Vitamin B12 deficiency is known to cause cognitive impairment and dementia in elderly individuals. Whether less severe variations in vitamin B intake contribute to variations in cognitive function in well-nourished populations is unclear. In prospective observational studies, elevated blood levels of homocysteine have been associated with an increased risk of cardiovascular disease, as well as Alzheimer's disease and other forms of dementia, although the findings have not been consistent. Supplementation with B vitamins, including folic acid, vitamin B6, and vitamin B12, is an effective method for reducing homocysteine levels and may thus help prevent cardiovascular disease, cognitive decline, and dementia. B vitamins may also prevent these conditions by additional mechanisms other than lowering homocysteine. Large-scale prevention trials of vitamin B supplementation are lacking, however.
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Antioxidant vitamins such as vitamin E, vitamin C, and beta-carotene may be important in protecting the brain from oxidant injury. Many large prospective observational studies suggest that high intakes of, or high blood concentrations of, these antioxidants may help protect against cognitive decline or dementia. However, evidence from randomized clinical trials that antioxidants promote cognitive vitality or prevent dementia is lacking. Several relevant trials are ongoing, however, and should help resolve this question. It is worth noting that there is a divergence of findings from observational studies and randomized clinical trials on the role of vitamin E, the most intensively studied of the antioxidant vitamins in epidemiologic settings to date, in cardiovascular disease prevention. Although many prospective observational studies suggest that a high vitamin E intake may prevent cardiovascular disease, randomized trials have failed to find that vitamin E supplementation reduces the risk of future cardiovascular events in primary or secondary prevention settings.
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Essential fatty acids include the long-chain omega-3 and omega-6 polyunsaturated fatty acids (PUFAs). The main omega-3 fatty acid is alpha-linolenic acid (ALA), found in certain plant oils, and its derivatives eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are found in fish. The main omega-6 fatty acid is linoleic acid (LA). Laboratory, epidemiologic, and intervention studies suggest that the ratio of omega-6 to omega-3 fatty acids may affect cardiovascular and cognitive health. This ratio is approximately 20:1 in the typical U.S. diet; however, the optimal ratio is thought to lie somewhere between 1:1 and 4:1. For most Americans, this means greatly reducing the omega-6 fatty acids they consume, and increasing the number of omega-3 fatty acids. Though sparse, data from large-scale epidemiologic studies suggest that PUFA intake may play a role in preventing cognitive dysfunction or dementia.
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Prospective observational studies suggest that light-to-moderate alcohol use may prevent cognitive decline and dementia.
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