Depression and dementia risk explained

Depression can affect our health and behaviour in ways that may increase dementia risk. For example, it can make it more likely that we:

• have high blood pressure
• are less physically active
• drink more alcohol
• eat a less healthy diet
• become socially isolated
• get less sleep.

Depression may also affect the brain more directly. It can cause higher levels of a hormone called cortisol. Over time high levels of cortisol have been linked to brain shrinkage, and problems with memory and thinking.

Researchers are still working to understand this link. For example, they are studying:

• at what point in our life it affects us
• how long we have it for
• whether we have one or more episodes of depression in our lifetime.

For every 100 cases of dementia worldwide, almost half (45%) could be prevented or delayed. Three of those 45 cases are linked to untreated depression in midlife.

So, if everybody could effectively prevent or treat depression, there'd be three fewer cases of dementia for every 100 people who develop it now.

Find out more: Addressing 14 health and lifestyle factors could prevent nearly half of global dementia cases - Alzheimer's Research UK

Some research has explored whether treating depression reduces dementia risk. At the moment, there is some evidence suggesting that people with mild depression who are treated are less likely to develop dementia compared to those who are untreated.

However, more research into treating depression to prevent dementia needs to be done.

Treating and managing depression is still important. It supports mental wellbeing and daily life, even if its effect on dementia risk is not yet clear.

Most of what we know about dementia risk comes from observational studies, which follow groups of people over time to look for patterns in health and lifestyle.

For example, studies like Insight 46 have followed people for decades to understand how health across life is linked to brain health and later on.

This is often the most practical way to study dementia risk. Many risk factors cannot be tested in trials in a fair or ethical way. Dementia also develops over many years, so studies can take a long time to show results.

Observational studies can show when something like our diet or occupation are linked to a higher or lower chance of dementia. But they cannot prove that something directly causes dementia, or that changing it will definitely reduce risk. These studies look at patterns across large groups, not what will happen to any one person.

Randomised controlled trials give stronger evidence because participants are assigned to different groups at random. This design makes it more likely that differences in outcomes, for example our risk of dementia, are caused by the intervention, for example a diet change, rather than by other factors.

But these studies are harder to do in dementia research. They often take many years and may test several changes at once, making results harder to interpret.

Newer trials are helping to build a clearer picture of what may support brain health. No single study can give a definite answer, but it is all research findings together that help us understand what is important for our brain health and risk of dementia.