The early method of assessing vitamin C nutritional status was by testing the extent of saturation of the body's reserves by giving a test dose of 500 mg (2.8 mmol) and measuring the amount excreted in the urine. In a subject with high status, more or less all of the test dose is recovered over a period of 5 or 6 h.
More sensitive assessment of status is achieved by measuring the concentration of the vitamin in whole blood, plasma, or leukocytes. Criteria of adequacy are shown in Table 2. The determination of ascor-bate in whole blood is complicated by nonenzymic oxidation of the vitamin by hemoglobin, and most studies rely on plasma or leukocyte concentrations of ascorbate.
A problem arises in the interpretation of leukocyte ascorbate concentrations because of the different capacity of different classes of leukocytes to accumulate the vitamin. Granulocytes are saturated at a concentration of about 530pmol/106 cells, while mononuclear leukocytes can accumulate 2.5 times more ascorbate. A considerable mythology has developed to the effect that vitamin C requirements are increased in response to infection, inflammation, and trauma, based on reduced leukocyte concentrations of ascorbate in these conditions. However, the fall in leukocyte ascorbate can be accounted for by an increase in the proportion of granulocytes in response to trauma and infection (and hence a fall in the proportion of mononuclear leukocytes). Total leukocyte ascorbate is not a useful index of vitamin C status without a differential white cell count.
There is increased formation of 8-hydroxyguanine (a marker of oxidative radical damage) in DNA during (short-term) vitamin C depletion, and the rate of removal of 8-hydroxyguanine from DNA by excision repair, and hence its urinary excretion, is affected by vitamin C status. This suggests that measurement of urinary excretion of
Table 2 Plasma and leukocyte ascorbate concentrations as criteria of vitamin C nutritional status
Deficient Marginal Adequate
Whole blood mmol/l <17 17-28 >28
8-hydroxyguanine may provide a biomarker of optimum status, as a basis for estimating requirements.
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