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Hello. My name is Susan
Fairweather-Tait.
I'm a Professor of
Human Nutrition in
the Norwich Medical School
at the University of
East Anglia in the UK,
and I'm going to
be talking to you
about Bioavailability
of Micronutrients,
that's minerals and vitamins.
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In European legislation,
the term bioavailability is
defined as available to
be used by the body,
that is, for specific
micronutrient-dependent
physiological functions.
The problem with bioavailability
is that it is impossible
to quantify apart from iron,
which is the one exception.
Fractional absorption
is often taken
as a surrogate measure
of bioavailability.
But it only works
for micronutrients
where retention is
close to absorption.
That is where excretion of
the absorbed mineral or
the vitamin is very low.
A common approach is to
measure relative
bioavailability,
that is, the bioavailability
of the micronutrient
from the mineral or
vitamin of interest,
as compared to a
reference source.
For chemically related compounds
having vitamin activity,
these are the vitamers,
i.e., they're capable of
contributing to
the physiological
requirements for the vitamin.
The term bioequivalence
is often used.
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Now there are various steps in
the bioavailability pathway,
and each of these
can be measured,
and they can all
give you information
about how bioavailable
a micronutrient is.
First of all, we
have the ingestion
of the micronutrient;
that's the total intake.
Then we have digestion
in the stomach
with salivary amylase,
gastric acid and pepsin.
This is often referred
to as solubilisation.
Further along in the
small intestine,
we get digestion with
pancreatic enzymes,
bile and intestinal secretions,
and this is sometimes referred
to as bioaccessibility.
That means it's
the amount that is
available for absorption
into the body.
We then get uptake into mucosal
cells in the intestine,
and this is the
apparent absorption,
and this can be measured as
the difference between
the total intake
and the amount excreted
in the faeces.
Once it's absorbed, you get
transport into the blood,
this is true absorption,
and that's obviously
the difference between
apparent absorption
minus the micronutrients
excreted in sloughed
mucosal cells.
Then the excretion of absorbed
micronutrients in the urine,
if you take that away,
you get the retention.
That's the difference
between the amount ingested
and the amount excreted
in both faeces and urine.
Finally, when you get delivery
to the target cells
for utilisation,
this is the actual
bioavailability.
You can see it's quite
a complicated process
and there are various
steps that you can measure
and you can draw
conclusions from.
But as I said,
bioavailability itself is
difficult to actually quantify.
Now various factors
influence bioavailability.
