184 Introduction to Human Nutrition
An intake of 40 mg/day is more than adequate to
maintain a total body content of 900 mg of vitamin
C (the British RNI). At a higher level of habitual
intake, 60 mg/day is adequate to maintain a total body
content of 1500 mg (the former US RDA). Making
allowances for changes in the rate of metabolism with
different levels of intake, and allowing for incomplete
absorption of the vitamin gives the Netherlands RDA
of 80 mg/day.
The current US reference intake (75 mg for women
and 90 mg for men) is based on intakes required to
saturate leukocytes with vitamin C.
Assessment of vitamin C status
Urinary excretion and saturation testing
Urinary excretion of ascorbate falls to undetectably
low levels in defi ciency, and therefore very low excre-
tion will indicate defi ciency. However, no guidelines
for the interpretation of urinary ascorbate have been
established.
It is relatively easy to assess the state of body reserves
of vitamin C by measuring the excretion after a test
dose. A subject who is saturated will excrete more or
less the whole of a test dose of 500 mg of ascorbate
over 6 h. A more precise method involves repeating
the loading test daily until more or less complete
recovery is achieved, thus giving an indication of how
depleted the body stores were.
Blood concentrations of ascorbate
The plasma concentration of vitamin C falls relatively
rapidly during experimental depletion studies to
undetectably low levels within 4 weeks of initiating a
vitamin C-free diet, although clinical signs of scurvy
may not develop for a further 3–4 months, and tissue
concentrations of the vitamin may be as high as 50%
of saturation. In fi eld studies and surveys, subjects
with plasma ascorbate below 11 μmol/l are consid-
ered to be at risk of developing scurvy, and anyone
with a plasma concentration below 6 μmol/l would
be expected to show clinical signs.
The concentration of ascorbate in leukocytes is
correlated with the concentrations in other tissues,
and falls more slowly than plasma concentration in
depletion studies. The reference range of leukocyte
ascorbate is 1.1–2.8 mol/10^6 cells; a signifi cant loss of
leukocyte ascorbate coincides with the development
of clear clinical signs of scurvy.
Without a differential white cell count, leukocyte
ascorbate concentration cannot be considered to give
a meaningful refl ection of vitamin C status. The dif-
ferent types of leukocyte have different capacities to
accumulate ascorbate. This means that a change in the
proportion of granulocytes, platelets, and mononu-
clear leukocytes will result in a change in the total
concentration of ascorbate/10^6 cells, although there
may well be no change in vitamin nutritional status.
Stress, myocardial infarction, infection, burns, and
surgical trauma all result in changes in leukocyte dis-
tribution, with an increase in the proportion of gran-
ulocytes, and hence an apparent change in leukocyte
ascorbate. This has been widely misinterpreted to
indicate an increased requirement for vitamin C in
these conditions.Possible benefi ts of high intakes of
vitamin C
There is evidence from a variety of studies that high
vitamin C status and a high plasma concentration of
the vitamin is associated with reduced all-cause
mortality.
At intakes above about 100–120 mg/day the body’s
capacity to metabolize vitamin C is saturated, and
any further intake is excreted in the urine unchanged.
Therefore, it would not seem justifi able to recom-
mend higher levels of intake. However, in addition
to its antioxidant role and its role in reducing
the tocopheroxyl radical, and thus sparing vitamin
E, vitamin C is important in the absorption of
iron, and in preventing the formation of nitrosa-
mines. Both of these actions depend on the presence
of the vitamin in the gut together with food, and
intakes totaling more than 100 mg/day may be
benefi cial.Iron absorption
Inorganic dietary iron is absorbed as Fe^2 + and not as
Fe^3 +; ascorbic acid in the intestinal lumen will both
maintain iron in the reduced state and chelate it, thus
increasing the amount absorbed. A dose of 25 mg of
vitamin C taken together with a meal increases the
absorption of iron by around 65%, while a 1 g dose
gives a ninefold increase. This occurs only when
ascorbic acid is present together with the test meal;
neither intravenous administration of vitamin C nor
intake several hours before the test meal has any effect
on iron absorption. Optimum iron absorption may