Food Composition 287infl uencing bioavailability, tables of food composition
cannot give a single value for a nutrient’s bioavailabil-
ity. Most research until now has centered upon inor-
ganic constituents, particularly iron, but the concept
is applicable to virtually all nutrients. Iron incorpo-
rated into heme is more readily absorbed than iron in
the nonheme form, and these two forms of iron are
sometimes listed separately in food composition
tables. Yet, such information does not take into
account, for example, the effect of ascorbic acid
(vitamin C) and organic acids (citric, malic, tartaric,
and lactic acid) on nonheme iron absorption. Iron
absorption is also increased in a state of iron defi -
ciency and research has shown that vitamin A and iron
intake has to be increased simultaneously to alleviate
anemia. In the coming years, it can be expected that
much more work will be carried out on bioavailability
than in the past, because of its key role in relating
functional nutritional status to nutrient intake.
Future research will probably also focus more on
the measurement of the bioavailability of food con-
stituents. Several vitamins and minerals, such as
calcium, iron, zinc, and a number of B vitamins, are
already being studied, with limited attention to carot-
enoid bioavailability. Inhibitors of absorption and the
effects of processing and storage on the foodstuffs
must be determined. As bioavailability is also infl u-
enced to a large extent by the meal in which a food
constituent is consumed, this means that more infor-
mation will be needed not only on daily food con-
sumption but also on intake of other constituents at
individual meals.
There is an increasing demand from users of food
composition tables for information on the glycemic
index (GI) of food, which is used as a tool in the
selection of food in the management of diabetes, as
opposed to the previous system of carbohydrate
exchange. The GI is a food-specifi c measure of the
relative tendency of carbohydrate in food to induce
postprandial glycemia. The body’s response to a 50 g
carbohydrate dose induced by either glucose or white
bread is taken as the reference and assigned a value of
a 100. Responses to all other foods are rated in com-
parison and listed in tabular format. New datasets
with complementary values, based on the GI and
available carbohydrate content of food, have been
proposed, of which one is a measure of the relative
glycemic response to a given mass of whole food and
the other is the mass of a food responsible for a given
specifi c glycemic response. A more recently proposed
identifi cation of a food’s GI value lies in indicating
the specifi c food’s category of GI as high, medium, or
low. Accurate numerical values of a food’s GI are dif-
fi cult to obtain as various factors, including human
subject variability both between and within subjects
during analysis as well as response during ingestion
of the food, can differ signifi cantly.
Both bioavailability and GI are food indices that
are infl uenced not only by the characteristics of the
food, but also by the response of the individual to the
food (i.e., absorption, metabolism, and excretion of
the metabolites). For example, quantitative analysis of
carotenoids alone could lead to a misinterpretation of
vitamin A value. Therefore, the bioavailability of test
foods in a single mixture may be investigated using
the digestive system of nutrient-depleted rats (i.e.,
measuring retinol accumulation factor as a measure
of total carotenoid bioavailability), or in humans
using the relative dose–response test. Advances in
analytical chemistry such as improvements in analyti-
cal methods, information science, computer hardware
and software will assist in fi lling these gaps in special-
purpose databases in the future.How to calculate a recipe not included in
the database
If the composition of a composite or mixed dish is
not known, it can be estimated by calculation from a
standard recipe and applying appropriate nutrient
retention factors and, in some cases, adjusting for
changes in moisture content due to cooking loss or
gain during cooking. The following guidelines are
suggested.
● Identify the ingredients of the recipe from the most
appropriate foods available in the food composi-
tion database table.
● Quantify the ingredients in mass (g).
● Calculate the nutrient values for the specifi c amount
of each ingredient.
● Add up the nutrient values of the individual
ingredients.
● Calculate the nutrient composition for 100 g of the
recipe.
● Apply suitable retention factors to the mineral and
vitamin nutrient values if the recipe food is cooked.
Note that if individual ingredients are in a cooked
form this step is not necessary.