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Published on: June 9, 2019
by Women’s Brain Health Initiative:
Seasonal rhythms are known to affect several aspects of human behaviour and physiology. For example, moods fluctuate throughout the year in individuals with Seasonal Affective Disorder (SAD), and seasonality seems to impact timing of onset in schizophrenia and brain responses to cognitive tasks, as seen with magnetic resonance imaging (MRI) technology.
A group of researchers – Dr. Andrew Lim and colleagues – wondered if the seasons might also have an impact on cognitive function and its underlying neurobiological factors, specifically in older adults. They analyzed data from nearly 3,400 participants from five different study groups in the United States, Canada, and France to explore that possibility, and shared their findings in the September 2018 issue of PLOS Medicine. Their findings included the following:
Cognitive function fluctuated seasonally in healthy older adults.
Healthy older adults who completed cognitive testing in the summer and fall performed better than those who were tested in the winter and spring. “Peak cognition occurred near the fall equinox,” explained the lead author on the study, Dr. Lim of Sunnybrook Health Sciences Centre and the University of Toronto. “The difference in performance between the fall and spring was quite large, equivalent to approximately four years of aging.” These findings were consistent even after the researchers accounted for potential confounding variables such as mood, amount of physical activity, and sleep quality.
Older adults with Alzheimer’s disease experienced seasonal fluctuations in cognitive function, too.
When the researchers analyzed data from participants who had Alzheimer’s disease, a seasonal pattern in cognition was found, similar to the pattern discovered in the healthy older adults. “This suggests that there is a seasonal component to cognition that is preserved during the progression of Alzheimer’s disease,” said Dr. Lim. Individuals with Alzheimer’s disease do not appear to get steadily worse as time goes on; instead, their cognitive abilities seem to fluctuate throughout each year.
The odds of meeting criteria for mild cognitive impairment (MCI) or dementia were higher in the winter/spring.
The seasonal fluctuations observed in cognitive function appeared to translate into an effect on diagnosis of MCI and dementia.
“We found that the odds of meeting diagnostic criteria for mild cognitive impairment or dementia were 30% higher in the winter and spring, compared to the summer and fall,” said Dr. Lim.
Amyloid-beta levels also fluctuate from season to season.
One of the hallmarks of Alzheimer’s disease is the accumulation of amyloid-beta (Aβ) proteins in the brain. Data was available on Aβ levels in the cerebrospinal fluid from a subset of participants in the study – 321 individuals both with and without Alzheimer’s disease. “When we examined the cerebrospinal fluid data, we discovered that Aβ levels fluctuated from season to season, in alignment with the shifts we found in memory and thinking,” noted Dr. Lim.
Gene expression showed seasonal patterns in line with cognitive fluctuations.
After analyzing post-mortem brain tissue from a subset of deceased participants, the researchers found that season was also associated with the brain expression of four cognition-associated gene modules – yet another factor that seems to vary in rhythm with the seasonal cognitive changes that were discovered.
Limitations of the Findings
While these findings are interesting and novel, they should be interpreted with a degree of caution, keeping in mind the limitations of the research. For starters, the data was “cross-sectional,” which means that different sets of people took the cognitive tests in the different seasons (i.e. each participant was tested once each year at about the same time of year). Comparisons between seasons reflect the group of participants collectively and do not represent known shifts within individuals. In other words, participants were not tested multiple times per year with their own scores in different seasons compared directly – this type of research would be ideal to conduct in the future.
Another study limitation is that all of the participants were from countries located in the northern hemisphere, so it is not known whether these findings would apply in other parts of the world. It would be interesting for future research to examine the effects of time of year on cognition in the southern hemisphere and equatorial regions.
Implications of this Study
Health Care Implications
The findings suggest potential wisdom in adjusting health care availability seasonally. “There may be value in increasing the level of dementia-related clinical resources available in the winter and early spring months,” said Dr. Lim, “when symptoms are likely to be most pronounced.”
Neuropsychologists and physicians should keep these findings in mind when diagnosing MCI and dementia. Repeated cognitive assessments, conducted at different times of year, would provide the most comprehensive data on cognitive performance, perhaps improving the accuracy of diagnosis.
Future Research Implications
There are a variety of theories about what factors might be behind these seasonal fluctuations related to cognition. Some of the theories suggest potential non-drug interventions. For example, if light or temperature influence the fluctuations, perhaps light therapy or temperature modification would work to sustain the summer-fall cognitive peak year-round. Vitamin D supplementation may help as well. Other factors that might play a role include seasonal variations in physical activity, sleep, diet, and psychological state – each of which would suggest different possible drug and/or non-drug interventions. More research is required to explore all of these options.
The seasonal differences found in this research are sizeable; in fact, they are as big as the effect researchers would look for in clinical trials assessing potential interventions and treatments. If these seasonal effects are not considered during research design for clinical trials, they could skew the results, either making a treatment look more effective or less effective than it truly is.
It will be helpful for family and professional caregivers to understand that the brain of an individual with Alzheimer’s disease changes with the seasons. Cognition will probably fluctuate throughout each year, as opposed to steadily declining over time. Additional support likely will need to be provided during the winter and spring. Caregivers can reasonably hope that there may be some improvement each summer and fall.
Source: MIND OVER MATTER V8
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