As researchers continue to investigate cognitive decline in humans, a number of possible risk factors and potential protective factors have been identified. However, a new systematic review has found that there is little evidence to support the associations between certain factors and cognitive decline. The researchers concentrated on two questions originally developed for a conference organized by the Office of Medical Applications of Research at the National Institutes of Health (NIH). First, they wanted to know what factors were associated with reducing the risk of cognitive decline in older adults. Second, they also wanted to investigate if any interventions could either improve or maintain cognitive ability. The researchers selected 127 observational studies, 22 randomized controlled trials, and 16 systematic reviews for the analysis.
Many of the studies defined cognitive decline in different ways, with no one agreed-upon definition. The studies reviewed covered a range of factors, including nutrition; medical considerations and medication; social, economic, and behavioral factors; toxic environmental exposures; and genetics. There was insufficient evidence to support an association between cognitive decline and most of the factors. However, individuals who smoked or had diabetes were more likely to demonstrate cognitive decline in observational studies. Similarly, an increased risk of cognitive decline was also seen with depression and having metabolic syndrome (high blood pressure, high blood sugar, unhealthy cholesterol levels, and excess abdominal fat).
There was also little evidence to support a variety of interventions to improve cognitive ability, including nutritional factors. However, observational studies showed a decreased risk with eating a Mediterranean diet and vegetables. Physical activity was associated with a decreased risk for cognitive decline when both observational and randomized clinical trial data were combined. Cognitive training was associated with decreased risk of cognitive decline in one intervention study.
All of the above findings, except for cognitive training, were classified as having low quality evidence supporting the association between cognitive decline and the specific factor. This means that future research may show that there is no association with the factor and cognitive decline. There was inadequate evidence to assess any associations between cognitive decline and such things as trace metals, fat/high-calorie intake, toxic/environmental exposures, and occupational level. The study was supported in part by the Agency for Healthcare Research and Quality (HS10066).
See "Systematic review: Factors associated with risk for and possible prevention of cognitive decline in later life," by Brenda L. Plassman, Ph.D., John W. Williams, Jr., M.D., M.H.Sc., James R. Burke, M.D., Ph.D., and others in the August 3, 2010, Annals of Internal Medicine 153(3), pp. 182-193 and W66-W69.