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. A 2015 meta-analysis showed no antitumor effect nor improvement in quality-of-life measures from high-dose vitamin C. This review included trials of oral and intravenous vitamin C. Cardiovascular disease In 2017, an independent study evaluating 15,445 participants found no evidence to show vitamin C decreases the risk cardiovascular disease. These results supported one 2013 review which found no evidence that antioxidant vitamin supplementation reduces the risk of myocardial infarction, stroke, cardiovascular mortality, or all-cause mortality (it did not provide subset analysis for trials that just used vitamin C). However, another 2013 review found an association between higher circulating vitamin C levels or dietary vitamin C and a lower risk of stroke. A 2014 review found a positive effect of vitamin C on endothelial dysfunction 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. 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. A review of nutrient status in people with Alzheimer's disease reported low plasma vitamin C, but also low blood levels of folate, vitamin B12, and vitamin E. Iron deficiency One of the causes of iron-deficiency anemia is reduced absorption of iron. Iron absorption can be enhanced though ingestion of vitamin C alongside iron-containing food or supplements. The instability of vitamin C during cooking and/or storage limits the number of foods suitable for this. Vitamin C helps to keep iron in the reduced ferrous state, which is more soluble and more easily absorbed. 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