Introduction to Human Nutrition

26 Introduction to Human Nutrition

interpretation of calculated body composition values
in situations where body water distribution can be
disturbed, as is the case, for example, in dialysis
patients and in ascites. In general, prediction formulae
based on impedance values are strongly population
specifi c, and age and gender are important contribu-
tors. Differences between populations and individuals
are partly caused by differences in body build (e.g.,
relatively long legs), which is not surprising, as the legs
contribute most to total body impedance relative to
other parts of the body (Box 2.10).
Currently available impedance analyzers vary in
their electrical features and in their principles. Many
companies have developed impedance analyzers
for personal use, anticipating considerable interest
among the public in determining their body fat per-
centage. There are instruments that measure imped-
ance from foot to foot while standing on a weighing
scale and provide not only body weight but also body
fat percentage. Other instruments measure imped-
ance from hand to hand and allow the reading of
body fat percentage, using a built-in software program
in which weight, height, age, and gender have to be
entered. Combinations of foot-to-foot and hand-to-
hand impedance analyzers are also marketed.
As for all other impedance analyzers, the incorpo-
rated formulae are population specifi c and have a
prediction error of 4–5%. This means that, apart
from a systematic error (prediction formula is not
valid), the value can be as much as 10% off in extreme
cases. This kind of error is similar to the possible error
in skinfold thickness measurements, and hence
impedance is no better than skinfold thickness mea-
surements. The advantage of impedance analyzers is
that there is no need to undress and measurements
are less prone to observer bias.

Total body electrical conductivity

Total body electrical conductivity (TOBEC) was devel-

oped in the 1970s. The principle of the method is that

conductive material (body water and dissolved elec-

trolytes) that is placed in an electromagnetic fi eld will

cause an inductive current, which is related to the

amount of conductive material. In practice, the subject

lies on a stretcher, which enters the inner space of an

electric wire coil, through which a high-frequency

current (2.5–5 MHz) passes. The measurement is very

quick (it takes only seconds), painless, and without

any risk to the subject. The reproducibility of a mea-

surement is within 2% and the error in the predicted

FFM was found to be about 3 kg in a group of adult

subjects, which is similar to, for example, skinfold

thickness measurements or impedance measurements.

The TOBEC method is especially suitable for mea-

surements in infants and young children, in whom

bioelectrical impedance measurements are diffi cult or

impossible to perform, owing to movement. The main

disadvantage of the method is the high price.

Creatinine excretion and

N-methyl-histidine excretion

In the muscle cell, creatine phosphate, necessary for

the energy metabolism of the cell, degenerates to cre-

atinine at a constant daily rate of about 2%. It is

assumed that 1 g of excreted creatinine is equivalent

to 18–22 kg of muscle mass. As the cell cannot recycle

creatinine, the kidneys excrete it. Since metabolized

creatine phosphate is not the only source of urinary

creatinine (e.g., creatinine in ingested meat is also

excreted immediately), the validity of the method is

dubious. A day-to-day coeffi cient of variation in the

excretion of creatinine of almost 20% is reported,

when the subject is “free living” and the urine is

sampled over constant periods of 24 hours. The high

variation is due to the ingestion of creatinine with

nonvegetarian meals, differences in physical activity

levels, and variation in creatinine excretion within the

phase of the menstrual cycle. After careful standard-

ization, which includes a controlled diet, the day-to-

day variability in excretion can be decreased to about

5%. To obtain a reliable assessment of the creatinine

excretion, sampling of urine over multiple 24 hour

periods is necessary.

The excretion of 3-methylhistidine has also been

proposed as a measure for muscle mass. FFM deter-

Box 2.10

The relative validity of impedance prediction formulae can be

demonstrated by a simple calculation example. A man, aged 35

years, of height 170 cm, weight 75 kg, and measured impedance

(from foot to hand) 400 Ω, has a predicted fat-free mass (FFM) of

64.7 kg according to Lukaski et al. (1986) and a predicted FFM of

60.5 kg according to Segal et al. (1988). Both prediction formulae

were developed in US populations and were cross-validated. The

instrument used was the same and the method of reference in both

studies was underwater weighing.