Recommendations for Vitamin C Intake

To the Editor: The article by Dr Levine and colleagues on recommendations for

vitamin C intake provides strong rationale for raising the recommended intake for vitamin C from the current level of 60 mg/d to as high as 200 mg/d. However, several statements made by the authors may create misconceptions. In the United States it is unlikely that the consumption of 5 servings of fruits and vegetables daily would provide 210 to 280 mg of vitamin C. The fruits and vegetables commonly consumed in the US diet are low in vitamin C, typically only 10 to 20 mg per serving. For example, the total amount of vitamin C in 1 apple, 1 banana, a lettuce salad, a serving of corn, and a serving of green beans is only 30 to 35 mg. Although the campaign to consume "5-a-day" is commendable, consumers need to be aware of the importance of including 1 or 2 vitamin C-rich fruits and vegetables in their diet daily, a list that includes citrus, cantaloupe, strawberries, broccoli, cauliflower, and peppers. 

Attention to diet is important because recent national survey data indicated that 10% to 13% of Americans are vitamin C deficient and at high risk for developing scurvy (plasma vitamin C <11 µmol/L). The authors' recommendation for the daily intake of vitamin C (100-200 mg/d) would provide for tissue saturation but not plasma saturation. This is an important distinction since an accepted functional marker for vitamin C nutriture is not available. It is not known whether plasma saturation confers added physiological benefits for humans beyond that achieved at tissue saturation. The cofactor functions of vitamin C occur intracellularly and are likely influenced more by tissue levels than plasma levels. However, the antioxidant and reduction actions of vitamin C in extracellular fluids could be maximized by plasma saturation, as some literature suggests. 

Finally, gram doses of vitamin C are well tolerated by healthy individuals, and epidemiological data indicate that individuals who regularly supplement their diets with vitamin C may be at lower risk for all-cause cancer deaths, colon and bladder cancer, lens opacities, and kidney stones.5 The tolerable upper intake level, as defined by the Food and Nutrition Board of the Institute of Medicine, is not designed to protect individuals with pathologies exacerbated by nutrient supplementation.6 Hence, the upper intake level for vitamin C should not be defined based on a potential adverse effect of vitamin C supplementation in patients with preexisting hyperoxaluria, as indicated by the authors. 

For more information

Carol S. Johnston, PhD
Arizona State University
Tempe

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