Summary: Cognitive reserve measured through IQ predicted the steepness of cognitive decline in aging men, but not women.
Source: University of Tasmania
Tasmanian researchers are one step closer to understanding why women are more likely to develop Alzheimer’s disease, with their research recently published in the journal Neurology.
Professor Jane Alty and Aidan Bindoff from the University’s Wicking Dementia Research and Education Center led a team of researchers to determine if cognitive reserve (education and IQ) slowed down age-related cognitive decline equally in males and females.
“We know women have a higher age-adjusted incidence of Alzheimer’s disease than men, but the reasons remain unclear. It is not simply related to women living longer than men,” Professor Alty said.
“One proposed contributing factor is that, historically, women had less access to education and therefore may have accumulated less cognitive reserve.”
Cognitive reserve refers to the ability to buffer the effects of physical changes in the brain so it does not have a direct effect on function.
“People who have developed higher cognitive reserve over their lifetime (through more education and other cognitively stimulating activities such as employment and hobbies) generally do not show as marked decline in their memory and thinking functions,” Professor Alty said.
Researchers measured cognitive reserve using total years of education and by measuring their IQ, accessing data through the Wicking Center’s Tasmanian Healthy Brain Project (THBP).
The THBP is a long-term cohort study, recruiting healthy Australians aged 50–80 years without cognitive impairment that began about 10 years ago.
The THBP aimed to determine if university education later in life reduced age-related cognitive decline and significantly decreases risk, or delays the onset, of dementia.
Data from 562 participants (383 females and 179 males) was analyzed for Professor Alty’s study.
The study’s results showed that cognitive reserve, measured through IQ, moderated the steepness of age-related cognitive decline in males, but not in females.
“Males with higher estimated IQ had a less rapid (less steep) age-related cognitive decline than their lower IQ male peers—this is what we expected,” Professor Alty said.
“However, we did not see these same protective effects in females—so those with higher cognitive reserve declined in their memory and thinking tests as they got older at the same rate as females with lower cognitive reserve.”
The study’s results also showed education did not significantly moderate cognitive trajectories in either males or females.
“The study’s findings do not appear to support the hypothesis that historical sex disparities in accessing education contribute to the higher female incidence of Alzheimer’s disease.
“They do suggest that there are sex-specific effects of cognitive reserve though, with males benefiting more—this highlights that further research studies should assess males and females separately when investigating how we can best protect people against Alzheimer’s disease and age-related cognitive decline,” Professor Alty said.
Sex-Specific Protective Effects of Cognitive Reserve on Age-Related Cognitive Decline
Background and Objectives
Females have a higher age-adjusted incidence of Alzheimer disease than males but the reasons for this remain unclear. One proposed contributing factor is that, historically, females had less access to education and, therefore, may accumulate less cognitive reserve. However, educational attainment is confounded by IQ, which in itself is a component of cognitive reserve and does not differ between sexes. Steeper age-related cognitive declines are associated with increased risk of dementia. We, therefore, evaluated the moderating effects of 2 proxies for cognitive reserve, education and IQ, on the steepness of age-related declining cognitive trajectories in unimpaired older males and females.
The Tasmanian Healthy Brain Project, a long-term cohort study, recruited healthy Australians aged 50–80 years without cognitive impairment. Baseline cognitive reserve was measured using educational history and IQ, measured by the Wechsler Test of Adult Reading, Full Scale Predicted IQ (WTAR-FSIQ). Cognitive trajectories for language, executive function, and episodic and working memory over 5 years were extracted from neuropsychological assessments. The adjusted effects of education, estimated IQ, and APOE allelic variant on cognitive trajectories were compared between males and females.
Five hundred sixty-two individuals (mean [SD] age 60 [6.7] years; 68% male; 33% APOE ε4+) were followed up over 5 years with 1,924 assessments and 24,946 cognitive test scores (annualized attrition rate 6.6% per year). Estimated IQ correlated with years of education (p < 0.001). Estimated IQ interacted with sex to moderate age-related cognitive trajectories (p = 0.03; adjusted for education); lower IQ males experienced steeper declining trajectories than higher IQ males, but lower IQ females had similar steepness of declining trajectories to higher IQ females. Education was not associated with rate of cognitive decline (p = 0.67; adjusted for WTAR-FSIQ). There were no significant differences in age-related cognitive trajectories between APOE genotypes in either sex.
IQ, a measure of cognitive reserve, predicted the steepness of declining cognitive trajectories in males only. Education did not explain as much variation in cognitive trajectories as IQ. Our findings do not support the hypothesis that historical sex disparities in access to education contribute to the higher female incidence of Alzheimer disease.