Leave no trace of cognitive decline Original paper

Environmental exposure to trace elements occurs worldwide, and there is some evidence suggesting this long-term exposure plays a role in the development of neurodegenerative diseases. Fingernails retain a high content of most trace elements, making them useful for cost-effective biomonitoring.

In this 5-year cohort study, 20 elderly individuals from 2 Portuguese geographic areas of interest (one next to a vast chemical complex, the other near numerous closed mines) were biomonitored for trace elements through fingernail analysis. Neuropsychological assessments (Mini Mental State Examination and Montreal Cognitive Assessment) were completed at baseline and 5 years. This data was then compared to an education-matched control group of 20 elderly Portuguese individuals who completed the same cognitive assessments as part of a separate cohort.

At 5 years, the neuropsychological scores had declined more in the study group. Moreover, the study group had more “dementia-like” performances compared to the estimated prevalence in the Portuguese population. Fingernail findings for trace elements were generally within the referenced values for non-exposed individuals (i.e., no toxicity was detected). Interestingly, less cognitive decline was found in individuals with higher levels of nickel or selenium in their fingernails.

There were a few limitations to the study group, one being dropouts. The original study group had 76 individuals, but 42 passed away before the five-year mark and 14 others simply failed to participate in the follow-up tests. Additionally, while attempts were made to use a control group that closely matched the final study group, the former was 9 years younger (on average) than the latter. There was also no fingernail data for the control group.

The nickel finding is somewhat surprising, as nickel exposure is usually associated with neurotoxicity. However, given that the “high” nickel levels found in this study were still within normal (nontoxic) range, it may be that nickel has beneficial effects on cognition only at a certain concentration level. The selenium finding, on the other hand, lines up with previous research. Low selenium levels are reported in Alzheimer's disease patients and may increase the risk of cognitive impairment.