Assessment of vitamin C status
Urinary excretion of ascorbate falls to undetectably low levels in deficiency, and therefore very low excre-tion will indicate deficiency. 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.
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 field 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 6μmol/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/106 cells; a significant 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 reflection 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/106 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.