|
Introduction
Before we get to the practical application, let’s
discuss some concepts. Remember that, like most medical technology and practice,
the use of BIA is evolving. If you understand some of the concepts behind such
testing, you will be better able to keep up with the changes in technology,
equations, and clinical applications of body composition evaluation through
single-frequency tetrapolar BIA testing.
Interest in body composition evaluation may
originate in the desire to have a more precise view of how well the body can
function (including physical performance),[1]
its nutritional status, and to overcome limitations of evaluating more general
measures, such as weight and body mass index (BMI).
| Body weight can vary as much as 0.1 kg per day.
Variations in body weight of more than 0.5 kg per day suggests an imbalance
in water or energy, or both. Starvation can result in weight losses of up to
0.4 kg per day, allowing for survival down to 70% of desired weight.[2]
Slower semi starvation allows a patient to survive to 55-50% of their
desirable weight. A minimum survivable weight ranges from 48%-55% of desired
body weight, or a BMI range of 13-15. Once weight loss exceeds 10% of
baseline in less than six months, several organ systems are likely to be
affected. 0% weight loss over the same period is likely to produce
alterations in body functions in almost every human being. In addition
to soft tissue loss, bone loss is a feature of weight loss. Early
osteoporosis becomes a concern with repeated episodes of weight loss, as
seen in “yo-yo” dieters. |
 |
|
On the other end of the spectrum, obesity alters
body composition by not only adding fat mass to the body, but also muscle, bone,
and extracellular water (ECW). Muscle and bone mass experience “compensated
increases” in response to the required effort and force to carry additional
weight during early obesity. If this phenomenon is followed, as it often is, by
immobility, compensated muscle gain may be lost. The result is a condition
referred to as “sarcopenic obesity” which may be viewed as a “muscle wasted, but
overfat” condition.
Although extremes in BMI, less than 18 and more
than 25-27, seem to correlate with health problems, it is difficult to predict
volume of fat and lean tissues in patients who do not fit the profile of normal,
healthy people. Body mass index has been described as “imprecise, non-linear,
and biased by age, especially in women.”[3]
Body composition evaluation beyond weight has been
correlated with morbidity and mortality.[8]
Interest in body composition during disease or injury is focused on lean tissues
or fat-free mass of the body.[9]
Lean mass accounts for bone, skeletal muscle, visceral organs, extracellular
fluids, and other tissues. Lean tissues are responsible for most of the
important metabolic processes that sustain life and function and provide the
bulk of support for processes related to the body during injury or disease.
| weight loss does not adequately
describe available stores Although unintentional weight
loss is an excellent indicator that something is amiss, it may not
adequately describe the available energy and nitrogen stores that are
important to preserve and are the basis for many clinical decisions.[4]
Weight is subject to error in patients with significant injury or acute
or chronic disease. |
| Alterations in fluid status (accumulation of fluid)
during loss of skeletal muscle protein and adipose tissue, and even the
slow rate of organ atrophy can confound the use of weight as a linear
model for body composition change.[5]
Unable to reflect non-linear deterioration, such as fluid shifts (edema
or ascites), tumor growth, organomegaly, transient glycogen losses, or
sarcopenic obesity, even
weight change is an
inadequate measure
to evaluate changes in body composition
and clinical status.[6]
[7] |
Thus, we look to other methods to differentiate
body compartments according to properties or functions in the physically normal,
healthy person as the baseline for understanding the value of body composition
evaluation. The “hydrostatic” or underwater weighing
process is used to determine the difference between body weight and
underwater body weight to establish body density.
Lean tissues are denser than
fat. Therefore, the more a person weighs underwater compared to “land” measures
of body weight, the more fat-free mass they are likely to have.
The greater the
difference in weights, the more fat mass the individual may have. Fat tissue is
less dense (0.901 grams/cc) compared to bone tissue (3.0 grams/cc), water (1.0
grams/cc), and other fat-free tissues. The average density of FFM is
approximated at 1.1 grams/cc. FFM density may be estimated as lower in persons
who are obese or experience overhydration. Conversely, FFM density is higher in
dehydration and can cause an underestimation of fat tissues. However, actual
density of FFM may vary according to age, gender, and race and can even vary
widely between individuals of similar ages, gender, and race. Additionally,
hydrostatic weight may overestimate fat in children under 10 years old and women
over 60 years old because of reduced bone density. Even gold standards have
their limitations! Anthropometry beyond BMI,
including fat-folds and circumferences, helps to describe body
dimensions and to estimate body fat and lean.[10]
[11]
In fact, anthropometry, done well and to exacting standards, may be considered a
type of criterion or “gold standard” measure, and correlates well with
hydrostatic weight for body density.[12][13]
[14]
Anthropometric measures have been used in regression equations validated through
hydrodensitometry (hydrostatic or underwater weight) to predict levels of fat in
the body. If done well, anthropometry can predict body density (r>/= 0.80).
|
For more information on anthropometric
measurement methods, take the free
Anthropometry
course on this site. |
Fat patterns
(such as those identified by waist-to-hip ratio, waist circumferences, and
abdominal fat-fold measures) can help to identify health risks of hypertension,
hypercholesterolemia, diabetes, cardiovascular disease, and others.[15]
[16]
[17]
Anthropometry, too, has its limitations.[18]
For instance, based on the assumption that most of the body’s fat is
subcutaneous, alterations produced by disease, genetics, and other variations
from the reference healthy, lean population will limit the usefulness of such
measures in short-term evaluations. There may be marked differences from the
“norms” in fat distribution both subcutaneously and internal/external fat
compartments as well as differences in landmark response to lost and gained
weight.[19]
In these instances, anthropometry is helpful to characterize an individual’s fat
patterning, additional information on health risks associated with obesity,[20]
and monitor changes over times by conducting trending measurements.
Estimations of fat-free mass can be made through
dual x-ray absorptiometry (DXA) testing.[21]
|
Fat
free mass testing can be accomplished through a variety of methods,
including
|
 |
anthropometry |
|
|
densitometry |
|
|
total body potassium counts (for body
cell mass) |
|
|
isotope dilution (for total body water) |
|
|
electromagnetic scanning |
|
|
dual x-ray absorptiometry |
Muscle mass may be determined by
|
|
anthropometry |
|
|
urinary creatinine or 3-methylhistidine
excretion |
|
|
dual x-ray absorptiometry |
|
|
magnetic resonance imaging |
|
|
computerized tomography |
|
|
bioelectrical impedance analysis |
|
Originally used primarily to evaluate bone density,
DXA measures can quantify and describe regional distribution of denser lean
tissues compared to less-dense fat tissues.[22]
Thus, with regional evaluation, visceral and subcutaneous fat compartments can
be estimated. Also, bone tissues can be differentiated from soft lean tissues,
providing a clue about an individual’s risk for disease and ability to sustain
soft lean tissue functions.
Total body nitrogen
estimation can provide information about the protein content of the
body. A combination of body weight, height, and age has been used to predict
normal body potassium, water, and fat in humans.[23]
This measure assumes a constant proportion of body nitrogen exists between
intracellular and extracellular spaces. However, because this ratio can change
during disease states,[24]
more direct measures of body potassium content may be a more valid estimation of
protein stores.[25]
Potassium counting (total body potassium or TBK+) is considered the best
reflection of the metabolic tissues, contained in the sub-compartment of
fat-free mass called body cell mass (BCM), because it is estimated that 97% of
this element is contained in
intracellular spaces.[26]
[27]
| |
Before we
continue, it is important that we take a few minutes and explain the different
body compartments and their composition. Please follow the link below for a
short explanation of body compartments. |
|
|
|
