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Hello. My name is Susan
Fairweather-Tait.
I'm Professor of Human Nutrition
in the Norwich Medical School
at the University of
East Anglia in the UK.
I'm going to be
talking about niacin,
which is one of the B vitamins
sometimes called vitamin B3.
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Niacin is a generic term
which covers nicotinic
acid and nicotinamide,
and it's the precursor of
the nicotinamide nucleotide
coenzymes NAD and NADP.
These are involved in more than
200 oxidation/reduction
reactions
in the metabolism
of carbohydrates,
fatty acids, and amino acids.
Tryptophan, an
essential amino acid,
can be converted to niacin,
and this actually is the
main source of the vitamin
rather than preformed
dietary niacin.
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Food sources which are rich
in niacin include liver,
meat and meat products, fish,
peanuts and whole grains.
Foods that are a good
source of tryptophan
include milk, cheese and eggs.
The content of
niacin in foods is
expressed as niacin
equivalents, and this is where
one niacin equivalent
equals the milligrams of
niacin in the food
plus the milligrams of
tryptophan in the
food divided by 60.
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In order to be absorbed,
the nucleotides, NAD and NADPH,
which are in fact the main forms
of niacin in animal foods,
and esterified niacin,
which is the main
form in plant foods,
need to be hydrolysed in the
intestine into nicotinamide.
Both nicotinic acid
and nicotinamide
are absorbed from the
small intestine by
a sodium-dependent
saturable process
and by a sodium-independent
process in the colon.
The absorption is quite
variable, it's from 23-70%.
It's actually the highest
from animal foods,
where most niacin is
free nicotinamide,
and it's lowest from cereals,
where niacin is mostly
bound as niacytin,
which is actually
unavailable for absorption.
Certain food preparation
and processing practices
may influence the content
and the bioavailability
of niacin in food.
