Bottom Line: Vitamin C is important for maintaining general health but it may reduce the effectiveness of chemotherapy drugs. There is no proof that high doses of vitamin C can treat cancer.
Vitamin C is a known antioxidant. This means that it reacts with and neutralizes free radicals in the body that can cause cellular and DNA damage. It is also important for the production of many essential molecules in the body, including proteins. Vitamin C plays an important role in the formation of collagen, which is the main structural protein in the skin. Therefore, vitamin C helps promote wound healing. Since the body cannot synthesize vitamin C, it must be gained from outside sources. In addition, scientists think that vitamin C might help reduce inflammation. However, vitamin C can make many chemotherapy drugs less effective. Patients should speak with their doctor before using vitamin C.
To treat asthma A handful of small clinical trials have tested the short-term effectiveness of vitamin C in protecting against exercise-induced asthma, with conflicting results. A small clinical trial found vitamin C helpful in protecting against air pollutant-induced asthma.
To improve their athletic performance Clinical trials generally show moderate benefits, including a reduction of muscle soreness.
To prevent and treat the common cold and upper respiratory infections Numerous clinical trials have shown that high-dose vitamin C (~3 grams per day) does not prevent the common cold and other upper respiratory infections, but can reduce the duration and severity of symptoms.
To prevent and treat cancer Clinical trials in cancer patients show no benefit from high-dose vitamin C. Vitamin C can make many chemotherapy drugs less effective.
To treat heart disease High dietary intake and blood levels of vitamin C have been associated with lower risks of heart disease. But data from clinical trials show that vitamin C does not help prevent heart disease.
To treat cataracts Long-term clinical trials show that antioxidant intake has no effect on development of cataracts or age-related visual loss.
To lower high blood pressure Clinical trials support this use.
To treat Parkinson's disease One small pilot study conducted 10 years ago showed that vitamins C and E could slow the progression of Parkinson's disease, but larger trials are needed to support these results.
To improve wound healing Clinical trials indicate that intravenous vitamin C benefits severely burned patients. Other clinical trials show conflicting results regarding the use of vitamin C for pressure sores or other wounds. Deficiencies in vitamin C are known to impair wound healing.
Cancer treatment: In a 1979 clinical trial, researchers evaluated the hypothesis that high-dose oral vitamin C might be effective in treating advanced cancer. One hundred and twenty three patients with advanced solid tumors that had not responded to previous therapy enrolled in the study and were randomly assigned to take either 10 grams vitamin C or a placebo pill daily. At the end of the study, patients taking vitamin C had the same average survival (7 weeks), similar symptoms, and equal performance status as those taking the placebo pill, indicating that high-dose vitamin C is not effective in treating advanced cancers.
A second clinical trial in 1985 examined the idea that high-dose oral vitamin C could treat cancer. One hundred patients with advanced colorectal cancer in good condition who had not received any prior chemotherapy were randomly assigned to take either 10 grams of vitamin C or a placebo pill daily. They followed this therapy until their cancer progressed or until they were no longer able to oral medications. There was no significant difference between patients taking vitamin C and those taking placebo in terms of time to progression or overall survival. In addition, no patients taking vitamin C experienced tumor regression. These results further refute the hypothesis that high-dose vitamin C can treat cancer.
A recent review of 14 clinical trials shows that supplementation with antioxidants B-carotene, vitamins A, C, and E does not seem to prevent gastrointestinal cancer and may actually increase overall mortality.
Alzheimer's disease: A population study in the Netherlands followed 5393 people aged 55+ for six years to compare their dietary antioxidant intake (vitamins C and E, flavonoids, and beta-carotene) to their risk of developing Alzheimer's disease. After statistical analysis that took into account risk factors such as age, alcohol intake, and smoking, people who had a high dietary intake of vitamins C and E were found to have a lower risk of Alzheimer's disease. However, it is likely that some of these associations may be due to chance alone. In addition, Alzheimer's disease develops over a course of 15+ years, indicating that following patients for only six years is not long enough. Most of the 146 patients probably had already begun developing the disease before the study even began, which in turn could adversely affect their diet.
Common cold: A 1975 study evaluated the well-known theory that vitamin C can prevent and treat the common cold. Three grams of vitamin C or an identical placebo were randomly given daily to 190 otherwise healthy volunteers. If a cold was suspected in a volunteer, they were randomly given an additional 3 grams of vitamin C or placebo. No significant adverse events were reported during the study. From study results, vitamin C was not able to prevent colds, but did cause a small but significant reduction in their duration and severity.
This product is regulated by the FDA as a dietary supplement. Unlike approved drugs, supplements are not required to be manufactured under specific standardized conditions. This product may not contain the labeled amount or may be contaminated. In addition, it may not have been tested for safety or effectiveness.
High dose vitamin C supplementation may alter the results of urine glucose testing strips, and may cause a false negative guaiac (blood in stool) test.
Large doses of vitamin C may lead to copper deficiency.
Vitamin C increases the absorption, transport, and storage of iron.
You have kidney impairment or are on chronic hemodialysis
You have hemochromatosis (Vitamin C increases iron absorption, transport, and storage in the body.)
You have known G6PDH (glucose-6-dehydrogenase) deficiency
You are a cancer patient undergoing radiation therapy or chemotherapy (These therapies use free radicals to kill cancer cells, and vitamin C or other antioxidants may neutralize these free radicals and lessen their effect).
Recent laboratory experiments and animals studies suggest that cancer cells preferentially uptake vitamin C. This raises the possibility that high-dose vitamin C may be detrimental to cancer patients undergoing radiation or chemotherapy.
It is controversial whether antioxidants like vitamin C can lessen or negate the effects of chemotherapy and radiation therapy. Because these therapies work by creating free radicals that kill cancer cells, some physicians have suggested that high levels of antioxidants can neutralize these free radicals and thereby protect cancer cells from these therapies. So what protects healthy cells may protect cancer cells as well. This question is still not fully understood and patients who are interested in taking more than the RDA of any antioxidant should consult with their doctor.
Vitamin C, also known as ascorbic acid, is a water-soluble vitamin that is found in many fresh fruits and vegetables. It is an essential vitamin that is not synthesized in humans and deficiency has been linked to scurvy. Vitamin C is also necessary for a variety of physiologic functions that include formation of collagen, catecholamines, and carnitine in addition to tyrosine metabolism, and peptide synthesis (1). Vitamin C is also one of the commonly used dietary supplements for immune stimulation, for wound healing, as an antioxidant, and for cancer prevention.
Vitamin C is believed to promote cardiovascular health but findings from the recent Physicians' Health Study II indicate no such benefit for preventing cardiovascular disease (25). Furthermore, supplementation with vitamins C and E increased risk of mortality alnd nonfatal myocardial infarction in patients with coronary artery disease (11). It is not clear if vitamin C helps reduce blood pressure as data are conflicting (12)(13). Vitamin C may reduce the severity and duration of respiratory infections in otherwise healthy patients, although no prophylactic benefit has been demonstrated. It may also offer protection against Alzheimer's disease (10)(15).
The role of vitamin C in cancer treatment and prevention is unclear. Results from in vitro and animal studies showed that cancer cells preferentially uptake vitamin C (7)(24). But supplementation with vitamins C, E, and beta carotene was not beneficial in preventing cancer incidence or affecting cancer mortality (26). Further, supplementation with vitamin C along with vitamins A E, and beta-carotene did not prevent gastrointestinal cancer (18), did not lower the risk of prostate cancer (21), and may actually increase overall mortality (18). A study done in end-stage cancer patients showed no benefit from high-dose vitamin C (14). Use of intravenous vitamin C has been examined in cancer patients (22) and clinical trials are underway to assess its efficacy (23).
Vitamin C can render many chemotherapy drugs less effective (24). However, inadequate amounts of antioxidants including vitamin C were associated with increased adverse effects due to chemotherapy in children with acute lymphoblastic leukemia (19). Other potential adverse effects are gastrointestinal in nature, although hypoglycemia and hypotension are documented with doses higher than one gram per day. Patients with history of oxalate kidney stones, renal insufficiency, G6PDH deficiency, hematochromatosis, or those undergoing chemotherapy should consult their physicians before taking vitamin C supplements.
A water-soluble antioxidant, vitamin C reacts directly with superoxide, hydroxyl radicals, and singlet oxygen molecules. Ascorbic acid promotes wound healing by serving as a reductant in the pathway for the formation and polymerization of collagen. Vitamin C deficiency thus results in impaired wound healing and scurvy (1). Vitamin C is important for the synthesis of catecholamines and carnitine, in addition to the conversion of folic acid to folinic acid. It is also involved with tyrosine metabolism and peptide synthesis. During a state of infection, vitamin C may reduce inflammation caused by oxidizing chemicals released by phagocytic leukocytes following activation (6). Vitamin C also may inhibit free radical inactivation of nitric oxide, thereby improving endothelium-dependent vasodilation (8). Dehydroascorbic acid, the oxidized form of vitamin C, may have cytoprotective effect by preserving mitochondrial membrane potential (24).
Absorption: Intestinal absorption occurs through a sodium-dependent active transport process that is saturable and dose-dependent. Approximately 70-90 percent of the usual dietary intake of ascorbic acid (30-180 mg/day) is absorbed, although absorption falls to 50 percent or less with increasing doses above 1 g/day. The bioavailability of vitamin C from food or supplements is not significantly different. Distribution: Dehydroascorbic acid is the form of the primarily crosses the membranes of cells, after which it is reduced intracellularly to ascorbic acid. High levels of the vitamin are maintained in the pituitary and adrenal glands, leukocytes, eye tissue and humors, and the brain, while relatively low levels are found in plasma and saliva. Metabolism / Excretion: Due to homeostatic regulation, the biological half-life varies widely from 8 to 40 days and is inversely related to body pools. Renal excretion increases proportionally with higher intakes. With large intakes of the vitamin, unabsorbed ascorbate is degraded in the intestine, a process that may account for the diarrhea and intestinal discomfort occasionally reported by people ingesting large doses. (4)
Common: Nausea, diarrhea, stomach cramps and possible hypoglycemia and hypotension with doses greater than 1 gram. Reported (oral): In patients with a history of kidney stones, increased oxalate kidney stone formation occurs with possible nephrolithiasis, acute renal failure, or renal insufficiency. In patients with G6PDH deficiency, hemolytic anemia may occur at high doses (1). Reported (oral): Severe cyanide poisoning following ingestion of 3 grams of amygdalin with concurrent use of 4800 mg of vitamin C per day (20). Toxicity: Excessive use of chewable tablets may break down tooth enamel, increasing the risk of dental caries (3)
Iron: Ascorbic acid increases iron absorption and modulates transport and storage in the body. Significant in patients with hematochromatosis. (3)(4) Chemotherapy Drugs: Vitamin C can reduce the effectiveness of many antineoplastic agents, include vincristine, doxorubicin, methotrexate, cisplatin, and imatinib. (24)
High dose ascorbic acid supplementation may alter results of urine glucose testing strips, and may cause a false negative guaiac (occult blood) test (5).
Sesso, HD, Buring JE, Christen WG, et al. Vitamins E and C in the prevention of cardiovascular disease in men. The Physicians' health study II Randomized Controlled Trial. JAMA. 2008;300(18):2123-2133. This long-term study sought to evaluate the role of vitamin C or E supplementation in reducing the risk of cardiovascular disease, and involved over 14,000 men aged 50 or older. The participants were randomized to 400 IU of vitamin E every other day or its placebo and 500 mg of vitamin C daily or its placebo. The primary outcome of the study was onset of major cardiovascular events including, nonfatal myocardial infarction, nonfatal stroke, and death from cardiovascular disease. At the 8-year follow-up, researchers observed the same number of cases in both the vitamin and the placebo groups, with increased risk of hemorrhagic stroke in the vitamin E group. These findings suggest no significant effects of vitamins in lowering the risk of cardiovascular disease as widely believed. It should be noted that both the vitamins used in the study were synthetic and at much higher levels than those achieved via dietary intake, which may have led to the effects observed. More studies at different doses and involving other populations are warranted to fully assess the role of vitamin supplementation for cardiovascular health.
Engelhart MJ, et al. Dietary intake of antioxidants and risk of Alzheimer disease. JAMA 2002;287:3223-9. A population-based, prospective cohort study evaluating antioxidant intake and risk of developing Alzheimer disease. Subjects (n=5393) were at least 55 years old, free of dementia, noninstitutionalized, and had reliable dietary assessment at baseline. Dietary intake was assessed by self-reported checklist and interview with dietitian using the semiquantitative food-frequency questionnaire (SFFQ). After mean follow-up of 6 years, 146 patients developed Alzheimer disease. After adjustments for age, sex, baseline Mini-Mental State Examination score, alcohol intake, education, smoking habits, pack-years of smoking, body mass index, total energy intake, presence of carotid plaques, and use of supplements, high intake of vitamin C and vitamin E were correlated with lower risk of Alzheimer disease. However, the apparent association may be caused by the influence of a preclinical illness on diet or diet recall/reporting, since the SFFQ itself may indirectly assess cognitive functioning. Furthermore, the multiple comparisons made suggests that some associations may be due to chance and that the critical P value should be set lower than .05.
Kim MK, et. al. Lack of Long-Term Effect of Vitamin C Supplementation on Blood Pressure. Hypertension 2002;40:797-803. A double-blind randomized controlled trial where 224 Japanese subjects in a region with high gastric cancer and stroke mortality levels took either 50 mg or 500 mg per day for five years. The study failed to show any significant decrease in systolic or diastolic blood pressure. Subjects were serologically diagnosed with atrophic gastritis and were slightly less likely to smoke than the general population. Atrophic gastritis or a more health-conscious subject population might have played a role in the outcome, however the size, quality and duration of the trial makes the results compelling.
Bjelokovic G, Nikolova G, Simonetti RG, et al. Antioxidant supplements for preventing gastrointestinal cancers. Cochrane Database Syst Rev 2004;(4):CD004183. This is a systematic review of 14 randomized clinical trials. Participants of the studies did not have gastrointestinal diseases but were at high risk of developing gastrointestinal cancer. The outcome measures were incidence of gastrointestinal cancers, overall mortality, and adverse effects. According to the findings of this study, supplementation with beta-carotene, vitamin A, vitamin C, and vitamin E, when taken alone or in combination do not affect incidence of gastrointestinal cancers. Further, they may increase the overall mortality. The effects of Selenium supplementation need confirmation in future randomized trials. Patients should consult their physicians before initiating vitamin supplementation, especially during and after chemotherapy.
