<P> Vitamin C distributes readily in high concentrations into immune cells, has antimicrobial and natural killer cell activities, promotes lymphocyte proliferation, and is consumed quickly during infections, effects indicating a prominent role in immune system regulation . 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 . </P> <P> 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 or high risk (smokers and asbestos - exposed) people . A 2014 meta - analysis found that vitamin C intake might protect against lung cancer risk . 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 of 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 . </P> <P> 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 (mmol / L), 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 (H O) 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 . Research continues in this field, but a 2014 review concluded: "Currently, the use of high - dose intravenous vitamin C (as an anticancer agent) cannot be recommended outside of a clinical trial ." 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 ." </P> <P> A 2013 meta - analysis found no evidence that vitamin C supplementation reduces the risk of myocardial infarction, stroke, cardiovascular mortality, or all - cause mortality . However, a second analysis found an inverse relationship between circulating vitamin C levels or dietary vitamin C and the risk of stroke . </P>

Which of the following uses structure in c