM = 0.0196 X TBK Bwt 0 + C + H + NTBO. TBC. TBH, TBN. +
(0.555 x TBK) FFM = TBK/68.l ECS = TBCa/0.l77 - 0.0261 X TBN + Ca +
P + K + Na
TBK. TBCa. TBNa. ECF = (0.9 X TBCI)/ + Cl + RTBP. TBCI. Nae. Ke
Plasma Cl
Molecular level FFM = TBW/O.732 SM = 1 1 .8 X Cr Bwt = L + A +
ProTBW, mineral. creatinine. FFM = 24.1 X Cr + 10.1 + M + G + R
3-MH + 20.7Cellular level Bwt = CM + ECF
ECF. plasma volume + ECSTissue-system level Bwt = adipose
tissue
Volume of subcutaneous + skeletal muscle
and visceral adipose + bone + vi5Cerstissue + blood + R
Whole-body level TBK = (27.3 X Bwt) Fat% = (4.95 X BV/ ECF =
0.135 X Bwt SM = 5(0.0553 Body surface = 0.007184Bwt, S. By,
circumference. + (11.5 X 5) - (21.9 Bwt - 4.5) X 100 + 7.35 X CTG2
+ 0.0987 X 50725 x BW#{176}425
skinfold x Age) + 77.8 x FG2 + 0.0331x CCG2) - 2445
* A. water (kg): BCM. body cell mass (kg): BV. body volume (L):
Bwt. body weight (kg): CCG. corrected medial calf girth (cm): CM.
cell mass (kg): Cr, 24-h urinecreatinine (g): CTG. corrected thigh
girth (cm): ECF. extracellular fluid (kg): ECS. extracellular
solids (kg): FFM. fat-free body mass (kg): FG. forearm girth (cm):
G.glycogen(kg): Ke. exchangeable potassium: L. lipid(kg): M.
Mineral(kg): 3-MH. 24-h urine 3-methylhistidine: Nae. exchangeable
sodium: plasma Cl. plasma concentrationofchlorine (mmol/L): Pro.
protein (kg): R. residual (kg): S. stature (cm): SM. skeletal
muscle (kg): TB. total body element (kg): and TBW. total body water
(kg).
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FIVE-LEVEL BODY COMPOSITION MODEL 27
Re/atiomi to 1flCt/lOdOl()tl
At present. body-composition methods are primarily cate-gorized
into technique-specific groupings such as dilution meth-ods and
neutron-activation analysis. According to the five-levelmodel,
however, the methods can be organized in a more sys-tematic
fashion.
Direct measurement !fitt/lOds. There are some direct
methods,such as anthropometric, biochemical, and radioisotopic
tech-niques that can be used to estimate components of body
com-position. Direct methods can be organized according to the
five-level model as follows:
1) On the atomic level, TBK can be directly determined
bywhole-body #{176}Kcounting ( 12): total body sodium.
chlorine.phosphorus, and calcium by delayed-y neutron activation
(3):total body nitrogen by prompt--) neutron activation (23):
andtotal body carbon by inelastic neutron scattering (4).
2) On the molecular level, TBW can be directly estimated
byseveral isotope-dilution techniques (10), and osseous mineralcan
be quantified by dual-photon absorptiometry (24).
3) On the cellular level, extracellular fluid (and plasma
vol-ume) can be directly determined by several
isotope-dilutiontechniques (25).
4 ) On the tissue-system level, the volumes of subcutaneousand
visceral adipose tissue can be directly determined by com-puterized
axial tomography and by magnetic resonance imagingtechniques
(26).
5) On the whole-body level, anthropometric indices such asBwt,
body volume, stature, circumferences, and skinfold thick-nesses can
be estimated directly (18).
According to this analysis there are not many direct methodsused
in the study ofhurnan body composition. Moreover, mostofthe direct
methods are concentrated on the atomic and whole-body levels. There
are only a few direct techniques on the mo-lecular, cellular, and
tissue-system levels.
Indirect neasuremnent methods. These estimate unknowncomponents
of body composition by combining direct mea-surernent techniques
with the established steady-state relation-ship between the
directly measurable and unknown components.Indirect methods greatly
expand the number of body compart-rnents that can be evaluated. At
present, some important corn-partments can be assessed only by
indirect methods. For ex-ample, although total body fat is a major
compartment of in-terest. there are no practical methods ofdirectly
evaluating thefat compartment in vivo. All ofthe presently used
methods areindirect and based on direct measurements at different
levels asfollows:
1) from direct method on the atomic level (10), fat = Bwt-
TBK(mmol)/68. 1 : 2) from direct method on the molecularlevel (2,
10), fat = Bwt - TBW/0.732: 3) from direct methodon the molecular
and whole-body levels, fat = 2.057 X BV- 0.786 x TBW - 1 .286 X
Bwt, where BV is body volume inliters (20); and 4) from direct
methods on the whole-body
level,fat=4.95XBV-4.5OXBwt,andfat=0.7l5XBwt- 12.1x stature2 (in m)
(19, 20).Thus it can be seen that indirect methods are not only
based onthe direct methods, but also are dependent on the
steady-stateproportions between known and unknown components as
de-termined in sample populations.
Direct and indirect body-composition methodology can beoutlined
according to the five-level model as shown by the ex-
The Study of Body Composition
Components on Levels I, II, III, IV, and VBody composition
rules
I _ IL Methodology j IBiological effects
FIG 5. The three areas ofbody-composition research.
amples presented in Table 6. The table demonstrates that mostof
the principal elements and anthropometric indices can bedirectly
measured and that many of the indirect methods havebeen developed
from the direct methods on the atomic andwhole-body levels.
respectively ( I 0, 17, 22, 25, 27-29). Con-versely, the table
shows that there are only a few direct methodson the cellular and
tissue-system levels, so the relevant indirectmethods are also very
limited. This is one ofthe weak areas inbody-composition
methodology and could constitute an im-portant topic for future
research.
Definition of body composition research
The study of body composition spans > 100 y, and the termbody
composition is widely used. However, it is unclear whatthis branch
of science represents and what exactly is meant bythe term body
composition. The five-level model presented inthis paper not only
builds an appropriate structure for body-composition research. but
is conducive to clearly define humanbody composition as a branch of
human biology that studiesvarious body compartments and their
quantitative steady-staterelations or rules. Body-composition
research includes three in-terconnecting areas: studying the
proportions of various corn-ponents and their steady-state
associations among the atomic,molecular. cellular, tissue-system,
and whole-body levels: study-ing the methods of measuring various
components in vivo; andstudying the influences of biological
factors on various levelsand components (Fig 5).
Conclusion
The five-level model grows from a need to organize both
therapidly developing methodologies and physiological conceptsthat
relate to the study ofhuman body composition. The modelis intended
to be a foundation on which future studies can refineor expand
selected definitions or equations. The five-level modelserves in
this organizational capacity and also stimulates abroader view of
body-composition research as a whole. C]
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