Introduction to Human Nutrition

20 Introduction to Human Nutrition

equations for body composition based on DXA.
However, as with other methods, DXA relies on
certain assumptions (Box 2.6) and there are many
publications showing that the error in body composi-
tion measurements using DXA can be considerable
(Figure 2.3). Moreover, identical machines, even
using the same software versions, can give different
results in scanning the same person.

Multicompartment models

Two-compartment models, consisting of fat mass
and FFM, lack validity in many situations where the
composition of the body is “abnormal.” Examples
already mentioned are pregnancy, morbid obesity,
and the elderly. A combination of techniques often
results in more valid estimates, as is the case when,
for example, body density and body water are com-
bined. In this particular case, the body is divided into
three compartments:

Body weight = Fat mass + Body water

• Dry fat-free mass
  In this three-compartment model the variation of
  the water content in the FFM is accounted for. There
  are fewer assumptions in this model, leading to more
  valid results. Modern techniques such as DXA enable
  the valid and precise measurement of bone mineral,
  from which total body mineral can be estimated.
  When the mineral content of the body is com-
  bined with body density and body water, a four-
  compartment model of the body is generated:

Body weight = Fat mass + Wa t e r

+ Minerals + Protein

In this model, most of the variation in the amounts
of the chemical components is accounted for, result-
ing in a very reliable body composition measure (Box
2.7). Four-compartment models can also be obtained
using other techniques. For example, the measure-
ment of calcium, phosphorus, and nitrogen with
IVNAA in combination with TBW provides informa-
tion for a model consisting of fat, minerals, protein,
and water. In the literature, models based on six
compartments are also described. However, they do
not provide much additional information and the
increased technical error negates the methodological
advantage.
More-compartment models enable the best possi-
ble estimate of body composition for populations as

Figure 2.3 Dual-energy X-ray absorptiometer (DXA) scan using a

HOLOGIC whole-body DXA (QDR-4500). Subcutaneous body fat, bone,

and muscle are distinguished by different colors.