High blood glucose (hyperglycaemia) may increase the risk of dementia, a new observational study has found. 

Notably, dementia risk increased with the severity of hyperglycaemia and the presence of conditions affecting large blood vessels, including ischaemic heart disease, ischaemic stroke, and chronic kidney disease.

The study, “Risk of incident dementia according to glycaemic status and comorbidities of hyperglycaemia: a nationwide population-based cohort study,” was published in Diabetes Care. 

Diabetes is linked to a higher risk of dementia because it increases the production of abnormal proteins typically found in people with dementia. Prediabetes ­– a condition where blood glucose levels are higher than normal but lower than the criteria for diabetes – may also increase dementia risk because the brain is exposed to abnormally high sugar levels. However, evidence for this theory is lacking.

It is also established that diseases affecting large blood vessels such as IHD increase the dementia risk. However, it is unknown whether the presence of such conditions with hyperglycaemia further increases dementia risk. 

Now, a team of researchers in South Korea sought to investigate whether dementia risk was associated with hyperglycaemia and if underlying conditions affecting large blood vessels influence dementia risk.  

The researchers used data from a South Korean health insurance claims database containing health examinations results for adults over 40 years.  The health examination results included age, weight, blood pressure measurements and other laboratory assessments. Adults also completed medical history and health-related lifestyle questionnaires.  

Participants were grouped into five categories based on their blood glucose levels: Normal, prediabetes, new-onset diabetes, diabetes occurring for less than five years and diabetes with five years or more. 

A total of 8,400,950 who had health examinations between 2009 and 2010 were included in the study. The mean age of the cohort was 54 years, and 48.4% were men. The majority (58.3%) had normal blood glucose levels, 24.3% had prediabetes, and 17.7% had diabetes.  

After a median follow-up of 5.94 years, 4.2% developed dementia (3.2% with Alzheimer’s dementia and 0.5% with vascular dementia). 

The risk of dementia increased as hyperglycaemia worsened and was highest in people with persistent diabetes. Compared to those with normal blood glucose, the risk of dementia increased by 45% in people with newly diagnosed diabetes, 32% in people with diabetes for less than five years and 62% in people with diabetes for at least five years. 

The risk of dementia further increased in people with hyperglycaemia and chronic kidney disease, ischaemic heart disease or ischaemic stroke. Those with diabetes and ischaemic stroke had the highest risk of dementia.  

Interestingly, people with diabetes and no other underlying conditions had a higher risk of dementia than people with normal blood glucose levels and ischaemic heart disease or chronic kidney disease. However, people with normal blood glucose levels and ischaemic stroke had a higher risk of dementia than those with diabetes and no underlying conditions, 90% vs 46%, respectively. 

“The association of ischaemic stroke with dementia was greater than that of hyperglycaemic. But for the other [co-existing conditions], we found that presence or absence of hyperglycaemia was a more important factor contributing to incident dementia,” the researchers wrote.

The researchers noted the inability to verify participants’ blood glucose levels with oral glucose tolerance tests or HbA1c measurements as a limitation.

Still, ”the results of this population-based study suggest that even a mild degree of hyperglycaemia and diabetes and other [co-existing conditions] could be important risk factors contributing to the incidence of dementia…” the researchers wrote.

“It is also important for clinicians to know that prediabetes and diabetes are not discrete conditions’ they are part of a hyperglycaemic continuum that ends with diabetes,” they wrote. 

More long-term studies are needed to determine how prolonged hyperglycaemia increases dementia risk. However, “active intervention in patients with prediabetes and other [co-existing conditions] can help improve population-level health outcomes in an ageing society,” the researchers concluded