Vitamin C, also known as ascorbic acid and ascorbate, is a vitamin found in various foods and sold as a dietary supplement. It also functions as an antioxidant. There is, however, some evidence that regular use may shorten the length of colds. It is unclear whether supplementation affects the risk of cancer, cardiovascular disease, or dementia. It may be taken by mouth or by injection. The United States Institute of Medicine recommends against taking large doses. Vitamin C was discovered in 1912, isolated in 1928, and in 1933, was the first vitamin to be chemically produced. Vitamin C is available as an inexpensive generic and over-the-counter medication. Partly for its discovery, Albert Szent-Györgyi and Walter Norman Haworth were awarded the 1937 Nobel Prizes in Physiology and Medicine and Chemistry, respectively.
Foods containing vitamin C include citrus fruits, kiwifruit, broccoli, Brussels sprouts, raw bell peppers, and strawberries. In plants, vitamin C is a substrate for ascorbate peroxidase. This enzyme utilizes ascorbate to neutralize excess hydrogen peroxide by converting it to water . Deficiency Scurvy is a disease resulting from a deficiency of vitamin C. Without this vitamin, collagen made by the body is too unstable to perform its function and several other enzymes in the body do not operate correctly.
The human body can store only a certain amount of vitamin C, Uses Vitamin C has a definitive role in treating scurvy, which is a disease caused by vitamin C deficiency. Beyond that, a role for vitamin C as prevention or treatment for various diseases is disputed, with reviews reporting conflicting results. A 2012 Cochrane review reported no effect of vitamin C supplementation on overall mortality. It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system. Researchers continued to work on this question, but research interest and public interest spiked after Linus Pauling, two-time awardee of the Nobel Prize, started publishing research on the topic and also published a book "Vitamin C and the Common Cold" in 1970. A revised and expanded edition "Vitamin C, the Common Cold and the Flu" was published in 1976. The authors of the Cochrane review concluded that "...given the consistent effect of vitamin C on the duration and severity of colds in the regular supplementation studies, and the low cost and safety, it may be worthwhile for common cold patients to test on an individual basis whether therapeutic vitamin C is beneficial for them." The European Food Safety Authority found a cause and effect relationship exists between the dietary intake of vitamin C and functioning of a normal immune system in adults and in children under three years of age.
Cancer There are two approaches to the question of whether vitamin C has an impact on cancer. First, within the normal range of dietary intake without additional dietary supplementation, are people who consume more vitamin C at lower risk for developing cancer, and if so, does an orally consumed supplement have the same benefit? Second, for people diagnosed with cancer, will large amounts of ascorbic acid administered intravenously treat the cancer, reduce the adverse effects of other treatments, and so prolong survival and improve quality of life?
A 2013 Cochrane review found no evidence that vitamin C supplementation reduces the risk of lung cancer in healthy people or those at high risk due to smoking or asbestos exposure. A second meta-analysis found no effect on the risk of prostate cancer. Two meta-analyses evaluated the effect of vitamin C supplementation on the risk of colorectal cancer. One found a weak association between vitamin C consumption and reduced risk, and the other found no effect from supplementation.
A 2011 meta-analysis failed to find support for the prevention of breast cancer with vitamin C supplementation, but a second study concluded that vitamin C may be associated with increased survival in those already diagnosed.
Under the rubric of orthomolecular medicine, "Intravenous vitamin C is a contentious adjunctive cancer therapy, widely used in naturopathic and integrative oncology settings." With oral administration absorption efficiency decreases as amounts increase. Intravenous administration bypasses this. Doing so makes it possible to achieve plasma concentrations of 5 to 10 millimoles/liter, which far exceed the approximately 0.2 mmol/L limit from oral consumption. The theories of mechanism are contradictory. At high tissue concentrations ascorbic acid is described as acting as a pro-oxidant, generating hydrogen peroxide to kill tumor cells. The same literature claims that ascorbic acid acts as an antioxidant, thereby reducing the adverse effects of chemotherapy and radiation therapy. A 2015 review added: "There is no high-quality evidence to suggest that ascorbate supplementation in cancer patients either enhances the antitumor effects of chemotherapy or reduces its toxicity.
Evidence for ascorbate's anti-tumor effects was limited to case reports and observational and uncontrolled studies." Cardiovascular disease A 2013 meta-analysis found no evidence that vitamin C supplementation reduces the risk of myocardial infarction, stroke, cardiovascular mortality, or all-cause mortality. A meta-analysis of 44 clinical trials has shown a significant positive effect of vitamin C on endothelial function when taken at doses greater than 500 mg per day. The endothelium is a layer of cells that line the interior surface of blood vessels. Endothelial dysfunction is implicated in many aspects of vascular diseases. The researchers noted that the effect of vitamin C supplementation appeared to be dependent on health status, with stronger effects in those at higher risk of cardiovascular disease. Brain function A 2017 systematic review found lower vitamin C concentrations in people with cognitive impairment, including Alzheimer's disease and dementia, compared to people with normal cognition. The cognitive testing, however, relied on the Mini-Mental State Examination, which is only a general test of cognition, indicating an overall low quality of research assessing the potential importance of vitamin C on cognition in normal and impaired people. Other diseases Studies examining the effects of vitamin C intake on the risk of Alzheimer's disease have reached conflicting conclusions. Maintaining a healthy dietary intake is probably more important than supplementation for achieving any potential benefit. A 2010 review found no role for vitamin C supplementation in the treatment of rheumatoid arthritis.
Vitamin C supplementation does not prevent or slow the progression of age-related cataract. Side effects Vitamin C is a water-soluble vitamin, with dietary excesses not absorbed, and excesses in the blood rapidly excreted in the urine, so it exhibits remarkably low acute toxicity. Other symptoms reported for large doses include nausea, abdominal cramps and diarrhea. These effects are attributed to the osmotic effect of unabsorbed vitamin C passing through the intestine. "Reports of kidney stone formation associated with excess ascorbic acid intake are limited to individuals with renal disease". although one large, multi-year trial did report a nearly two-fold increase in kidney stones in men who regularly consumed a vitamin C supplement.
