18 Introduction to Human Nutrition
Research to date has generally shown good agreement
between underwater weighing and air displacement.
Air displacement is better accepted by the volunteers,
but some experience diffi culties because of the
breathing pattern to be followed or because of
claustrophobia.
Dilution techniques
Carcass analyses revealed that the amount of water in
the FFM is relatively constant at about 73%. Total
body water (TBW) can be determined by dilution
techniques. Dilution techniques are generally based
on the equation:
C 1 × V 1 = C 2 × V 2 = Constantwhere C is the tracer (deuterium oxide, tritium, or
(^18) O water) concentration and V is the volume.
When a subject is given a known amount of a tracer
(C 1 × V 1 ), which is known to be diluted in a given
body compartment, the volume of that body com-
partment can be calculated from the dose given and
the concentration of the tracer in that compartment
after equilibrium has been reached. Suitable tracers
for the determination of TBW are deuterium oxide,
tritium oxide, and^18 O-labeled water. Other tracers
can also be used, such as alcohol and urea, but they
are less suitable because they are partly metabolized
(alcohol) or because they are actively excreted from
the body (urea) during the dilution period. After
giving a subject the tracer and allowing around 3–5
hours for equal distribution throughout the body,
determination of the concentration of deuterium in
blood, saliva, or urine allows the calculation of TBW
(Box 2.5).
Alternatively, other tracers can be used, such as
tritium oxide and^18 O-labeled water, and the tracer
can be given intravenously, which is advantageous
when the subject has gastrointestinal disorders. The
reproducibility of the method is 1–3%, depending on
the tracer used and the analytical method chosen.
From TBW, the FFM, and hence fat mass, can be
calculated, assuming that 73% of the FFM is water:
BF% = 100 × (Weight − TBW/0.73)/Weight
The precision for estimations of body fat is about
3–4% of body weight. As with the densitometric
method, this error is due to violations of the assump-
tion used (i.e., that the relative amount of water in the
FFM is constant and equals 73% of the FFM). In sub-
jects with a larger than 73% water content in the FFM
(pregnant women, morbid obese subjects, and patients
with edema), the factor 0.73 will result in an overesti-
mation of the FFM. A three-compartment model of
the body that contains fat mass, water, and dry FFM
has a lower bias than a two-compartment model.
An overestimation of body fat by densitometry, for
example because of a relatively high amount of water
in the FFM, will be counteracted by an underestima-
tion using the dilution method (see also Box 2.6).
The use of tracers that do not cross the cell mem-
brane enables the determination of extracellular
Box 2.5
A person with a body weight of 75 kg is given an exactly weighed
dose of 15 g deuterium oxide. This deuterium oxide is allowed to
be equally distributed in the body water compartment for about
3–5 hours. Then, blood is taken and the deuterium concentration
in the sample is determined. Assuming the plasma level to
be 370 mg/kg, the “deuterium space” can be calculated as
15 000/370 = 40.5 kg. As deuterium exchanges in the body with
hydroxyl groups from other molecules, the deuterium space has to
be corrected for this nonaqueous dilution (4–5%). Thus, total body
water is 0.95 × 15 000/370 = 38.5 kg. Assuming a hydration of
the fat-free mass of 73%, the body fat percentage of this 75 kg
weight subject would be: 100 × [75 − (38.5/0.73)/75] = 29.7%.
Box 2.6
70 71 72 73 74 75
7 6 5 4 3 2 1 0
-1
Percent water in FFM
Bias in body fat percent
For the computation of body composition from dual-energy X-ray
absorptiometry, especially body fat and lean tissue, several
assumptions are made, one of which is a constant hydration of the
fat-free mass (FFM). The fi gure shows that the bias in calculated
body fat percentage depends on the hydration of the FFM.
Reference is a four-compartment model.