Requirements for niacin are related to energy intake
because of the involvement of NAD and NADP as
coenzymes in the oxidative release of energy from
food. Estimation of niacin requirement is complicated
by the conversion of tryptophan to the vitamin. The
effi ciency of the conversion is affected by a variety of
infl uences, including the amounts of tryptophan and
niacin ingested, protein and energy intake, hormonal
status, and vitamin B6 and ribofl avin nutriture. A
normal intake of protein will probably provide more
than enough tryptophan to meet the body’s requirement
for niacin without the need for any preformed
niacin in the diet.
A notable exception to the 60:1 conversion ratio of
L-tryptophan to niacin is the state of pregnancy, in
which the conversion is about twice as efficient. This
increased conversion is presumably due to the stimulation
by oestrogen of tryptophan oxygenase, which
is a rate-limiting enzyme in the biosynthetic pathway.
Conversion is also increased when contraceptive pills
Effects of high intake
Nicotinic acid administered orally at doses as low
as 100 mg per day causes peripheral vasodilatation,
with the appearance of skin fl ushing. In high doses,
nicotinic acid competes with uric acid for excretion,
leading to an increase in the incidence of gouty arthritis.
Of greatest concern is possible liver damage,
and in one report severe jaundice occurred at doses
of 750 mg per day for only 3 months. Nicotinamide
does not cause vasodilatation, but is otherwise two to
three times as toxic as the acid (Miller & Hayes, 1982;
Alhadeff et al., 1984).