Tuesday, July 3, 2007

Dietary sources and bioavailability of Niacin

Niacin can be synthesized in the human body from
the essential amino acid L-tryptophan. Approximately
60 mg of L-tryptophan yield 1 mg of niacin;
therefore, to calculate the niacin equivalent of a diet,
one adds one-sixtieth of the weight of tryptophan
present to the weight of preformed niacin.
Because of the contribution of tryptophan, foods
containing balanced protein are important contributors
to total niacin equivalent intake. Lean red meat,
poultry and liver contain high levels of both niacin
and tryptophan and, together with legumes, are important
sources of the vitamin. Peanut butter is an excellent
source of niacin. Cheese and eggs are relatively
poor sources of preformed niacin, but these highprotein
foods contain ample amounts of tryptophan
and therefore have a high niacin equivalent. Fruits and
vegetables provide useful amounts, depending upon
the dietary intake. Other useful sources are whole
grain cereals, bread, tea and coffee.

In mature cereal grains most of the niacin is present
as bound nicotinic acid and is concentrated in the
aleurone and germ layers. Milling to produce white
fl our removes most of the vitamin with the bran. In
the UK it is compulsory by law to add niacin to white
fl our (mostly 70% extraction rate) at 16 mg kg–1. All
fl our other than wholemeal (100% extraction) must
be enriched (Bender, 1978).
As discussed later, some plant-derived foods contain
niacin in chemically bound forms that result in
their bioavailabilities being low. Most food composition
tables give total niacin, and are compiled from the
results of analyses in which nicotinic acid is liberated
from unavailable bound forms by hydrolysis with
acid or alkali. Therefore, tabulated niacin contents
for many plant foods, particularly mature cereals,
over-estimate their value in providing biologically
available niacin.

The majority of the bound nicotinic acid in mature
cereal grains is biologically unavailable after conventional
cooking (Wall & Carpenter, 1988). Nicotinoyl
glucose itself is readily utilized, so why should this
compound be unavailable when present in plant
tissues? Mason & Kodicek (1973) suggested that its
incorporation within indigestible celluloses and
hemicelluloses prevents access of the gastrointestinal
esterases to the nicotinoyl ester bonds. Alternatively,
esterase activity may be poor: the methyl ester of nicotinic
acid was only 15% as effective as the free acid in
supporting the growth of rats (Wall & Carpenter,
1988). About 10% of the total niacin was released as
free nicotinic acid after extraction of sorghum meal
with 0.1 N HCl (Magboul & Bender, 1982). This suggests
that a small proportion of bound nicotinic acid
can be hydrolysed by gastric juice and made available.
Pellagra, the disease caused by a defi ciency of both
niacin and tryptophan, has commonly been found in
population groups having maize as their staple food.
The generally accepted explanation for this association
is the unavailability of niacin in maize, coupled
with a very low proportion of tryptophan in zein
(the major protein in maize). Mexican and Central
American peasants, and also Hopi Indians in Arizona,
rely upon maize as a staple food and yet do not experience
pellagra. The explanation for this paradox lies
in the way in which these people prepare the maize
for bread-making. In the traditional preparation of
Mexican tortillas (Cravioto et al., 1945), the maize is
soaked at alkaline pH in lime-water before baking and
this process releases the nicotinic acid from its bound
forms. In the making of piki bread the Hopi Indians
use wood ash, which is alkaline and also results in the
liberation of nicotinic acid. The availability of nicotinic
acid in tortillas baked from maize treated with
lime-water has been demonstrated in pigs by Kodicek
et al. (1959).

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