Daily Requirements
The North American Dietary Reference Intake recommends 90 milligrams per day and no more than 2 grams (2,000 milligrams) per day. Other related species sharing the same inability to produce vitamin C require exogenous vitamin C consumption 20 to 80 times this reference intake. There is continuing debate within the scientific community over the best dose schedule (the amount and frequency of intake) of vitamin C for maintaining optimal health in humans. A balanced diet without supplementation usually contains enough vitamin C to prevent scurvy in an average healthy adult, while those who are pregnant, smoke tobacco, or are under stress require slightly more. However, the amount of vitamin C necessary to prevent scurvy is less than the amount required for optimal health, as there are a number of other chronic diseases whose risk are increased by a low vitamin C intake, including cancer, heart disease, and cataracts. A 1999 review suggested a dose of 90–100 mg Vitamin C daily is required to optimally protect against these diseases, in contrast to the lower 45 mg daily required to prevent scurvy.
High doses (thousands of milligrams) may result in diarrhea in healthy adults, as a result of the osmotic water-retaining effect of the unabsorbed portion in the gastrointestinal tract (similar to cathartic osmotic laxatives). Proponents of orthomolecular medicine claim the onset of diarrhea to be an indication of where the body's true vitamin C requirement lies, though this has not been clinically verified.
| United States vitamin C recommendations | |
|---|---|
| Recommended Dietary Allowance (adult male) | 90 mg per day |
| Recommended Dietary Allowance (adult female) | 75 mg per day |
| Tolerable Upper Intake Level (adult male) | 2,000 mg per day |
| Tolerable Upper Intake Level (adult female) | 2,000 mg per day |
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