Type two diabetes and dementia risk explained

People with type two diabetes in midlife are more likely to develop dementia, particularly vascular dementia, compared to people without diabetes.

The risk is higher for people whose diabetes is severe or not well managed.

Research also suggests that diabetes developing earlier in adulthood, particularly in mid‑life, is linked to a higher risk of dementia later on being diagnosed in older age. This may be because the brain is exposed to the effects of diabetes for longer.

People with diabetes who experience repeated episodes of low blood sugar (hypoglycaemia) may also have a higher risk of dementia. Keeping blood sugar levels within a healthy range may help protect brain health.

For every 100 cases of dementia worldwide, almost half (45%) could be prevented or delayed. Two of those 45 cases are linked to type two diabetes in midlife.

So, if everyone could prevent or manage type two diabetes effectively, there'd be two 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

More research is needed to fully understand the complex relationship between diabetes and dementia risk. But current evidence suggests three main ways that type two diabetes may increase the risk of dementia:

1. Damage to blood vessels

Diabetes can damage both large and small blood vessels, including those in the brain. This can lead directly to vascular dementia and Alzheimer’s disease. It can also increase the risk of stroke, which in turn can lead to dementia.

2. Insulin resistance in the brain

Type two diabetes can make brain cells less responsive to insulin. This can affect how well brain cells use glucose for energy, which is important for memory, thinking and planning.

It may also be linked to the build-up of proteins called amyloid and tau, which are associated with Alzheimer’s disease.

3. Inflammation

Diabetes can lead to long-term inflammation in the body. Over time, this can damage brain cells. It is also linked to conditions like Alzheimer’s disease.

While we don’t yet know if treating diabetes directly lowers dementia risk, managing diabetes and related health conditions is likely to be better for brain health than leaving them untreated.

People with diabetes often have other health conditions that can also affect dementia risk, such as high blood pressure and high cholesterol.

Because these conditions often occur together, it can be hard to tell whether treating diabetes itself lowers dementia risk, or whether any benefit comes from improving overall heart and blood vessel health.

Some studies have found that people with type two diabetes who take common medications to control their blood sugar are less likely to develop dementia than people who are not treated. However, it is not yet clear whether this is due to the medication itself, or to better overall management of health.^,,,. You can find more information about these treatments from Diabetes UK.

Large scale clinical trials are ongoing to see if certain diabetes medications can reduce the likelihood of people getting dementia.

Despite this, we know that people with type two diabetes are at an increased risk of cardiovascular disease and managing diabetes helps to reduce this risk. Poor heart and blood vessel health is also linked to a higher risk of dementia.

The term ‘type 3 diabetes’ is not a medical diagnosis. It is an unofficial term some people use to describe the link between insulin resistance in the brain and Alzheimer’s disease.

It is used by some researchers to describe the theory that insulin resistance dysfunction in the brain may cause Alzheimer's disease. However insulin resistance hasn’t been shown to directly cause Alzheimer’s disease or other types of dementia.

For this reason, ‘type 3 diabetes’ is not recognised by doctors, and the term is not used in clinical care or diagnosis.

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.