People with above-average blood sugar levels may be more likely to develop a form of dementia, new research suggests.

Between five and seven million people in the UK are believed to have pre-diabetes, which leaves them more at risk of developing type 2 diabetes.

Now a University College London-led (UCL) study has found that people with the condition were 54% more likely to develop vascular dementia over an eight-year period on average.

We believe there is a potential connection that needs to be investigated further

Vascular dementia is a common type of dementia caused by reduced blood flow to the brain.

The study, published journal Diabetes, Obesity and Metabolism, analysed the data of 500,000 people, with an average age of 58, from the UK Biobank.

It found those with prediabetes were also 42% more likely to experience cognitive decline over an average of four years.

The increased risk to brain function remained after other factors, such as age, deprivation, smoking and body mass index, were taken into account, researchers said.

The study’s lead author Dr Victoria Garfield said while the results “cannot prove” higher blood sugar levels cause worsening brain health, the “potential connection” required further investigation.

Dr Garfield, from UCL’s Institute of cardiovascular science, said: “Our research shows a possible link between higher blood sugar levels, a state often described as ‘prediabetes’, and higher risks of cognitive decline and vascular dementia.

“As an observational study, it cannot prove higher blood sugar levels cause worsening brain health. However, we believe there is a potential connection that needs to be investigated further.

“Previous research has found a link between poorer cognitive outcomes and diabetes but our study is the first to investigate how having blood sugar levels that are relatively high – but do not yet constitute diabetes – may affect our brain health.”

Researchers investigated how different blood sugar levels, or glycaemic states, were associated with performance in cognitive tests over time, dementia diagnoses, and brain structure.

Participants were tested to determine their average blood sugar levels over the past two to three months, and divided into five groups on the basis of the results.

The researchers used data from repeated assessments of visual memory to determine whether participants had cognitive decline.

They found that prediabetes was associated with a higher likelihood of vascular dementia, but not Alzheimer’s disease.

People with diabetes, meanwhile, were three times more likely to develop vascular dementia than people whose blood sugar levels were classified as normal, and more likely to develop Alzheimer’s disease, the study found.

Those with prediabetes can reduce their risk of developing type 2 diabetes by eating a healthy, balanced diet, being more active, and staying at a healthy weight.