The Optimal Intake of Vitamin C
Last year, a group of scientists led by Dr. Mark Levine in the National Institutes of Health reported the results of their study on the optimal intake of vitamin C in the Proceedings of the National Academy of Sciences with the title, "Vitamin C pharmacokinetics in healthy volunteers: Evidence for a recommended dietary allowance." Their careful study concluded that the current RDA for vitamin C of 60 mg/day should be increased to 200 mg/day, exactly the same recommendation proposed by chemistry Nobel laureate Linus Pauling nearly twenty years ago. Dr. Levine gave vitamin C in doses ranging from 30 mg/day to 400 mg/day to seven healthy young men, 20 to 26 years old, who were hospitalized for four to six months. The largest doses, 1,000 mg/day and 2,500 mg/day, were given near the end of the study. Due to attrition, only six men received 1,000 mg/day (1 gram/ day) and only three received the largest dose of 2,500 mg/day. It is, therefore, difficult to draw strong conclusions about the effect of the highest doses.
The researchers measured some critical physiological and biochemical parameters, including bioavailability, absorption, and excretion. For bioavailability, doses of vitamin C were given orally and intravenously and the difference between resultant plasma concentrations was calculated. Absorption was complete at 200 mg and then declined with increasing doses, so that less than 50% of the 1,250 mg dose was absorbed. Urinary excretion of vitamin C was determined after oral and intravenous administration of the vitamin. About 25% of a 100 mg dose given either orally or intravenously was excreted within 24 hours. Excretion increased as the dose increased; at the higher doses, about half of the orally administered vitamin was excreted within 24 hours and nearly all of the vitamin given intravenously was excreted.
Plasma concentrations of vitamin C increased rapidly with doses up to 400 mg/day, but increased much more slowly at higher doses. For instance, blood levels achieved at 60 mg/day (the present RDA) nearly tripled after a dose of 400 mg/day. While increasing the dose from 400 mg/day to 2,500 mg/day resulted in an average increase in the blood level of vitamin C of about 20%, the physiological relevance of an increase of this magnitude is not known.
Dr. Levine's group measured the level of vitamin C attained in immune cells, such as neutrophils, monocytes, and lymphocytes, as a function of the dose. The maximum concentration in these cells was reached at the dose of 100 mg/day.
Other investigators have suggested that large doses of vitamin C may be contraindicated in cases of renal insufficiency and chronic hemodialysis. Not much is known about possible interactions between drugs, such as those used in cancer chemotherapy, and vitamin C, although some evidence suggests that large doses of vitamin C may not be advisable for patients taking anticoagulant drugs. However, the results of studies have been inconsistent and often inconclusive.
effects and therapy
In an article in The New England Journal of Medicine in 1985, Drs. Eaton and Konner estimated that the Paleolithic diet provided about 400 mg/day of vitamin C to our human ancestors, much more than the 90 mg/day or less presently consumed dietarily by most Americans. Dr. Gladys Block has estimated that 40% of Americans have vitamin C blood levels about half of the level obtainable by consuming a diet abundant in fruits and vegetables. Supplementation is an economical, safe, and effective means to achieve higher blood levels.
In "Evolution and the Need for Ascorbic Acid" (published in the Proceedings of the National Academy of Sciences in 1970), Linus Pauling calculated the amount of several vitamins in 110 raw plant foods, based on the amount of each plant food that provides 2,500 kcal of energy, which is the average daily need for humans. The thiamine (vitamin B1), riboflavin (vitamin B2), and nicotinic acid (vitamin B3) content in the amount of these foods that provide 2,500 kcal of energy is about 3 times the RDA for these vitamins, but the average amount of vitamin C for the 110 plant foods providing 2,500 kcal of energy is 2,300 mg, or 38 times the present RDA. (Of course, the value for vitamin C would decrease somewhat if meat, poultry, or fish were included in the diet to provide food energy or if the consumption of nuts or grains is high.) It appears that Dr. Levine's recommendation to increase the RDA for vitamin C to 200 mg/day is a step in the right direction.
Many micronutrients in food have beneficial effects, and interactions between them may be important. Most epidemiologic studies correlate a high consumption of fruits and vegetables with a decreased risk of developing age-related diseases like cancer, cataract, and heart disease. Evidence also supports special roles for supplemental vitamins C and E in reducing the incidence of heart disease and cancer. Indeed, much of the protective effect can be related to antioxidant vitamins, while much is certainly related to beneficial phytochemicals and other substances. It is a fascinating and very complicated picture, and Dr. Levine's study has contributed to our understanding. However, much more research needs to be done to determine the optimal intake of vitamin C for children, women, the elderly, those under stress or exposed to environmental toxins, and those suffering from chronic or acute illness.
Dr. Albert Szent-Gyorgyi, 1937 Nobel laureate in Physiology or Medicine for discoveries of biological oxidation processes, confided to Linus Pauling in 1966 that he took about 1,000 mg/day of supplemental vitamin C. Dr. Pauling himself took much larger doses. Both Szent-Gyorgyi and Pauling recognized the importance of vitamins many decades ago--in 1938, Linus Pauling said that investigation of substances manufactured in the body or ingested in food would have "transcendent significance to mankind," and in 1939, Szent-Gyorgyi wrote, "Vitamins, if properly understood and applied, will help us to reduce human suffering to an extent which the most fantastic mind would fail to imagine." Based on new experimental evidence, we may be well on the way to proving them right.
For information on vitamin C and its role in health and disease, see the Linus Pauling's Institute's Micronutrient Information Center.
Last updated May, 1997
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