158 Introduction to Human Nutrition
At intakes of 1.1–1.6 mg/day urinary excretion rises
sharply, suggesting that tissue reserves are saturated.
A more generous estimate of requirements, and the
basis of reference intakes, is the level of intake at
which there is normalization of the activity of the red
cell enzyme glutathione reductase; the activity of this
fl avoprotein is especially sensitive to ribofl avin nutri-
tional status. Normal values of the activation coeffi -
cient are seen in subjects whose habitual intake of
ribofl avin is between 1.2 mg/day and 1.5 mg/day.
Because of the central role of fl avin coenzymes in
energy-yielding metabolism, reference intakes are
sometimes calculated on the basis of energy intake:
0.14–0.19 mg/MJ (0.6–0.8 mg/1000 kcal). However,
in view of the wide range of ribofl avin-dependent
reactions, other than those of energy-yielding metab-
olism, it is diffi cult to justify this basis for the calcula-
tion of requirements.
Assessment of ribofl avin nutritional status
The urinary excretion of ribofl avin and its metabo-
lites (either basal excretion or after a test dose) can be
used as an index of status. However, ribofl avin excre-
tion is only correlated with intake in subjects who are
in nitrogen balance. In subjects in negative nitrogen
balance there may be more urinary excretion than
would be expected, as a result of the catabolism of
tissue fl avoproteins, and loss of their prosthetic
groups. Higher intakes of protein than are required
to maintain nitrogen balance do not affect the require-
ment for ribofl avin or indices of ribofl avin nutri-
tional status.
Glutathione reductase is especially sensitive to
ribofl avin depletion. The activity of the enzyme in
erythrocytes can therefore be used as an index of
ribofl avin status. Interpretation of the results can be
complicated by anemia, and it is more usual to use
the activation of erythrocyte glutathione reductase
(EGR) by FAD added in vitro. An activation coeffi -
cient of 1.0–1.4 refl ects adequate nutritional status,
whereas >1.7 indicates defi ciency.
Interactions with drugs and other nutrients
The phenothiazines such as chlorpromazine, used in
the treatment of schizophrenia, and the tricyclic anti-
depressant drugs such as imipramine, are structural
analogues of ribofl avin, and inhibit fl avokinase. In
experimental animals, administration of these drugs
at doses equivalent to those used clinically results in
an increase in the EGR activation coeffi cient and
increased urinary excretion of ribofl avin, with reduced
tissue concentrations of ribofl avin phosphate and
FAD, despite feeding diets providing more ribofl avin
than is needed to meet requirements. Although there
is no evidence that patients treated with these drugs
for a prolonged period develop clinical signs of ribo-
fl avin defi ciency, long-term use of chlorpromazine is
associated with a reduction in metabolic rate.
Ribofl avin defi ciency is sometimes associated
with hypochromic anemia as a result of impaired
iron absorption. A greater proportion of a test dose
of iron is retained in the intestinal mucosal cells
bound to ferritin, and hence lost in the feces, rather
than being absorbed, because the mobilization of iron
bound to ferritin in mucosal cells for transfer to
transferrin requires oxidation by a fl avin-dependent
enzyme.
Ribofl avin depletion decreases the oxidation of
dietary vitamin B 6 to pyridoxal; pyridoxine oxidase is
a fl avoprotein and is very sensitive to ribofl avin deple-
tion. It is not clear to what extent there is functional
vitamin B 6 defi ciency in ribofl avin defi ciency. This is
partly because vitamin B 6 nutritional status is gener-
ally assessed by the metabolism of a test dose of tryp-
tophan, and kynurenine hydroxylase in the tryptophan
oxidative pathway is a fl avoprotein; ribofl avin defi -
ciency can therefore disturb tryptophan metabolism
quite separately from its effects on vitamin B 6 nutri-
tional status.
The disturbance of tryptophan metabolism in
ribofl avin defi ciency, due to impairment of kynuren-
ine hydroxylase, can also result in reduced synthesis
of NAD from tryptophan, and may therefore be a
factor in the etiology of pellagra.8.8 Niacin
Niacin is not strictly a vitamin, since it can be synthe-
sized in the body from the essential amino acid tryp-
tophan. Indeed, it is only when tryptophan metabo-
lism is deranged that dietary preformed niacin
becomes important. Nevertheless, niacin was discov-
ered as a nutrient during studies of the defi ciency
disease pellagra, which was a major public health
problem in the southern USA throughout the fi rst
half of the twentieth century, and continued to be a
problem in parts of India and sub-Saharan Africa
until the 1990s.