Body Composition 19water (ECW). Commonly used tracers in this respect
are bromide salts or sodium-24. Intracellular water
(ICW) cannot be determined directly and is calcu-
lated as the difference between TBW and ECW.
Total body potassium
Chemical carcass analysis has revealed that the amount
of potassium in the fat-free body is relatively con-
stant, although the amount of potassium in
different tissues varies widely. The determination
of total body potassium (TBK) is relatively easy, owing
to the natural occurrence of three potassium isotopes
(^39 K,^40 K, and^41 K), in constant relative amounts, of
which^40 K is radioactive (gamma emission). Counting
the emission of the gamma rays from the body reveals
the amount of radioactive potassium, from which
TBK and hence FFM can be calculated. The chamber
in which the subject is scanned has to be carefully
shielded to avoid any background radiation (cosmic
radiation). The scanning of the body for potassium
lasts for 20–30 min and the reproducibility is 2–3%.
Several authors have shown that the amount of
potassium in the FFM is different between males and
females, is lower in obese subjects, and is probably
also age dependent. Thus, TBK is much more useful
as a measure of body cell mass (BCM) than as a
measure of FFM. However, this discrepancy can be
used to calculate the “quality” of FFM, defi ned as the
ratio of cellular to extracellular components of FFM,
or operationally as BCM/FFM. Thus, when TBK is
used to assess BCM, and another method such as
hydrodensitometry or DXA is used to assess FFM
independently, it can be shown that the quality of
FFM declines with age, along with the quantity (Figure
2.2). When potassium values are used to calculate
intracellular water, BCM, or FFM, assuming constant
amounts of potassium in these body components, the
same errors can occur as with densitometry and dilu-
tion techniques.
Although the technique is easy to apply in patients,
the high cost of the scanning instrumentation limits
its use other than in research settings.
Dual-energy X-ray absorptiometry
During DXA (also known as DEXA), the body or part
of the body is scanned with X-rays of two distinct
levels of energy. The attenuation of the tissues for the
two different levels of radiation depends on its chemi-
cal composition and is detected by photocells. The
instrument’s software generates a two-dimensional
picture of the body or the body compartment under
study. The software can calculate several body com-
ponents: bone mineral content and bone mineral
density, lean mass, and adipose tissue fat mass. These
calculations are possible for each of the body parts,
e.g., for legs, trunk, spine, femur, and arms. However,
the method cannot distinguish between subcutane-
ous adipose tissue and discrete adipose tissue sites
such as perirenal adipose tissue. The reproducibility
of DXA is very high, varying from about 0.5% for
bone mineral density to about 2% for total body com-
position. The reproducibility for regional body com-
position is less. The method is quick and easy to
perform and places very few demands on the subject.
The radiation dose (0.02 mSv) is only a fraction of the
radiation dose of a normal chest radiograph, and
hardly higher than the normal background. Apart
from repeated scanning, the radiation dose should
not be a limiting factor in terms of volunteers being
exposed to hazardous levels of radiation. A disadvan-
tage of the method is that the attenuation of the X-
rays depends on the thickness of the tissue. Therefore,
correction for the body size has to be made. Compared
with traditional methods, DXA scanning is easy and
widely available which, in turn, leads to predictionTBK:FFM321
25 45 65 85WomenMenAge (years)
Figure 2.2 Difference in total body potassium (TBK) content of the
fat-free mass (FFM) between men and women and the relationship
with age.