Subject: Setting the scale for testing the benefits of vitamin C

As I summarize the peer-reviewed articles published in the American Journal of Clinical Nutrition each month, I am sharing a letter to the editor I submitted recently. The references show the ongoing debate about an optimum dosage of vitamin C.

Dear Sirs: In their editorial on the report by Schleicher et al about serum vitamin C (1), Levine and Eck (2) indicate that their Figure 1 can be used to establish concentration-function relations for vitamin C, with an emphasis on working on the x-axis of that Figure.  The x-axis provided goes only from a daily dose of ascorbic acid of 0 to 200 mg.  In 1994, King et al (3) demonstrated that to achieve a continuously elevated concentration of vitamin C in urine in all subjects in their test group required consuming a minimum of 500 mg of vitamin C every 12 hr.  Levine et al (4) subsequently published results confirming that 500 mg twice a day saturates blood serum as well.  That study concluded that 200 mg daily in divided doses was sufficient for the seven healthy young men tested, based particularly on the saturation of white blood cells.  However, given many other variables to be considered to set the x-axis for typical people, such as age, weight, and tissue, and the recommendations of Linus Pauling (5), I believe it would be appropriate to extend the x-axis to 1,000 mg twice a day, a dosage set as the upper limit by the Food and Nutrition Board in 2000 (6).  Limiting the x-axis to 200 mg/day, and not emphasizing the importance of divided doses, may have contributed to clinical trials conducted on vitamin C that have used only a single daily dose, which will lead to substantial variation in serum concentration over each day, and to the outcome of the trials.

The author does not have a conflict of interest to declare.
REFERENCES
1. Schleicher RL, Carroll MD, Ford ES, and Lacher DA. Serum vitamin C and the prevalence of vitamin C deficiency in the United States: 2003–2004 National Health and Nutrition Examination Survey (NHANES). Am J Clin Nutr 2009; 90; 1252-63.
2. Levine M and Eck P. Vitamin C: working on the x-axis. Am J Clin Nutr 2009; 90; 1121-23.
3. King G, Beins M, Larkin J, Summers B, and Ordman AB. Rate of Excretion of Vitamin C in Human Urine. AGE 1994;17; 87-92.
4. Levine M, Conry-Cantilena C, Wang Y, Welch RW, Washko PW, Dhariwal KR, Park JB, Lazarev A, Graumlich JF, King J, and Cantilena LR. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance.  Proc Natl Acad Sci U S A 1996; 93; 3704–09.
5. Pauling, L. Orthomolecular psychiatry: varying the concentrations of substances normally present in the human body may control mental disease. Science 1968;160:265–71.
6. Food and Nutrition Board, Panel on Dietary Antioxidants and Related Compounds. Vitamin C: Dietary Reference Intakes for vitamin C, vitamin E, selenium, and carotenoids. Washington, DC: National Academy Press, 2000, 95–185.

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