Vitamins C and E are essential nutrients for humans. People who consume high levels of these vitamins through their diets have a lower risk of dementia, although it remains unclear whether the key factor is a healthy diet or the specific vitamins. Supplements do not appear to offer the same protection. While vitamin C is generally safe for healthy people, some trials have linked vitamin E supplements with a slightly shorter lifespan and an increased risk of cancer.
Multiple meta-analyses and systematic reviews have examined the effects of vitamins C and E on cognitive functions, but the results very depending on how the study was designed. Our search identified:
• 3 meta-analyses of numerous randomized controlled clinical trials examining the effects of vitamins C and E supplements
• 2 meta-analyses examining the association between cognitive function and dietary intake of vitamins C and E
• 1 randomized controlled study examining the effects of a supplement that includes both vitamins C and E on healthy older adults
• 3 randomized controlled studies examining the effects of vitamin E and other supplements on Alzheimer’s disease patients
• Numerous preclinical studies
In meta-analyses of observational research, a diet rich in vitamins C and E was linked to a 20 to 25 percent lower risk of Alzheimer's disease . However, results from each study vary based on the age and health status of the participants and the methods used to measure vitamin intake and track cognitive function. Some experts argue that the specific vitamins cannot be reliably separated from broader patterns of health and diet (i.e., benefit may be from a healthy diet rather than vitamins alone).
Even if the vitamins are protective, supplements are not a promising preventive measure. In two randomized controlled trials, vitamin E or vitamin E and C supplements did not help patients with mild cognitive impairment (MCI) . Supplementation with both vitamins C and E failed to slow cognitive decline for women over age 65  although another observational study reported more promising results . It is possible that supplements are helpful but only for people who do not consume enough vitamin E or C through their diets. Nevertheless, current evidence suggests that vitamin C or E supplementation, alone or in combination, is unlikely to promote cognition or prevent cognitive decline in most people.
Two observational studies have reported conflicting results on whether vitamins C and E selectively affect APOE4 carriers versus non-carriers . For more information on what the APOE4 gene allele means for your health, read our APOE4 information page.
Vitamin C and E deficiencies have been associated with Alzheimer's disease and dementia, but these could be due to the changes in nutrition and absorption that are common for such patients . Several meta-analyses of clinical trials reported that vitamin E supplementation has very little to no protective effects on cognitive function for dementia patients . However, a 2014 clinical trial of vitamin E and memantine reported that patients with mild-to-moderate Alzheimer’s disease taking 2000 IU/day of vitamin E experienced slower decline in their ability to carry out daily tasks . These results warrant further investigation. Very little is known about the effects of vitamin C supplementation alone.
While vitamins C and E are essential for good health, excessive supplementation carries some risks. Some studies have linked vitamin E supplementation of approximately 225 IU/day to a slightly increased risk of death , although there is conflicting evidence . In men with high selenium levels, vitamin E supplementation at 400 IU/day has been reported to nearly double the risk of prostate cancer .
NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.
A healthy diet naturally rich in vitamins C and E is likely to offer the most benefit. Citrus fruits, sweet peppers, and Brussels sprouts are rich in vitamin C. Vitamin E is plentiful in green leafy vegetables, sunflower seeds, and almonds. Because vitamin C absorption is limited to about 400 mg/day , higher doses are unlikely to be more beneficial.
Vitamin C and E supplements, which have less clear benefit in studies, are available over the counter as pills, chewable tablets, or topical creams. Alpha-tocopherol is the most extensively studied type of vitamin E, but recent research suggests that other vitamin E types, such as gamma-tocopherol and beta-tocotrienol, may also be important for proper health .
Vitamin C: Fact Sheet for Health Professionals, National Institutes of Health, Office of Dietary Supplements
Vitamin E: Fact Sheet for Health Professionals, National Institutes of Health, Office of Dietary Supplements
Article from Scientific American, "The Myth of Antioxidants"