282 Introduction to Human Nutrition
component. Estimates of variability must be based on
sampling and analyses specifi cally planned to yield
such data. The intended use of the data should deter-
mine the specifi city and level of precision for the
estimates.
For instance, it was found that the nutrient com-
position of whole milk in South Africa differed among
the fi ve localities investigated between winter and
summer, with the fat-soluble vitamins showing the
greatest variation of all the nutrients. Vitamin A is
commonly regarded as one of the micronutrients that
are defi cient in most developing countries and spe-
cifi cally in disadvantaged schoolchildren in South
Africa. Considering the results of the nutrient com-
position of whole milk, a recommendation was made
to the health authorities to fortify summer milk with
retinol in the South African school-feeding interven-
tion program, where milk is served as a mid-morning
snack to 5 million primary school children.
There is a growing recognition that the composi-
tion of commodities such as meat and cereals tends
to change over time. This necessitates updating food
composition data every 5–10 years. In most countries
this has not been possible. Changes in nutrient com-
position of red meat consumed are due to consumer
demand for leaner cuts, changes in breeding for faster
growth, and higher proportions of marketable meat
as well as changes in feed to meet scientifi c standards
or due to economic reasons.
11.3 Nutrients, nonnutrients and energy
Analytical methods
Judgment should be made on the availability of suit-
able methods of analyses for nutrients and whether
the resources, laboratory equipment, and experience
are adequate before deciding which nutrients should
be included in a nutrient database. If the methods
available are not well developed, one should recon-
sider the importance of the nutrient and whether it
justifi es using limited resources, in most instances
and countries, to develop the method and train the
staff accordingly. It will not be cost-effective to analyze
food for a particular nutrient, however high in prior-
ity, if methods yield confl icting values. This implies
that, as new or improved methods for measuring a
nutrient emerge, foods that are important in the food
supply and are known or suspected to be a good
source of that nutrient should be analyzed or re-
analyzed. Food regulation sometimes limits the choice
of methods.
The choice of method selected should be that which
most closely refl ects the nutritive content of the food-
stuff analyzed. A basic understanding of the chemis-
try of the nutrients, the nature of the food substrate
(the way in which the nutrient is distributed and
held in the food matrix) to be analyzed, the effect of
processing and preparation on both the food matrix
and the nutrient, and the expected range of concen-
tration of the nutrient determine the choice of
method. An understanding of the role of the nutrient
in the diet of individuals or populations is also a
prerequisite.
The basic principle is that the method used should
provide information that is nutritionally appropriate.
For instance, traditionally, carbohydrate was esti-
mated by difference, that is, by directly measuring the
percentage of protein (from the nitrogen content),
fat, ash, and water, and deducting these from 100 to
provide the percentage of carbohydrate. This method
is inadequate for all nutritional purposes as it com-
bines in one value all of the different carbohydrate
species: sugars, oligosaccharides, and polysaccharides
(starch and non-starch), together with all of the errors
in the other determinations, as the physiological
effects of all of the components are quite different.
Therefore, the sum of the individually analyzed car-
bohydrates is widely recommended today.
In studying the relationship between particular
foods and health or disease, the biological action of
related nutrients may be crucial information for par-
ticular uses of food composition data. For example, a
study on the role of vitamin A and carotenoids in lung
cancer requires more information than the vitamin A
activity expressed in retinol equivalents. At the very
least, vitamin A and provitamin A activity are required
separately. Information on provitamin A could be
divided into the various provitamin A carotenoids,
and it may also be desirable to have information on
other carotenoids present. This is also true for the
vitamers of other vitamins, including vitamin B 6
(pyridoxal, pyridoxal phosphate, and pyridoxamine),
folic acid (with a side-chain with one, three, or seven
glutamic acid residues), vitamin D (D 2 or D 3 ), vitamin
E (various tocopherols and tocotrienols) and vitamin
K (with various numbers of saturated and unsatu-
rated isoprene units in the side-chain).