Some people have the biological hallmarks of Alzheimer’s – proteins called amyloid and tau that gum up the brain – but have no disease symptoms. Researchers suggest that this could be because some people build up a “cognitive reserve” – the brain’s ability to find new ways to handle and overcome problems.
People with greater cognitive reserve seem to be better at staving off dementia symptoms, but when stress levels are high or persistent, they can weaken this reserve by making it less likely that they will socialise and less likely that they will be physically active – both of which are known to protect against dementia.
Stress itself has also been linked to faster cognitive decline and an increased risk of developing Alzheimer’s disease.
In a recent study, we examined the relationship of cognitive reserve with cognition, and Alzheimer’s disease biomarkers – the previously mentioned tau and amyloid. We assessed whether the potential benefits of cognitive reserve vary by stress.
For our study, we looked at 113 participants from a memory clinic in Sweden. They were part of the Cortisol and Stress in Alzheimer’s Disease cohort study.
There are many ways cognitive reserve can be built up, such as staying mentally active throughout life. This could be by spending more years in formal education, playing bridge, learning a new language or having a complex job. Being physically active and maintaining healthy social relationships are important too.
To get an overall measure of cognitive reserve, we created an index by combining different information on the level of lifelong education participants had acquired, the complexity of the longest-held job, and engagement in physical, leisure activity and social interactions in later life.
Stress
We also looked at participants’ stress levels. Both subjective and biological measures were taken.
Subjective stress was measured using a questionnaire. People rated how much they perceived their life to be uncontrollable and unpredictable, and whether or not they had too much to deal with during the previous month.
For an objective measure of stress, we used salivary cortisol, a stress hormone. Cortisol follows a rhythm. It typically increases rapidly as soon as we wake up, peaks 30 minutes later (known as “cortisol awakening response”), and then decreases during the remainder of the day. It is lowest at nighttime, as our body gets ready to sleep.
Salivary cortisol was taken at different times of the day to measure these patterns. Previous studies have shown that a disruption of the cortisol pattern may increase Alzheimer’s disease risk.
We found greater cognitive reserve improved cognition in memory clinic patients, but when we factored physiological stress (cortisol) into the equation, the beneficial association of cognitive reserve was weakened – in other words, cortisol seems to deplete cognitive reserve.
Interestingly, though, subjective stress did not change the relation in a similar manner. So subjective stress doesn’t seem to use up cognitive reserve in the same way as biological stress seems to. We don’t know why this is. It could be that subjective and biological measures assess different aspects of stress.
Participants who had a good balance of morning and evening cortisol levels improved their working memory, but this wasn’t true for those who had an imbalance. Working memory stores information for short periods but allows us to actively process and manipulate the information. For example, we rely on working memory to solve a maths problem.
If cortisol levels are too high in the evening, it affects sleep. And if they are too low in the morning, it can affect morning alertness. The right balance is essential.
In those with unusually high amounts of cortisol shortly after waking up, having a higher cognitive reserve was linked to increased tau – a protein that forms tangles in brain cells, thereby disrupting their function. It could be that tau protein accumulation might make a person more prone to be stressed or stress itself may bring about changes to tau. This might lower a person’s ability to control and avoid actions that support the development of cognitive reserve.
Higher chronic stress may lessen the cognitive advantages of stimulating activities and enriching experiences in later life. Adding stress management techniques, such as mindfulness and meditation into your daily routine may contribute to overall brain health and slow cognitive decline.