The Vitamins 167Doses of 50–200 mg of vitamin B 6 /day have an
antiemetic effect, and the vitamin is widely used,
alone or in conjunction with other antiemetics, to
minimize the nausea associated with radiotherapy
and to treat pregnancy sickness. There is no evidence
that vitamin B 6 has any benefi cial effect in pregnancy
sickness, or that women who suffer from morning
sickness have lower vitamin B 6 nutritional status than
other pregnant women.
Doses of vitamin B 6 of 100 mg/day have been
reported to be benefi cial in the treatment of the carpal
tunnel syndrome or tenosynovitis. However, most of
the reports originate from one centre and there
appears to be little independent confi rmation of the
usefulness of the vitamin in this condition.
Vitamin B 6 toxicity
In experimental animals, doses of vitamin B 6 of
50 mg/kg body weight cause histological damage to
dorsal nerve roots, and doses of 200 mg/kg body
weight lead to the development of signs of peripheral
neuropathy, with ataxia, muscle weakness, and loss of
balance. The clinical signs of vitamin B 6 toxicity in
animals regress within 3 months after withdrawal of
these massive doses, but sensory nerve conduction
velocity, which decreases during the development of
the neuropathy, does not recover fully.
Sensory neuropathy has been reported in seven
patients taking 2–7 g of pyridoxine/day. Although
there was some residual damage, withdrawal of these
extremely high doses resulted in a considerable
recovery of sensory nerve function. Other reports
have suggested that intakes as low as 50 mg/day
are associated with neurological damage, although
these studies were based on patients reporting symp-
toms rather than objective neurological examination.
There have been no reports of nerve damage in chil-
dren with vitamin B 6 -dependent homocystinuria,
or other inborn errors of metabolism, who take
200–300 mg/day.
8.10 Vitamin B 12
Dietary defi ciency of vitamin B 12 occurs only in strict
vegans, since the vitamin is found almost exclusively
in animal foods. However, functional defi ciency (per-
nicious anemia, with spinal cord degeneration) as a
result of impaired absorption is relatively common,
especially in older people with atrophic gastritis.
Structure and vitamers
The structure of vitamin B 12 is shown in Figure 8.13.
The term corrinoid is used as a generic descriptor for
cobalt-containing compounds of this general struc-
ture that, depending on the substituents in the pyrrole
rings, may or may not have vitamin activity. The term
“vitamin B 12 ” is used as a generic descriptor for the
cobalamins, that is, those corrinoids having the bio-
logical activity of the vitamin. Some of the corrinoids
that are growth factors for microorganisms not only
have no vitamin B 12 activity, but may be antimetabo-
lites of the vitamin.
Although cyanocobalamin was the fi rst form in
which vitamin B 12 was isolated, it is not an important
naturally occurring vitamer, but rather an artifact due
to the presence of cyanide in the charcoal used in the
extraction procedure. It is more stable to light than
the other vitamers, and hence is used in pharmaceuti-
cal preparations. Photolysis of cyanocobalamin in
solution leads to the formation of aquocobalamin or
hydroxocobalamin, depending on pH. Hydroxoco-
balamin is also used in pharmaceutical preparations,
and is better retained after parenteral administration
than is cyanocobalamin.
Vitamin B 12 is found only in foods of animal origin,
although it is also formed by bacteria. There are
no plant sources of this vitamin. This means that
strict vegetarians (vegans), who eat no foods of
animal origin, are at risk of developing dietary vitamin
B 12 defi ciency, although the small amounts of
vitamin B 12 formed by bacteria on the surface of
fruits may be adequate to meet requirements. Prepa-
rations of vitamin B 12 made by bacterial fermentation
that are ethically acceptable to vegans are readily
available.
There are claims that yeast and some plants (espe-
cially some algae) contain vitamin B 12. This seems to
be incorrect. The problem is that the offi cially recog-
nized, and legally required, method of determining
vitamin B 12 in food analysis is a microbiological assay
using organisms for which vitamin B 12 is an essential
growth factor. However, these organisms can also use
some corrinoids that have no vitamin activity. There-
fore, analysis reveals the presence of something that
appears to be vitamin B 12 , but in fact is not the active
vitamin and is useless in human nutrition. Biologi-
cally active vitamin B 12 has been identifi ed in some
preparations of algae, but this seems to be the result