Stability and bioavailability of vitamins

2.3 Stability and bioavailability of vitamins

2.3.1 Stability
Vitamin stability is an important issue when considering
the nutritional value of a food. Processing and
storage losses depend upon conditions such as pH,
temperature and moisture content. Niacin and biotin
are relatively stable, but the other water-soluble vitamins
are labile to varying extents and under different
conditions. Ribofl avin is notoriously susceptible to
decomposition by light. During domestic cooking
the water-soluble vitamins are easily leached out into
the cooking water or exuded from meat, but are not
lost if the cooking fl uids are consumed. In the case of
vitamin C, rapid heat treatment, such as the blanching
of fruits and vegetables or the pasteurization of fruit
juices, actually serves to prevent vitamin losses during
post-processing storage by inactivating enzymes that
promote the direct oxidation of ascorbic acid.


2.3.2 Bioavailability
The term ‘bioavailability’, as applied to vitamins
in human nutrition, refers to the proportion of
the quantity of vitamin in the food ingested that
undergoes intestinal absorption and utilization by
the body. Utilization encompasses transport of the
absorbed vitamin to the tissues, cellular uptake and
the subsequent fate of the vitamin. The vitamin may
be converted to a form which can fulfi l some biochemical
function. Alternatively, the vitamin may be
metabolized within the cell to a nonfunctional form
for subsequent excretion or simply stored within the
cell for future use. Any defi nition must be viewed as
an operational defi nition within the context of the
method used to determine bioavailability.
Bioavailability should not be confused with nutrient
stability. Whereas food processing can result
in the loss of a labile vitamin, the bioavailability of
the remaining amount of vitamin is not necessarily
altered. Bioavailability is infl uenced by a diverse range
of interacting parameters and therefore the amount of
bioavailable vitamin in a diet or individual food can
vary considerably.

Absorption of a vitamin depends on the chemical
form and physical state in which the vitamin
exists within the food matrix. These properties may
be infl uenced by the effects of food processing and
cooking, particularly in the case of niacin, vitamin B6
and folate. In foods derived from animal and plant
tissues, the B-group vitamins occur as their coenzyme
derivatives, usually associated with their protein apoenzyme.
In addition, niacin in cereals and vitamin B6
in certain fruits and vegetables occur largely as bound
storage forms. In milk and eggs, which are derived
from animal secretions, the B-group vitamins occur,
at least to some extent, in the underivatized form, a
proportion of which is associated with specifi c binding
proteins. Vitamins that exist naturally as chemically
bound complexes with some other material in
the food matrix exhibit lower effi ciencies of digestion
and absorption compared with the free (unbound)
vitamin ingested, for example, in tablet form.
Certain dietary components can retard or enhance
a vitamin’s absorption, therefore the composition of
the diet is an important consideration. For example,
the presence of adequate amounts of dietary fat is essential
for the absorption of the fat-soluble vitamins.
Carotenoids exhibit low bioavailability relative to
vitamin A due to the poor digestibility of fi brous
plant material. Other ingested substances such as alcohol
and drugs may interfere with the physiological
mechanisms of absorption.

Biological factors can infl uence the absorption of a
vitamin from a particular food or diet. For example,
the absorption mechanism in intestinal epithelium
can be adapted physiologically to meet changing
metabolic requirements and food deprivation. Malabsorption
may occur in the presence of gastrointestinal
disorders or disease. Other general factors that
infl uence absorption include the plane of nutrition,
metabolic requirements, age and state of health.
It is impracticable to determine true or absolute
bioavailability, and therefore almost all methods for
determining vitamin bioavailability in foods yield a
measurement of relative bioavailability. This is the
observed response obtained when the animal is fed
the test food or diet expressed as a percentage of the
response obtained by feeding a reference material of
high bioavailability. Rats or chicks have been used
extensively as experimental animals, but these animal
studies are now thought to have relatively little value
in predicting vitamin bioavailability for humans. This
is because of problems such as intestinal synthesis of
water-soluble vitamins by gut microfl ora, coprophagy
(faecal recycling) and metabolic differences between
Nutritional aspects of vitamins 11
animals and humans. The main emphasis nowadays
in the fi eld of nutrient bioavailability has turned to
the use of protocols with human subjects in order
to avoid the uncertain relevance of animal models
(Gregory, 1988).