Reliable
and relatively consistent scientific data showing a substantial health benefit. Contradictory, insufficient, or preliminary studies
suggesting a health benefit or minimal health benefit. For an herb, supported by traditional use but minimal
or no scientific evidence. For a supplement, little scientific support and/or minimal health
benefit.
Who is likely to be deficient?
Although scurvy (severe vitamin C deficiency) is uncommon in Western societies, many
doctors believe that most people consume less than optimal amounts. Fatigue, easy bruising, and bleeding gums are early signs of vitamin
C deficiency that occur long before frank scurvy develops. Smokers have low levels of vitamin
C and require a higher daily intake to maintain normal vitamin C levels. Women with preeclampsia have been found to have lower blood
levels of vitamin C than women without the condition.1 Women who have lower blood
levels of vitamin C have an increased risk of
gallstones.2
People with kidney failure have an increased risk of vitamin C deficiency.3
However, people with kidney failure should take vitamin C only under the supervision of a
doctor.
How much is usually taken?
The recommended dietary allowance (RDA) for vitamin C in nonsmoking adults is 75 mg per day
for women and 90 mg per day for men. For smokers, the RDAs are 110 mg per day for women and
125 mg per day for men. Most clinical vitamin C studies have investigated the effects of a
broad range of higher vitamin C intakes (100–1,000 mg per day or more), often not
looking for (or finding) the “optimal” intake within that range. In terms of heart disease prevention, as little as
100–200 mg of vitamin C appears to be adequate.4 Although some doctors
recommend 500–1,000 mg per day or more, additional research is needed to determine
whether these larger amounts are necessary. Some vitamin C experts propose that adequate
intake be considered 200 mg per day because of evidence that the cells of the human body do
not take up any more vitamin C when larger daily amounts are used.5
Some scientists have recommended that healthy people take multi-gram amounts of vitamin C
for the prevention of illness. However, little or no research supports this point of view and
it remains controversial. Supplementing more results in an excretion level virtually identical
to intake, meaning that consuming more vitamin C does not increase the amount that remains in
the body.6 On the basis of extensive analysis of published vitamin C studies,
researchers at the Linus Pauling Institute at Oregon State University have called for the RDA
to be increased, but only to 120 mg.7 This same report reveals that “. . .
90–100 mg vitamin C per day is required for optimum reduction of chronic disease risk in
nonsmoking men and women.” Thus, the multiple gram amounts of vitamin C taken by many
healthy people may be superfluous.
The studies that ascertained approximately 120–200 mg daily of vitamin C is correct
for prevention purposes in healthy people have typically not investigated whether people
suffering from various diseases can benefit from larger amounts. In the case of the common cold, a review of published trials found that
amounts of 2 grams per day in children appear to be more effective than 1 gram per day in
adults, suggesting that large intakes of vitamin C may be more effective than smaller amounts,
at least for this condition.8
Are there any side effects or interactions?
Some people develop diarrhea after as
little as a few grams of vitamin C per day, while others are not bothered by ten times this
amount. Strong scientific evidence to define and defend an upper tolerable limit for vitamin C
is not available. A review of the available research concluded that high intakes (2–4
grams per day) are well-tolerated by healthy people.9 However, intake of large
amounts of vitamin C can deplete the body of
copper1011 —an essential nutrient. People should be sure to
maintain adequate copper intake at higher intakes of vitamin C. Copper is found in many multivitamin-mineral supplements. Vitamin C
increases the absorption of iron and should be
avoided by people with iron overload diseases (e.g., hemochromatosis, hemosiderosis). Vitamin
C helps recycle the antioxidant, vitamin E.
It is widely (and mistakenly) believed that mothers who consume large amounts of vitamin C
during pregnancy are at risk of giving birth
to an infant with a higher-than-normal requirement for the vitamin. The concern is that the
infant could suffer “rebound scurvy,” a vitamin C deficiency caused by not having
this increased need met. Even some medical textbooks have subscribed to this
theory.12 In fact, however, the concept of “rebound scurvy” in infants
is supported by extremely weak evidence.13 Since the publication in 1965 of the
report upon which this mistaken notion is based, millions of women have consumed high amounts
of vitamin C during pregnancy and not a single new case of rebound scurvy has been
reported.14
A preliminary study found that people who took 500 mg per day of vitamin C supplements for
one year had a greater increase in wall thickness of the carotid arteries (vessels in the neck
that supply blood to the brain) than those who did not take vitamin C.15 Thickness
of carotid artery walls is an indicator of progression of atherosclerosis. Currently, no evidence supports a
cause-and-effect relationship for the outcome reported in this study. The vast preponderance
of research suggests either a protective or therapeutic effect of vitamin C for heart disease, or no effect at all.
People with the following conditions should consult their doctor before
supplementing with vitamin C: glucose-6-phosphate dehydrogenase deficiency, iron overload
(hemosiderosis or hemochromatosis), history of kidney stones, or kidney failure.
It has been suggested that people who form calcium oxalate kidney stones should avoid vitamin C supplements,
because vitamin C can be converted into oxalate and increase urinary oxalate.1617 Initially, these concerns were questioned because of potential errors in the
laboratory measurement of oxalate.1819 However, using newer methodology
that rules out this problem, recent evidence shows that as little as 1 gram of vitamin C per
day can increase the urinary oxalate levels in some people, even those without a history of
kidney stones.2021 In one case, 8 grams per day of vitamin C led to
dramatic increases in urinary oxalate excretion and kidney stone crystal formation causing
bloody urine.22 People with a history of kidney stones should consult a doctor
before taking large amounts (1 gram or more per day) of supplemental vitamin C.
Despite possible therapeutic effects of vitamin C in people with diabetes at lower intakes, one case of
increased blood sugar levels was reported after taking 4.5 grams per
day.23
Are there any drug
interactions?
Certain medicines may interact with vitamin C. Refer to drug interactions for a list of those medicines.
References (To view, roll mouse over the "References" heading; to hide, click on the heading)
1. Kharb S. Total free radical trapping antioxidant potential in
pre-eclampsia. Int J Gynaecol Obstet 2000;69:23–6.
2. Simon JA, Hudes ES. Serum ascorbic acid and gallbladder disease
prevalence among US adults. Arch Intern Med 2000;160:931–6.
3. Makoff R. Vitamin replacement therapy in renal failure patients.
Miner Electrolyte Metab 1999;25:349–51 [review].
4. Balz F. Antioxidant Vitamins and Heart Disease. Presented at the 60th
Annual Biology Colloquium, Oregon State University, February 25, 1999.
5. Levine M, Rumsey SC, Daruwala R, et al. Criteria and recommendations
for vitamin C intake. JAMA 1999;281:1415–23.
6. Levine M, Conry-Cantilena C, Wang Y, et al. Vitamin C pharmacokinetics
in healthy volunteers: evidence for a recommended dietary allowance. Proc Natl Acad
Sci 1996;93:3704–9.
7. Carr AC, Frei B. Toward a new recommended dietary allowance for
vitamin C based on antioxidant and health effects in humans. Am J Clin Nutr
1999;69:1086–107.
8. Hemilä H. Vitamin C supplementation and common cold symptoms:
factors affecting the magnitude of the benefit. Med Hypotheses 1999;52:171–8
[review].
9. Johnston CS. Biomarkers for establishing a tolerable upper intake
level for vitamin C. Nutr Rev 1999;57:71–7.
10. Sandstead HH. Copper bioavailability and requirements. Am J Clin
Nutr 1982;35:809–14 [review].
11. Finley EB, Cerklewski FL. Influence of ascorbic acid supplementation
on copper status in young adult men. Am J Clin Nutr 1983;37:553–6.
12. Wilson JD. Vitamin deficiency and excess. In Fauci AS, Braunwald E,
Isselbacher KJ, et al. (eds). Harrison’s Principles of Internal Medicine, 14th
ed. New York, McGraw Hill, 1998, 487.
13. Cochrane WA. Overnutrition in prenatal and neonatal life: a problem?
Can Med Assoc J 1965;93:893–9.
14. Gaby AR. The myth of rebound scurvy. Townsend Letter for
Doctors 2000;June:122.
15. Dwyer J, Nicholson LM, Shircore A, et al. Vitamin C intake and
progression of carotid atherosclerosis. The Los Angeles Atherosclerosis Study. American
Heart Association Annual Meeting. March 2, 2000 [abstract].
16. Piesse JW. Nutritional factors in calcium containing kidney stones
with particular emphasis on vitamin C. Int Clin Nutr Rev 1985;5:110–29
[review].
17. Ringsdorf WM, Cheraskin WM. Medical complications from ascorbic acid:
a review and interpretation (part one). J Holistic Med 1984;6:49–63.
18. Hoffer A. Ascorbic acid and kidney stones. Can Med Assoc J
1985;32:320 [letter].
19. Wandzilak TR, D’Andre SD, Davis PA, Williams HE. Effect of high
dose vitamin C on urinary oxalate levels. J Urol 1994;151:834–7.
20. Levine M. Vitamin C and optimal health. Presented at the February 25,
1999 60th Annual Biology Colloquium, Oregon State University, Corvallis, Oregon.
21. Levine M, Conry-Cantilena C, Wang Y, et al. Vitamin C
pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. Proc
Natl Acad Sci 1996;93:3704–9.
22. Auer BL, Auer D, Rodgers AL. Relative hyperoxaluria, crystalluria and
haematuria after megadose ingestion of vitamin C. Eur J Clin Invest
1998;28:695–700.
23. Branch DR. High-dose vitamin C supplementation increases plasma
glucose. Diabetes Care1999;22:1218 [letter].
The information presented in Aisle7 is for informational purposes only.
It is based on scientific studies (human, animal, or in vitro), clinical experience,
or traditional usage as cited in each article. The results reported may not necessarily occur
in all individuals. For many of the conditions discussed, treatment with prescription or over
the counter medication is also available. Consult your doctor, practitioner, and/or pharmacist
for any health problem and before using any supplements or before making any changes in
prescribed medications.
