The Vitamins 165Requirements of infants
Estimation of the vitamin B 6 requirements of infants
presents a problem, and there is a clear need for
further research. Human milk, which must be assumed
to be adequate for infant nutrition, provides only
some 2.5–3 μg of vitamin B 6 /g protein. This is very
much lower than the requirement for adults, although
there is no reason why infants should have a lower
requirement.
Based on the body content of 3.7 μg (15 nmol)
of vitamin B 6 /g body weight, and the rate of weight
gain, a minimum requirement for infants over the
fi rst 6 months of life is 100 μg/day to establish
tissue reserves, and an additional 20% to allow for
metabolic turnover. Even if the mother receives
daily supplements of 2.5 mg of vitamin B 6 through-
out lactation, thus more than doubling her normal
intake, the infant’s intake ranges from 100 μg/day to
300 μg/day over the fi rst 6 months of life. At 1 month
this is only 8.5 μg/g protein, rising to 15 μg/g by
2 months.
Assessment of vitamin B 6 status
Fasting plasma total vitamin B 6 (measured microbio-
logically), or more specifi cally pyridoxal phosphate, is
widely used as an index of vitamin B 6 nutritional
status. Despite the fall in plasma pyridoxal phosphate
in pregnancy, which has been widely interpreted as
indicating vitamin B 6 depletion or an increased
requirement, the plasma concentration of pyridoxal
phosphate plus pyridoxal is unchanged. This suggests
that determination of plasma pyridoxal phosphate
alone may not be a reliable index of vitamin B 6 nutri-
tional status.
About half of the normal dietary intake of vitamin
B 6 is excreted as 4-pyridoxic acid. Urinary excretion
of 4-pyridoxic acid will largely refl ect the recent intake
of the vitamin rather than the underlying nutritional
status.
Coenzyme saturation of transaminases
The most widely used method of assessing vitamin B 6
status is by the activation of erythrocyte transami-
nases by pyridoxal phosphate added in vitro. An acti-
vation coeffi cient for alanine transaminase >1.25, or
for aspartate transaminase >1.8, is considered to indi-
cate defi ciency.
The tryptophan load test
The tryptophan load test for vitamin B 6 nutritional
status (the ability to metabolize a test dose of trypto-
phan) is one of the oldest metabolic tests for func-
tional vitamin nutritional status. It was developed as
a result of observation of the excretion of an abnor-
mal colored compound, later identifi ed as the trypto-
phan metabolite xanthurenic acid, in the urine of
defi cient animals.
Kynureninase (see Figure 8.12) is a pyridoxal phos-
phate-dependent enzyme, and its activity falls mark-
edly in vitamin B 6 defi ciency, at least partly because it
undergoes a slow mechanism-dependent inactivation
that leaves catalytically inactive pyridoxamine phos-
phate at the active site of the enzyme. The enzyme can
only be reactivated if there is an adequate supply of
pyridoxal phosphate. This means that in vitamin B 6
defi ciency there is a considerable accumulation of
both hydroxykynurenine and kynurenine, suffi cient
to permit greater metabolic fl ux than usual through
kynurenine transaminase, resulting in increased for-
mation of kynurenic and xanthurenic acids.
Xanthurenic and kynurenic acids, and kynurenine
and hydroxykynurenine, are easy to measure in urine,
so the tryptophan load test [the ability to metabolize
a test dose of 2–5 g (150–380 μmol/kg body weight)
of tryptophan] has been widely adopted as a con-
venient and very sensitive index of vitamin B 6
nutritional status. However, because glucocorticoid
hormones increase tryptophan dioxygenase activity,
abnormal results of the tryptophan load test must be
regarded with caution, and cannot necessarily be
interpreted as indicating vitamin B 6 defi ciency.
Increased entry of tryptophan into the pathway will
overwhelm the capacity of kynureninase, leading to
increased formation of xanthurenic and kynurenic
acids. Similarly, estrogen metabolites inhibit kyn-
ureninase, leading to results that have been misinter-
preted as vitamin B 6 defi ciency.The methionine load test
The metabolism of methionine includes two pyri-
doxal phosphate-dependent steps: cystathionine
synthetase and cystathionase (see Figure 8.16).
Cystathionase activity falls markedly in vitamin B 6
defi ciency, and as a result there is an increase in the
urinary excretion of homocysteine and cystathionine,
both after a loading dose of methionine and under