So-called “antioxidant-rich foods” are often marketed as a preventive treatment of cognitive decline. While the hype on the potential benefits of antioxidant-rich food often extends beyond the evidence, there is reason to think that antioxidant supplementation may some day be found to be beneficial for cognitive health. Oxidative stress (basically, an imbalance in the metabolism of oxygens in the body) appears likely to contribute to a variety of age-related diseases such as Alzheimer’s Disease and cardiovascular disease; what remains to be seen is whether supplemental antioxidants can be productively used against oxidative stress in the brain. A recent review in the European Journal of Nutrition reviewed the existent research base on the potential cognitive benefits of dietary antioxidants, finding that there is little evidence for the use of dietary antioxidants and a need for better data on the role of antioxidants, diet, and oxidative stress.
The review focused on cohort studies of cognitive decline that measured the intake of at least one relevant dietary antioxidant in a population-based human sample. Studies included in the review examined antioxidants consumed as supplements or as part of participants’ diets, and measured changes in cognitive test scores at two or more points. Out of a pool of 850 potentially relevant studies, only 10 published studies met the criteria, and examined the association between cognitive decline and antioxidants including carotenes (such as beta-carotene, found in carrots and sweet potatoes), flavonoids (compounds found in citrus, berries, tea, and chocolate), selenium (found in nuts, meat, and many other high-protein foods), and vitamins C and E.
For the analysis, the authors evaluated the methodological quality of the relevant studies (such as how dietary intake was assessed, length of time between assessments, and statistical controls for relevant associated variables) and tabulated the results of the studies. Overall, studies were mixed in terms of outcomes and findings. While some studies (which often draw significant attention from popular media) suggest that dietary antioxidants can be beneficial, other studies often conflict with these findings. There was no category of antioxidants that were consistently seen to be beneficial. For example, one study found that individuals with high levels of dietary flavonoids experienced lower rates of cognitive decline, while the other flavonoid study included in the review reached the opposite conclusion.
The authors stated that two particularly high-quality studies found some evidence in support of dietary antioxidants. A study that measured selenium levels in blood samples found that a decrease in plasma selenium was associated with steeper declines in cognition and motor speed over a period of nine years, and a study based on self-reported intake of vitamins C and E found that subjects with high vitamin intakes had slower rates of cognitive decline after three years. However, benefits were not sustained for longer than three years in the vitamin study, and the blood-based selenium study did not assess participants’ diet or supplement intake.
In sum, there is insufficient evidence to recommend the use of specific nutritional antioxidant supplements as a preventive measure against cognitive decline, with the important caveat that deficient diets heighten the risk of cognitive and functional decline. Based on the existing findings, the researchers recommended that further study be conducted on some of the more promising antioxidants, which may someday be shown to be beneficial against cognitive decline.
Rafnsson SB, Dilis V, and Trichopoulou A. Antioxidant nutrients and age-related cognitive decline: a systematic review of population-based cohort studies. European Journal of Nutrition (2013). 52: 1553–1567. DOI: 10.1007/s00394-013-0541-7