Vitamin C describes all compounds exhibiting the biological activity of ascorbic acid. No reliable functional tests of ascorbic-acid deficiency have been established, but tests of plasma and leukocyte levels are the most practical and reliable methods for assessing vitamin-C status.
Plasma levels of ascorbic acid are highly influenced by recent dietary intake of vitamin C at the time of the test. Plasma ascorbic acid levels rise with increasing dietary intake until intakes are above 200 mg/day, at which point a plateau is reached at 1.2 to 1.8 mg/dL (68-102 mcmol/L). Normal plasma ascorbic acid is in the range of 0.4 to 1.5 mg/dL (23-84 mcmol/L). Marginally low ascorbic acid levels are between 0.2 and 0.4 mg/dL (11.4-23 mcmol/L) and deficiency occurs when below 0.2 mg/dL.
Factors that may affect ascorbic acid levels include acute stress due to hot or cold temperatures, surgery, trauma, infections, inflammatory diseases, the use of oral contraceptive pills, or cigarette smoking. Smoking has been found to increase the metabolic turnover of ascorbic acid; thus higher intakes are required for smokers than for non-smokers.
Leukocyte ascorbic acid tests are more reliable for assessing tissue storage and body stores, because changes in leukocyte levels are less likely to fluctuate with recent intake. The technique is difficult to perform, however, and requires large samples of blood.
Urinary excretion does not parallel vitamin C intake because the kidney is efficient at reabsorption when intakes are low and at clearance when intakes are high. At adequate intakes, urinary excretion can be up to 50 mg/day. When intake is less than 40 mg/day urinary excretion falls to less than 10 mg/day and with severe depletion levels are undetectable. This method can therefore be useful in detecting severe depletion.
Isotope dilution can be a reliable method of assessing vitamin-C status. After ingestion of a radioactive vitamin-C isotope, the specific activity of blood or urine ascorbate is measured for 24 or 48 hours afterwards.