Introduction to Human Nutrition

Body Composition 21

well as for individuals. Although some studies com-
paring body composition from four-compartment
models show that mean values generally agree with
simpler approaches, there are also studies showing
directional bias of two-compartment body composi-
tion models. For this reason, more-compartment
models should ideally be used as a reference (gold
standard). However, only a limited number of labo-
ratories can perform all of the necessary measure-
ments for the calculation of maximum compartment
models. Moreover, the data are expensive to collect,
and measurements are time-consuming and not very
practical in clinical situations.

Imaging techniques

CT scanning enables the visualization of tissues in
cross-sectional slices of the body. The thickness of
those slices can vary, but is normally about 1 cm.
During CT scanning a source of X-rays rotates per-
pendicularly around the body or a body segment,
while photodetectors, opposite to the source, register
the attenuation of the X-rays after they have passed
through the body in the various directions. The infor-
mation received by the photodetectors is used to gen-
erate images. Software enables the calculation of the

amounts of tissues with different attenuation, for

example adipose tissue against nonadipose tissue.

The CT technique was introduced for body composi-

tion assessments in the 1980s and is now widely used,

predominantly for measurements of body fat distri-

bution. Figure 2.4 shows a scan of the abdomen at the

level of the umbilicus, made by MRI, a technique that

gives comparable information. The precision of the

calculation of a tissue area or tissue volume from the

same scan(s) is very accurate, with an error of about

1%. Partial volume effects (pixels that contain tissue

with different attenuation) may infl uence the accu-

racy and reproducibility of the method.

A single CT scan provides only relative data, for

example in a scan of the abdomen the relative amount

of visceral adipose tissue to subcutaneous adipose

tissue. Multiple CT scanning allows the calculation of

tissue volumes. From adipose tissue volumes (tissue

level) and an assumed density and composition of the

adipose tissue, the amount of fat mass (molecular

level) can be calculated. Multiplying tissue volumes

with specifi c densities of these tissues (determined in

vitro) allows a recalculation of the body weight, a

necessary but not suffi cient exercise for validation of

a whole body technique. Research in this area has

shown that the CT technique allows the determina-

tion of total body composition, with an error of

estimate for fat mass of 3–3.5 kg (compared with

densitometry).

Figure 2.4 Magnetic resonance imaging scan at the L4 level in an

obese subject. The white areas in the image are adipose tissue. Sub-

cutaneous adipose tissue and intra-abdominal adipose tissue are sepa-

rated by the abdominal muscles.

Box 2.7

FMFM FMFM FMFM

M

M + P P

FFM TBW TBW

The fi rst bar represents a two-compartment model of body compo-

sition, in which the body is divided into fat mass and fat-free mass

(FFM). In the second bar, the FFM is divided into water and a “dry”

FFM, consisting of protein and mineral. The third bar shows a four-

compartment model in which the body is divided into water,

protein, mineral, and fat. The four-compartment model shown has

only minor assumptions and provides body composition data that

are very accurate.