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OCR for page 242
Methodologies for Measuring
Body Composition in Humans
HWAI-PING SHENG
"Nothing is measured with greater error than the human body."
Beneke, 1878
Studies of the composition of the human
body are relatively new in human biology.
Although interest in the effects of malnu-
trition on body tissues, morbidity, and mor-
tality clates back to the time of Hippocrates,
the primary research interest in body com-
position began in the 1940s. Since that time,
many procedures ant! techniques have been
developed to assess indirectly the various
components of the body. These techniques
have been discusser] and evaluated at a
number of symposia and in review articles
(Brozek, 1963, 1965; Brozek and Henschel,
1961; Garrow, 1982; Lohman, 1984; Na-
tional Academy of Sciences, 1968; Siri, 1961~.
This paper briefly reviews the indirect
methods that are currently available to
measure adipose tissue (fat) in the body.
These procedures are directed either to
whole-body measurement or to specific sites
and regions, in which case they extrapolate
data to whole-bocly fat content using pre-
viously determined relationships. Methods
range from the simple to the complex; most
make use of constants and assumptions de-
rivec3 from either the guinea pig data of
Pace and Rathbun (1945) or cadaver analy-
ses, which form the basis for the "reference
man" (Brozek et al., 1963~. Most indirect
242
methods were validated against another in-
direct method; few studies were validatecl
by direct cadaver analysis (Knight et al.,
1986) or carcass analysis of an animal (Lewis
et al., 1986; Sheng and Huggins, 1979~.
The terms lean body mass (LBM) andfat-
free mass (FFM) are used interchangeably
and sometimes cause confusion among in-
vestigators. Body mass can be considered
as the sum of adipose tissue and LBM or,
alternatively, of ether-extractable fat and
FFM. The terms would be synonymous if
adipose tissue were composed of pure fat
instead of approximately 80 to 85 percent
fat, 2 percent protein, en cl 13 to 18 percent
water. Thus, the distinction between LBM
and FFM is not critical in a fairly lean
incliviclual, but it is important in an obese
individual in whom the contribution of the
nonfat component to the adipose tissue can
be large. The terms fat en c! FFM will be
used in this paper.
WHOLE-BODY MEASUREMENTS
Fat-Soluble Gases
Because most anesthetic gases are rare
inert gases (for example, cyclopropane, xe
OCR for page 243
MEASURING BODY COMPOSITION IN HUMANS
none, and krypton) and are highly soluble
in fat but not in water, it is theoretically
possible to use the dilution principle to
calculate total body fat by measuring the
absorption of these gases. Measurements of
fat by this technique have been reported
by several investigators who used either the
absorption phase (Lesser and Zak, 1963) or
both the absorption and Resorption phases
of the gas (Mettau et al., 1977) to calculate
body fat. Results for small animals agreed
with those obtainer] by carcass analysis;
results for humans were in the range of
published data for fat (Mettau et al., 19771.
The disadvantages of this method include
the necessity of a closed respiratory system
and the length of time required to attain
equilibrium conditions, both of which in-
convenience the subject. Attempts to re-
duce the duration of the experiment by
extrapolation of the early phase of measure-
ments have been relatively unsuccessful.
Body Compartmentalization into Two
Components
Most indirect methods compartmentalize
the body in numerous ways, depending on
the purpose of the study and the require-
ments of the investigators. In its simplest
form, body mass can be considered to consist
of two phases: fat and FFM. In the research
setting, body fat content is often determined
by deriving FFM from a set of measure-
ments and then calculating body fat content
as part of the body mass not accounted for
by FFM. The concept of a fat-free body was
originally suggested in 1915 by Dubois and
Benedict, who proposed that the FFM was
metabolically important and had a constant
chemical composition. Research on FFM
was accelerated markedly during the 1940s
by Behnke et al. (1942), who attempted to
measure the amount of"primary energy-
exchanging mass" of tissues in the body,
which they called the LBM. The LBM or
FFM has been estimated by several meth-
ods, all of which, as summarized by Wedg
243
wood (1963), assume that LBM has a con-
stant density, LBM has a constant proportion
of water, bone is a constant proportion of
LBM, and cell water is a constant proportion
of cell mass. It is also frequently assumed
that LBM has a constant percentage of
potassium.
Determination of Fat by Densitometry
Densitometric determination of body fat
is considered by many investigators as the
"reference method" or the "standard" against
which other indirect methods are compared.
Equations have also been developed to
predict body fat from anthropometric meas-
urements using fat data obtained by densi-
tometry. The estimation of body fat from
densitometry was pioneered by Behnke et
al. (1942), who reasoned that if the densities
of the two body components (fat ant] LBM)
were known and if the density of the whole
body could be measured, then the propor-
tional masses of fat and LBM could be
calculated. Although the concept of densi-
tometry is theoretically sound, an accurate
measurement of body density and, for the
two-compartment approach, the known
densities for body fat and FFM are required.
Theoretically, body density can be meas-
ured with an accuracy of + 0.001 to + 0.025
g/ml (Sir), 1961), but in practice this is
difficult to achieve. Body density is calcu-
lated using an Archimedean principle:
Body density = Body mass/body volume.
Many different methods have been devel-
oped to measure body volume, but as yet,
none appears to yield a satisfactory level of
accuracy.
The original, and still most widely used,
physical method to measure body volume
uses either underwater weighing (Gnaedin-
ger et al., 1963) or water displacement (Garn
and Nolan, 1963~. This measurement can
be made with a relatively simple apparatus,
but it supers from two practical problems:
(1) subject cooperation is required because
OCR for page 244
244
whole body submersion is essential, and (2)
residual volumes of air in the lungs and the
gut have to be measured separately. A1-
though the residual volume of air in the
lungs can be measured easily, no adequate
method is available for measuring air in the
gut.
Photogrammetry has been suggested as a
tool for the measurement of body volume
(Pierson, 1963), but thus far has not proved
successful because photomapping requires
complicated mathematics and highly skilled
personnel. An adclitional drawback in this
method is the inclusion of the residual
volumes of air in the lungs and gut.
Diethelm et al. (1977) and Garrow et al.
(1979) have reported the successful use of a
combination of water displacement (to meas-
ure a partially submerges! body) and air
displacement (to measure the nonsub-
merged head region). Certain technical dif-
ficulties, such as volume of air in the gut
and thermodynamic problems with the air-
displacement method, have yet to be re-
solved. Bocly volume measured by air dis-
placement is theoretically simple but tech-
nically difficult. In theory, the volume of
air clisplaced by an infant placer! in a rigid
chamber can be measured by either the
helium dilution method or by measuring
the pressure cli~erence as described by
Boyle's law (Faulkner, 1963; Fomon et al.,
1963; Gnaedinger et al., 1963; Lim, 1963;
Taylor et al., 1985~. If the chamber volume
is 30 liters and if a piston changes the volume
by 0. 3 liters, a 2-liter premature infant
would only change the incremental pressure
over that of the empty chamber by 0. 76 cm
of water (or 0.073 percent). To measure
such small pressure changes accurately is
difficult, since a change of temperature from
36 to 37°C at a constant volume and an
ambient pressure of 760 mm Hg would
cause a pressure rise of 3.34 cm of water.
This technical difficulty can be resolved by
the development of a differential dynamic
system where identical volume changes in
two identical chambers are induced by two
APPENDIX
yokel! pistons (Taylor et al., 1985~. Any
differential pressure, as measured by a ma-
nometer between the two chambers, would!
be due entirely to the difference in air
volume between the chambers. This system
would require a resolution of the differential
pressure of 1 percent (instead of 0.073
percent) for a 1 percent change in body
volume (Taylor et al., 1985~. Body volume
measurements obtained by this system are
generally reasonable, although widely di-
vergent values are producer! occasionally,
probably because of pressure fluctuations
from respiratory movement and tempera-
ture changes (Taylor et al., 1985~. When
the technical difficulties are resolved, this
method may be particularly suited for in-
fants, because corrections for residual vol-
umes of air in the lungs and gut are not
necessary.
The acoustic plethysmograph is another
method being explored to measure body
volume (Deskins et al., 1985~. It makes use
of the Helmholtz principle that resonant
frequency is inversely proportional to the
volume of the resonating chamber; that is,
the volume of an object placed inside the
resonating chambers can be calculated from
the difference in resonant frequencies. The
acoustic plethysmograph can be constructed
and operated relatively inexpensively and
can be easily used to measure holly volume
in infants. Its disadvantages include a lack
of ability to measure the residual volume of
air in the lungs and gut.
Even with the assurance that body (lensity
can be measured with great "precision" for
a given in~liviclual, and perhaps with great
"accuracy," the application of the densito-
metric approach to measure FFM and fat
is not without error. The values of 0.9 for
the density of body fat and 1.10 or 1.095
for the density of mixed tissues of the FFM
are used in the calculations (Brozek et al.,
1963~. Although the density of body fat
varies at different body sites and from con-
sumption of different diets, the variations
reported are less than 2 percent (Pearson
OCR for page 245
MEASURING BODY COMPOSITION IN HUMANS
et al., 1968~. Therefore, their contribution
to the error in the estimation of fat is small.
However, there is an increasing realization
that it is invalid to assume the chemical
constancy of FFM (WedgwoocI, 1963~; thus,
the value of 1.095 for the density of FFM
(derived from cadaver analysis) must be used
cautiously. The greatest change in the chem-
ical composition of FFM occurs during the
growth of the infant, resulting in an increase
of FFM density from 1.064 in infants (Fomon
et al., 1982) to 1.095 for the reference man
(Brozek et al., 1963~. The extent of error in
fat estimation can be calculated for an infant:
Fat content was estimated as 11 percent of
body weight when a density value of 1.064
was used for FFM and 23 percent when
1.095 was used. Thus, reported percentages
of body fat must be viewer] with caution.
As Brozek et al. (1963) concluded after a
detailed review of the method, "It appears
that no universally valic] formulas for clen-
sitometric estimation of the fat content can
be offered."
Determination of FFM by Hydrometry
A value for body fat may be derived simply
from the total body water (TBW) measure-
ment baser] on the assumption that FFM
has a constant water content of 73.2 percent:
FFM = TBW/0.732.
Measurement of TBW is theoretically sim-
ple, requiring the estimation of dilution
spaces of small-molecular-weight substances
or tracer doses of isotopically labeled water
(Schoeller et al., 1980; Sheng and Huggins,
1979~. However, increasing evidence sug-
gests that tritiated water overestimates TBW
to varying degrees in animals in various
nutritional ant! physiological states, espe-
cially in rapidly growing young animals
(McManus et al., 1969; Sheng and Huggins,
1979~. The degree of overestimation of TBW
would affect the degree by which body fat
was underestimatecI.
Use of a "constant" for the hydration of
245
FFM has been questioned. In the derivation
of this constant from eviscerated guinea pigs
and several other species of animals, Pace
and Rathbun (1945) recognized that the
constant-73.2 percent can be applied only
to adult animals, a provision that has occa-
sionally been overlooked. Even in adult
animals, fatter animals tend to have a higher
FFM water content. Moulton (1923) rec-
ognized in 1923 that relative water content
was reduced during early growth in a num-
ber of animal species. The animal reaches
chemical maturity only when its relative
water content stabilizes, ant] the age at
which stabilization occurs depends on the
species. This concept has been challenged
by various investigators and lately by Shie! :Is
et al. (1983), who could! finch no evidence of
a constant chemical composition in the fat-
free body portion of pigs. The pigs in the
growth study reached a body weight of 150
kg. Consequently, care must be exercised
when the value 73.2 percent is applied in
the young; otherwise, underestimation of
the fat will result.
Determination of Fat from Potassium
FFM can be estimated from potassium
(K) by the following equation:
FFM = Total body K/68.1.
In this equation, FFM is assumed to have
a constant proportion of K throughout life:
68.1 mmol/kg of FFM, a value derived from
cadaver analysis (Kirton and Pearson, 1963).
This assumption, however, does not apply
in all circumstances; evidence has shown
that infants have a lower concentration of
K (Forbes and Hursh, 1963) and that adult
K concentrations may differ between pop-
ulations and ethnic groups (Meneely et al.,
19631.
Total body K has been measured with
the dilution of OK, a radioisotope of K (Corsa
et al., 1950). Alternatively, body K can be
estimated by measuring the naturally oc-
curring radioisotope 40K which constitutes
OCR for page 246
246
approximately 0.012 percent of the natural
K in humans (Forbes, 1962~. The high-
energy gamma ray emitted from 40K can be
measured with highly sensitive, but expen-
sive, whole-body counters. Proper calibra-
tion of this system has permitted quantifi-
cation of the K concentration in the human
body from which FFM, and hence fat, can
be estimated.
Total Body Electrical Conductivity and
Impedance
Total body electrical conductivity (TO-
BEC) and bioelectrical impedance analysis
(BIA) have recently been used to assess
adiposity (Cochran et al., 1986; Harrison
and Van Itallie, 1982; Lukaski et al., 1985;
Segal et al., 1985~. These methods are
cliscussed in greater detail in the paper by
Boileau in this volume. Briefly, these two
techniques use the basic principle that lean
tissue conducts an electrical current better
than fat tissue. Values for FFM obtained by
these techniques compare favorably with
those obtained by other indirect methods,
such as anthropometry, clensitometry, hy-
c3rometry, and total body K (Cochran et al.,
1986; Harrison ant] Van Itallie, 1982; Lu-
kaski et al., 1985; Segal et al., 1985), and
with direct carcass analysis of animals (Fior-
otto et al., 1987~. However, as discussed by
Cohn (1985), data that validate these tech-
niques are incomplete and additional studies
are needed.
Multicompartmentalization of the Body
The above methods to estimate body fat
from FFM assume that the chemical com-
position of FFM is constant, an assumption
that undoubtedly introduces an error whose
boundaries are not well defined`. In contrast,
the recent development of more sophisti-
cated acid complex techniques for the ele-
mental analysis of the belly allows a more
accurate estimate of body fat without such
an assumption. This multicompartmental
APPENDIX
approach (Anderson, 1963; Cohn et al.,
1984, 1985) was first used by Moore et al.
(1963), who objected to using FFM as a
reference standard because it contained a
significant amount of extracellular tissues,
primarily skeleton and extracellular fluid.
Moore suggested that the term "body cell
mass" (BCM), which is a more homogeneous
mass responsible for basal metabolism, re-
place the term FFM. The calculation of
BCM is based on the assumptions that
nearly all K is in the cells, the ratio of K to
nitrogen (N) is constant (3 mmol of K/g of
N), and N is a constant proportion of BCM.
Thus, BCM can be calculated from meas-
ured K multiplied by a coefficient factor of
8.33. The three-compartment approach, as
conceived by Moore and colleagues, clivides
the body mass into fat, BCM, and extracel-
lular tissue (ECT) compartments.
The concept of BCM only recently re-
ceived the attention it deserves. The de-
velopment of total body neutron activation
analysis allows the estimation of extracel-
lular tissues-the solid phase estimated from
body calcium (Ca) and the aqueous phase
from body chlorine (Cat. Bo(ly cell mass can
be estimated from body K by measuring
40K (Cohn et al., 1984, 1985~. Body fat then
can be calculated after estimation of the
BCM and ECT compartments (Cohn et al.,
1984, 1985~. This approach, although theo-
retically superior to the two-compartment
approach, also uses assumptions derived
from cadaver analysis-that is, a constant
proportion of body Ca in the extracellular
solids, a constant ratio of K/N, and a constant
proportion of N in the BCM. Body Ca can
be measured accurately by a well-calibrated
neutron activation system; a relatively small
error is introduced into the final estimation
of fat by assuming a constant proportion of
body Ca in extracellular solids. The potential
error resulting from the use of the K/N ratio
of 3 mmol/g may be substantial (Sheng and
Huggins, 1973~.
Expansion of the three-compartment ap-
proach to the four-compartment approach
OCR for page 247
MEASURING BODY COMPOSITION IN HUMANS
reduces the body into its four elemental
phases: fat, water, protein, and minerals.
An accurate estimate of fat is possible if
water (measurer! by dilution technique),
protein (calculated from body nitrogen
measured by prompt gamma neutron acti-
vation analysis), and body minerals (calcu-
lated from body Ca measurer] by delayed
neutron activation analysis) are accurately
measured (Cohn et al., 1984, 1985~. The
only assumption made is that Ca is a constant
proportion of body minerals. Any error
introduced into the body fat estimation by
this assumption is small because of the small
proportion of minerals in the whole body (4
percent). The predominant clisadvantages
of neutron activation analysis are its com-
plexity, cost, and the radiation exposure,
however minimal, to growing infants and
adults of childbearing age.
The clensitometric method has been ap-
plied recently to the pediatric population
using the four-compartment approach in
which total body water was measured with
a tracer, and the mineral content and the
densities of fat, water, protein, ant! minerals
were obtainer! Tom relevant literature (Sheng
et al., 1984~. The use of literature values
for mineral content and the various densities
to estimate fat appeared to introduce only
a small error. Body volume was measured
using either the pressure-differential method]
(Dell et al., 1987; Taylor et al., 1985) or the
acoustic plethysmograph (Deskins et al.,
1985~.
As discusser! earlier, the overestimation
of TBW by tritium, particularly in the infant,
may introduce error to the four-compart-
ment approach of estimating fat. Recently,
Lewis et al. (1986) reported that TOW in
the infant baboon can be measured accu-
rately by nuclear magnetic resonance (NMR).
NMR's potential for the analysis of body
composition appears promising; also with
NMR imaging, regional distribution of body
fat can be analyzed (Fuller et al., 19851.
Further developments of this technique
may result in the measurement of total body
247
fat without the use of assumptions as in the
compartmental approaches.
REGIONAL FAT MEASUREMENT
Progress has been made in the develop-
ment of methods to measure composition
at various regions of the body. Body fat is
calculated from these measurements using
equations that establish a relationship be-
tween these measurements and body fat
estimated by another indirect method. Such
methods, which will not be discussed in
detail, range from simple anthropometric
measures used primarily for population
studies to sophisticated computerized meth-
ocis in a research setting. For all these
methods, validation has been primarily with
another indirect method; that is, the values
obtained were compared with reported body
fat values in the literature or compared
against values obtained by other indirect
methods performer] on the same individual.
The least expensive and most frequently
used method uses calipers to measure skin-
fold thicknesses at specific sites. Other
methods include soft-tissue radiography
(Garn, 1957) and ultrasonography (Borkan
et al., 1982), both of which use expensive
and nonportable instruments. More re-
cently, infrared interactance has been pro-
posed as a rapid, safe, and noninvasive
method to measure subcutaneous fat in both
research and field settings (Conway et al.,
1984~. Numerous studies have attempted to
validate the extrapolation of subcutaneous
fat thickness measured at a number of sites
on the body to total body fat and to establish
subcutaneous fat thickness as a "standard"
for the assessment of total body fat (Durnin
and Rahaman, 1967~. Although the thickness
of subcutaneous fat is roughly proportional
to the total weight of body fat, body fat
calculated by this method may be inaccurate
and misleading because of the variation
among population norms. Equations are
being developed to overcome this difficulty;
specific formulas for body fat estimation are
OCR for page 248
248
suggested for specific population groups
(Lohman, 1981).
Interest in adapting complex diagnostic
tools to estimate body fat is increasing.
Images depicting fat and muscle of body
regions can be obtained with computerized
axial tomography (Borkan et al., 1983;
Heymsfield en c] Noel, 1981; Sjostrom et al.,
1986), dual-photon absorptiometry (Got-
fredsen et al., 1986; Mazess et al., 1984),
and nuclear magnetic resonance (Fuller et
al., 1985~. Sophisticated software allows to-
tal body fat to be computed from a series
of cross-sectional fat areas along the length
of the body. Although all these techniques
show great potential in the estimation of
body fat, they are expensive and relatively
unavailable for routine measurements. Fur-
thermore, a degree of radiation exposure is
involved with both computerized axial to-
mography and dual-photon absorptiometry
methods.
SUMMARY
Many indirect methods of varying degrees
of complexity are available for estimation of
body fat. Most of the methods have been
validated for predictability and precision
using other indirect methods. The reference
method most commonly used is that based
on densitometry to estimate body fat from
a two-compartment approach (Sheng et al.,
1984~. The accuracy of most of these meth-
ods has been validated only in a few in-
stances by direct carcass analysis. The final
choice of an indirect method ultimately
depends on its cost, the objective of the
experiment, and the physical conditions
under which it is to be used.
ACKNOWLEDGMENTS
This work is a publication of the U.S.
Department of Agriculture/Agricultural Re-
search Service Children's Nutrition Re-
search Center, Department of Pediatrics,
Baylor College of Medicine and Texas Chil
APPENDIX
dren's Hospital, Houston, Texas. This proj-
ect has been partially funded by the U.S.
Department of Agriculture, Agricultural Re
search Service, under Cooperative Agree
ment 58-7MNl-6-100.
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OCR for page 250
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APPENDIX
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OCR for page 251
Utilization of Total Body Electrical
Conductivity in Determining Body Composition
R. A. BOIL~EAU
Assessment of body composition is an
important part of evaluating nutritional sta-
tus, health, and physical fitness. In general,
body composition analysis uses concepts and
measurement techniques that permit par-
titioning of bocly weight into two or more
components. The simplest conceptual mode!
partitions body weight into a fat weight
component ant] a fat-free or lean body
weight component. These are of particular
interest in relation to human nutrition and
health, since obesity is a major health prob-
lem in Western societies, both among chil-
dren (Coates et al., 1982; Ylitalo, 1981) and
adults (Buskirk, 1971; McArdle et al. ,1981~.
Furthermore, body composition analysis has
many applications for the animal scientist,
including nondestructive monitoring of meat
production.
Measurement of human body composi-
tion has been a somewhat perplexing prob-
lem because of the necessity to use nonin-
vasive techniques and the lack of a substantial
data base characterizing the chemical com-
position of the body for validation purposes.
Hence, the status of our understanding of
human body composition has developed
from use of indirect measurement tech
.
251
piques, the conceptual framework of which
is baser] on the chemical analysis of only a
few cadavers. The reference techniques
judged to be most accurate, precise, and
conceptually sound include clensitometrY.
hydrometry, and body potassium (40K) spec-
troscopy. The methodology of these tech-
niques has been reviewed in a number of
reports (Behnke and Wilmore, 1974; Boi-
leau and Lohman, 1977; Boileau et al., 1985;
Keys and Brozek, 1953; Lohman et al.,
1984a).
Other techniques, consiclered to be less
precise but applicable in large population
studies, involve skinfold thickness ant] other
anthropometric measurements. More re-
cent technologies have spawned develop-
ment of several new techniques including
total body neutron activation analysis (Cohn
et al., 1974), computerizes! axial tomography
(Borkan and Hults, 1983), nuclear magnetic
resonance imaging (Lohman, 1984), whole-
body impedance (Nyboer, 1972), and total
bocly electrical conductivity analysis (Har-
rison and Tan Itallie, 19824. The focus of
this report is on total body electrical con-
rluctivity (TOBEC) as a technique for body
composition analysis.
, ~
OCR for page 252
252
BACKGROUND AND MEASUREMENT
PRINCIPLE
Electrical conductivity analysis is a method
of compositional analysis that uses an in-
strument (U.S. Patent 3,735,247, 1973 -
called electronic meat measuring equip-
ment (EMME - to measure the fat and lean
content of live swine (mode} SA-1~. EMME
SA-1 was later mollifier! for measurement
of packaged meat and in vivo measurements
of humans (EMME/TOBEC HA-1. Data
presented in the literature are primarily
based on the EMME/TOBEC HA-1, which
is the prototype of the new TOBEC HA-2.
Application of the electrical conductivity
method to the two-component body com-
position mode} is based on the concept that
the fat-free body (FFB) component conducts
electrical current more readily than the fat
component. This is due to the higher water
and electrolyte content found in the tissues
ant] extracellular water making up the FFB.
Electrical conductivity of various biological
materials indicates that constituents asso-
ciated with the FFB (for example, muscle,
liver, and blood) have conductivity values
of about 4 mmho-cm versus conductivity
values for fat of about 0.3 mmho-cm in the
2.5- to 5.0-MHz range, an FFB/fat ratio of
about 13 (GecIdes and Baker, 1968; Pethig,
1979~. Van Itallie et al. (1985) have sug-
gested that the FFB/fat conductivity ratio
may be as high as 20 to 1.
Current flow inclucec] in a biological sys-
tem is a Unction of conductive and dielectric
properties. The conductive properties are
related to the intra- and extracellular ionic
content, and the dielectric effect is associ-
ated primarily with capacitance relater} to
cell membranes. Impedance to current flow
in the system results in an irreversible loss
of energy as heat. This energy loss is related
to the conductive mass. The dielectric or
capacitance properties of current flow in a
biological system must also be considered;
these represent the reactive part of imped-
ance in which energy transfer is reversible
APPENDIX
due to temporary storage of electrical en-
ergy. Capacitance is partly determiners by
the geometry of the conductor, which may
produce an effect whereby capacitance in-
creases as cross-sectional area, length, or
both increase. While theoretically both elec-
trical properties define the flow of current
in a conductive mass, the conductive prop-
erties appear to exert a more dominant
effect in estimating FFB mass. A detailed!
treatment of the electrical properties of
biological tissues can be fount] in Pethig
(1979~.
There are two basic bioe~ectrical tech
~ . ~. . . . .
niques used to measure whole-bo(ly con-
~luctivity for body composition assessment:
(1) direct injection of current and (2) non-
contact total holly electrical conductivity
(TOBEC). This discussion focuses on TO-
BEC. In this technique current applied to
a coil induces an electromagnetic field in
which the body is statically situated (HA-1)
or scanned (HA-2~. The conducting mass
(subject) passing through the electromag-
netic field of the cod! absorbs heat energy,
thereby perturbing the electrical field of
the coil. The loss of energy cletected in the
cod! is an index of the concluctive mass of
the body. The power dissipated in the
subject at any one time is less than 1 ,uW/
cm2-less than 1/lOOth of the standard set
by the American National Standards Insti-
tute for human exposure. The oscillating
current frequency applied to the coil is an
important aspect of the measurement, since
the degree of separation in the conductivi-
ties of FFB and fat has been shown to be
frequency dependent (Pethig, 1979~. The
first TOBEC model (HA-1) used 5-MHz
oscillating cod] current and required a 0.5-
second measurement on the statically situ-
atec3 subject (Harrison and Van Itallie, 19821.
The new TOBEC instrument (HA-2) is a
scanning crevice in which the subject moves
on a motor-driven sled through a 2.5-MHz
cod] electromagnetic field at a constant rate.
It requires about 40 seconds for one meas-
urement, during which conductivity is
OCR for page 267
LIVE ANIMAL AND CARCASS COMPOSITION
from those obtained by analysis of the
slaughtered animals by 0.8 to 1.7 kg in
seven ewes containing 5.2 to 21.4 kg of fat.
The standard! deviation was + 1.2 percent-
age units.
The D2O dilution method has no com-
mercial application and is used on a limited
basis as a research tool to estimate body
composition. It is relatively simple for sci-
entists to use but too complex for industrial
application. It is a good way to estimate
total body water but is limited in its level
of accuracy for total body fat.
Application of a kinetic technique to solve
an anatomic problem with no clocumenta-
tion of the congruity of the kinetic and
anatomic models is clearly limited. Simu-
lation analysis indicates that the kinetic
mode! is very insensitive to changes in
anatomic pool sizes, but very sensitive to
changes in exchange rates of water among
pools (R. W. Russell and R. B. Reed,
personal communication, 1986~.
COMPUTERIZED TOMOGRAPHY
The Nobel Prize was awarder! to A. M.
Cormack and G. N. Houndsfield for the
development of the computerized tomog-
raphy (CT) technique. The concept is based
on presentation of anatomic areas of the
body by computer] synthesis of an image
from x-ray transmission data obtained in
many different directions through the plane
under consideration (Cormack, 1980;
Houndsfield, 1980~. An x-ray tube rotates
around] an object, and the computer recon-
structs from a series of pictures a slide
through the object. By this technique, the
density (CT number) of different body tis-
sues at different distances from the x-ray
tube can be calculated.
One of the first applications of this tech-
nique for estimating composition of meat-
producing animals was reported by Skjer-
vold (1982) from the Agricultural University
of Norway. Their study of 23 pigs indicated
that it was possible to obtain a good predic
267
lion of the body composition on the basis
of the relative CT distribution from one
tomographic plane. Skjervold also reported
the CT numbers of different body tissues.
Lung tissue had values of-200 to -100;
fat tissue, - 100 to 0; muscle tissue, +30
to + 100; and bone, + 400 to + 500.
Allen and Vangen (1984) used comput-
erizec! tomography to estimate the body
composition of 207 pigs ranging in weight
from 59 to 120 kg. The values they obtained
are similar to those reported by Skjervold
(1982).
European researchers have been active
in evaluating CT for use in estimating body
composition of meat-proclucing animals, but
only limited research is being clone in the
United States. Researchers at the Meat and
Animal Science Department of the Univer-
sity of Wisconsin are cooperating with med-
ical college faculty ant! are currently col-
lecting data from pigs.
The main drawbacks to computerized
tomography are expense, the time required
to obtain an estimate, and the necessity to
anesthetize the animal before scanning. Even
with these limitations, however, improved
techniques are expected that will make
computerized tomography acceptable for
scanning animals for genetic selection of
breeding stock. This method therefore has
great potential for future use in the livestock
industry.
NUCLEAR MAGNETIC RESONANCE
IMAGING
The nuclear magnetic resonance (NMR)
method for estimating belly composition is
based on a strong static magnetic field and
pulsed radio waves that in(luce resonance
of protons in the measured body. The signals
emitted are a reaction of the body to the
high-frequency disturbance. Therefore, they
are a product of the matter itself, with
intensities (lepending on the proton spin
densities and molecular structures. The NMR
signal does not continue indefinitely. En
OCR for page 268
268
vironmental influences cause the individual
flipped magnetic moments to get out of
phase and return to the orientation they
had before the radio frequency pulse was
applied. The time requires] to reestablish
original conditions has been defined as spin
lattice relaxation time T1 ant] spin-spin
relaxation time T2. Procedures to determine
T1 are known as inversion recovery, and for
T2 as spin-echo methods. Both systems
produce a data matrix of the size 12~128
or 25~256 that contains in x-ray CT the
normalizer] Houncisfield units ranging from
-1,000 (air) to more than 1,400 (compact
bone). There are several ways to produce
images. On a cievice with seven colors, the
total data space in a matrix is subdivided
into seven regions, with each region rep
resenting a different color. Fat, muscle,
bone, and connective tissue are always pre
sented if the total data space is mapped onto
seven colors (Groenevelc] et al., 1984~.
Fuller et al. (1984) user] the Aberdeen SOLUBLE SHORT-LIVED
r RADIOACTIVE GAS TRACERS
NMR Imaging machine to evaluate pigs for
body composition. Only three pigs were
evaluated. Images were obtained at nine
sites along the body, three each of the
shoulder, midback, ant! rump. Good images
reportedly were obtained of the muscle, fat,
ant! bone portions of the sites scanned.
Nuclear magnetic resonance imaging has
great potential, but very limited data are
available on its usefulness for predicting
body composition traits of meat-producing
animals. The equipment is very expensive,
and the method is very complex; its future
will depend on the amount of resources
available for its development as an agricul
tural tool.
APPENDIX
the NIR method. Reaclings were taken at
specific sites on the ham, shoulder, and side
of the pig. Carcass composition was deter-
mined by analysis ofthe soft tissue dissected
from the eviscerates] carcass for lipid, pro-
tein, and water content. Multiterm regres-
sion correlations were generates] for carcass
fat as a percentage of live body weight. For
the carcass, percent fat correlated best with
NIR readings taken on the ham. The meas-
urements taken from the carcass accounted
for about 50 to 60 percent of the variation.
The values for the live pig were lower.
These relationships indicate that with re-
finement in instrumentation and technique.
this method may be useful in predicting
body composition. It is simple, ant] the
equipment is not Drohibitivelv expensive.
More research is needed, however, before
NIR can be consiclered for commercial use.
, . , ,¢ .
~. ~
NEAR-INFRARED REFLECTANCE
Near-infrared reflectance (NIR) is widely
used to predict the composition of various
plant materials and may have potential ap-
plication for estimating carcass composition.
Mitchell et al. (1986) used 20 pigs for each
weight group of SO, 60, and 90 kg to evaluate
A range of halogenated gases with a par-
ticular affinity for adipose tissue could be
considered for predictors of body composi-
tion. The commonly used anesthetic halo-
thane (2-bromo-2-chloro-1, 1,1-trifluoro-
ethans) is an example. The label can be 11C,
OF, 77Br, or 38C1. This idea was reported
by Ettinger et al. (1984), who suggested
that an animal can be given labeled halo-
genated gases in concentrations small enough
to have no noticeable anesthetic effect but
large enough that the gases are taken up by
the adipose tissue. The amount taken up
could be measured by a conventional whole-
body counter or a whole-body scanner. The
hypothesis has not been tested, but a good
theoretical basis exists for the concept.
SUMMARY OF THE PRACTICABILITY/
COST-BENEFIT COMPARISON OF
BODY COMPOSITION MEASURES
More than 30 techniques for estimating
live animal or carcass composition were
OCR for page 269
LIVE ANIMAL AND CARCASS COMPOSITION
reported in this review. The cost of the
equipment to measure belly composition
can range from 1 dollar to over 1 million
dollars. Accuracy, precision, ant] practicality
are also considerations. Many promising
techniques have been rejected for commer-
cial use, not because of costs but because
of practicability.
One of the least costly techniques avail-
able for estimating fat thickness in cattle
and swine is the ruler back fat probe. Its
accuracy is as high as the best ultrasonic
techniques ant] almost as high as the com-
puterized tomography methods recently cle-
veloped for meat-proclucing animals. The
cost of a ruler probe can range from 1 to 50
dollars, but still, the device is not used
extensively in the meat industry because
personnel are concerned about its practi-
cality. (This concern is not really valid; a
trained person can probe an incliviclual pig
or steer in less than a minute when the
animal is restrainecI.)
The scientific community must under-
stanc] that most producers and buyers of
livestock in the United States prefer the
use of live weight ant! visual assessment
methods for estimating belly composition
because of their practicality, low cost, and
rapidity in making the measurements. This
must reflect the limited interest of the U. S.
livestock industry in reducing fat in meat-
proclucing animals by objective methods.
One reason for this is the small margin paid
by the packing industry for trim, well-
musclecT animals versus fat, less muscular
ones. We need] an improved marketing
program that will pay farmers for producing
trim, muscular animals. A system of this
type will encourage the use of more objec-
tive methods for the selection of breeding
animals and the marketing of animals for
meat production.
From the research standpoint, many tech-
niques are available to estimate body com-
position, but their accuracies are not out-
stancling. Most can account for 60 to 80
percent of the variation in muscle, fat, or
269
bone of the carcass. Thus, more accurate
methods are needed for researchers working
in the body composition field. Based on
recent literature, it may be possible to
improve accuracy with such new methods
as computerized tomography and nuclear
magnetic resonance imaging. The cost of
the equipment currently prevents their
widespread use, but with further research
on new methods, we may, in the near future,
develop the ultimate technique- one that
is cost-effective, simple, and accurate.
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APPENDIX
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OCR for page 271
LIVE ANIMAL AND CARCASS COMPOSITION
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APPENDIX
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OCR for page 273
Altering Carcass Measurements and
Composition of the Pig
V. C. SPEER
GENETICS AND SELECTION
The pig and fat are closely linked in the
mind of the consumer, much to the detri-
ment of the pig. But in reality the amount
ant] type of fat in a pig carcass is quite
similar to that in other red meat animals. A
dramatic change in carcass measurements
and composition has come about with the
development of the modern lean-type pig.
The lean-type pig utilizes and deposits pro-
tein more efficiently than a fat-type pig,
yielding a carcass with more lean tissue.
This change came about through selection
(genetics) during the period 195S to 1970.
In the late 1960s, the incidence of sudden
pig death, or Porcine Stress Syndrome (PSS),
became an acute problem among heavily
muscles! pigs developed through genetic
improvement (Cassens et al., 1972~. The
improvement in muscling since 1970 has
cleclinec3 for the barrows submitted to the
Iowa Swine Testing Station (Evans, 1986),
largely because of the association of PSS
with heavy muscling.
SEX
At typical slaughter weights for pigs, the
intact male (boar) yields a carcass with the
least fat and most lean, followed by the
female (gilt). The castrated male (barrow)
yields a carcass with the most fat ant] least
lean. During the growth phase and until
male aggressiveness (ranting) develops, the
boar will gain weight the most rapidly and
most efficiently. There is the potential prob-
lem of strong odor or flavor in the meat
from boars slaughterer] at typical market
weights in the Uniter! States. Carcasses from
boars are readily accepted! in some other
countries (for example, Australia and Eng-
land) but are slaughtered at light weights
to reduce the possibility of boar taint in the
carcass.
W EIGHT
Beyonc! a live weight of about 90 kg, the
rate of lean tissue deposition reaches a
plateau and, in many pigs, actually declines
as fat deposition increases. Furthermore,
(laity gain seems to decline slightly, although
the daily feed requirement increases.
NUTRITION
Protein
Ashton et al. (1955) and Jensen et al.
(1955) reporter] that an increase in protein
273
OCR for page 274
274
levels in corn-soybean meal diets produced
only minimal carcass responses in fat-type
pigs. Genetically improved pigs fed similar
corn-soybean meal diets 10 years later (Iohn
son, 1965) were more responsive to protein
level: An increase in dietary protein level
yielded a greater reduction in back fat depth
and a greater increase in ham and loin
percent. Responsiveness to protein level as
related to type (fat versus lean) is evident
in the U.S. Department of Agriculture's
selection study, reported by Davey and
Morgan (1969~.
Amino Acids
In the typical protein level study, the
levels and ratios of the essential amino acids
change, so it is difficult to determine whether
responses are due to protein level or to one
or more amino acids. In the typical corn
soybean meal diet, lysine is the first limiting
amino acid as protein level is reduced.
Carcass measurements improved in re
sponse to increases in lysine levels when all
other diet components were held constant
(Asche et al., 1985~.
Energy
Diet density (energy level) will affect
carcass measurements of pigs that are fed corn.
act libitum. Feecling pigs a diet with acldecl
fat (3,600 keel of metabolizable energy tME]/
kg) versus a corn-soybean meal diet (3,100
keel of ME/kg) reduced the feed require
ment but increased the back fat measure
ment (Wagner et al., 1963~.
Calorie/Protein Ratio
Increasing the energy content of a corn
soybean meal diet by adding fat may depress
daily gain, and feet! efficiency may not
improve as much as expected. Carcass meas
urements are also adversely affected. To
counteract these adverse performance and
APPENDIX
carcass criteria, the diet can be formulated
to contain a constant calorie/protein ratio.
Daily gain and feed efficiency were shown
by Allee et al. (1976) to improve markedly
when the protein level in the diet was
adjuster! proportionately to the energy level.
Carcass back fat, however, increased com-
parecl with those pigs given a control diet
(1.35 versus 1.22 inches). Generally, diets
are formulated to constant calorie/protein
ratios using metabolizable energy values for
the ingredients. Because fat has a propor-
tionately lower heat increment than normal
energy sources such as grain, its energy
value is underestimated. Perhaps if diets
were formulated to contain constant calorie/
protein ratios using net energy values for
the ingredients when fat is included in the
flirt, the adverse effects on carcass meas-
urements would be corrected.
Grain Source
The two most commonly fed grain sources
for pigs are corn and barley. Corn is better
than barley in terms of performance criteria,
but barley is superior to corn with regard
to carcass measurements (Greer et al., 1965~.
Much, if not all, of the positive carcass
response to barley is related to its lower
energy composition compared with that of
RESTRICTED FEED INTAKE
Reducing the feed intake of growing-
finishing pigs will improve carcass measure-
meets (Braude, 1972; Greer et al., 1965;
Speer, 1966~. Restricted or controlled feed-
ing is commonly practiced in pig production
in Europe, but because daily gain is reduced
it has not been adopted by U. S. producers.
The improved feed efficiency reported by
Braude (1972) in response to restricted feed-
ing compared to ad libitum feeding was not
evident from the studies of Speer (1966) and
Greer et al. (1965~.
OCR for page 275
ALTERING CARCASS MEASUREMENTS
TEMPERATURE
At environmental temperatures higher
than ideal, the pig reduces feed intake and
expends energy in an attempt to stay cool.
The result is an adverse effect on production
criteria but an improvement in carcass meas-
urements (Stahly and Cromwell, 1979~. At
lower environmental temperatures, the pig
increases feed intake and once again ex-
pends energy to maintain body tempera-
ture. With respect to carcass measurements,
the increased energy expenditure to main-
tain body temperature may counteract the
effect of increased feed intake.
HORMONES AND RELATED
COMPOUNDS
Diethyistilbestrol
Plimpton and Teague (1972) implanted
diethylstilbestrol in boars weighing 70 kg
and then slaughtered them at about 110 kg
live weight. This procedure retained the
positive carcass attributes of the young boar,
while the effects of objectionable odor and
flavor of boar meat were reduced.
Diethylstilbestrol and
Methy~testosterone
A combination of diethyistilbestro! (2.2
mg/kg of diet) and methy~testosterone (2.2
mg/kg of diet) added to the feed improved
the feed efficiency and carcass measure-
ments of growing-finishing pigs (Baker et
al., 1967~. This product was never approved
by the Food and Drug Administration for
use in the United States, but it was approved
and marketed in Great Britain.
Epinephrine and Epinephrine-Like
Stimulators
Cunningham et al. (1963) user] epineph-
rine to increase fat mobilization, lipolysis,
275
and nitrogen deposition in the pig. Results
were encouraging, but the required daily
injection was a distinct disadvantage. In
subsequent studies, Cunningham and Friend
(1964) and Cunningham (1968) added ni-
cotine or caffeine to the feed in an attempt
to stimulate epinephrine-like responses. Both
compounds seemed to improve carcass
measurements of growing-finishing pigs.
Similarly, the action of the beta-adrenergic
agonists clenbuterol and cimaterol Jones et
al., 1985; Moser et al., 1984) improved
carcass measurements in growing-finishing
pigs. The beta-adrenergic agonists are orally
active, making them easier to use than
eplnep. crane.
Growth Hormone
Daily injections of porcine growth hor-
mone by Machlin (1972) have been shown
to improve daily gain, feed efficiency, and
carcass measurements. Chung et al. (1985)
used a porcine preparation that was more
highly purified than Machlin's and found
similar responses in growing pigs, but at a
much lower dosage rate. Bacterially synthe-
sized human growth hormone is also active
in stimulating growth rate and carcass im-
provement (Baile et al., 1983). Both the
natural and bacterially synthesized hor-
mones must be administered by daily in-
jections, which is a distinct disadvantage for
IMMUNOLOGY
Immunization of growing boars against
androstene steroids the compounds re-
sponsible for boar taint and odor-controls
these undesirable characteristics without
significantly affecting other characteristics
such as weight gain and feed efficiency
(Brooks et al., 1986; Williamson et al., 1985~.
A reduction in an(lrostene steroids might
also be attained through selection, since
Booth et al. (1986) detected positive cor
OCR for page 276
276
relations between the bulbourethral and
submaxillary gland weights and concentra-
tions of 3-cx-androstenol and 5-~-androsten-
one in market weight boars.
Encouraging results with immunology have
been obtained in lambs by autoimmunizing
the lambs against somatostatin (Spencer and
Garssen, 1983~. Somatomedin concentra-
tion increased (nonsignificant) and growth
rate improved compared with control lambs.
A similar approach has been reported by
Flint and Futter (1986), in which rats im-
munized against their fat cells were found
upon postmortem examination to have about
30 percent less carcass fat than untreated
rats.
TISSUE COMPOSITION
The type of dietary fat fed to the pig will
influence the fat composition of the carcass.
The percentage of unsaturated fat in back
fat samples reflects the type of oil fed (Ellis
and Isbell, 1926~. Changing carcass fat com-
position can be accomplished more readily
in the pig than in any other large farm
animal.
The amount and type of fat found in the
lean tissue of the pig longissimus dorsi
muscle will respond to differences in diet
and management. Restricted feeding re-
duces the fat content and the level of un-
saturation in the muscle, as does feeding
barley instead of corn (Greer et al., 1965~.
And increasing the protein level or reducing
the energy concentration of the diet reduces
the fat content of the longissimus dorsi lean
tissue (Wagner et al., 1963~. From these
examples, it seems that carcass back fat and
the fat content of lean tissue are positively
correlated. If this is true, then as producers
in the United States strive for leaner ani-
mals, they could encounter some of the
problems that have surfaced in England.
According to a technical report of the Meat
and Livestock Commission of the United
Kingdom (Phelps, 1985), the marked re-
duction in back fat that has occurred in
APPENDIX
England's pig population has been accom-
panied by an increase in retailer and con
sumer complaints that the very lean car
casses produce meat that looks unattractive,
lacks succulence and flavor, and has a ten
dency to be tough.
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Representative terms from entire chapter:
body fat
