Alzheimer’s as “Type 3 Diabetes”? Why Metabolic Health Matters for the Brain

When we think of insulin, we usually think of blood sugar. But the hormone also acts in the brain, influencing signalling pathways involved in nerve cell function, learning and memory. The connection between insulin signalling and Alzheimer’s has therefore been a focus of research for years.

Why is Alzheimer’s sometimes called “type 3 diabetes”?

The term describes the hypothesis that impaired insulin signalling in the brain may contribute to Alzheimer’s disease. Researchers refer to this as brain insulin resistance: cells become less responsive to the hormone’s signals.

A study of human brain tissue found reduced responsiveness to insulin in Alzheimer’s, including in people without known diabetes. Changes in insulin signalling were associated with poorer memory performance. This supports the relevance of these mechanisms.

How does type 2 diabetes fit in?

A pooled analysis of studies involving approximately 2.3 million people found that type 2 diabetes was associated with a higher risk of developing dementia, including Alzheimer’s and vascular dementia. This association does not mean an inevitable outcome: diabetes does not automatically lead to dementia, and Alzheimer’s can develop without diabetes.

BUT: Why metabolic health matters early

In insulin resistance, cells in the muscles, liver and fat tissue become less responsive to insulin. Initially, the pancreas can compensate by producing more insulin. Blood glucose may therefore remain normal even though the body already needs more insulin. A single normal glucose result does not necessarily rule out early insulin resistance.

This explains why it makes sense to assess metabolic health as a whole. Insulin resistance and prediabetes frequently cause no symptoms. Fasting glucose and HbA1c are key tests, sometimes supplemented by an oral glucose tolerance test. Blood pressure, blood lipids, waist circumference and personal medical history provide additional context.

What can we influence?

The Lancet Commission includes diabetes, high blood pressure, elevated LDL cholesterol and obesity among 14 modifiable dementia risk factors. Its modelling suggests that up to 45% of cases worldwide could potentially be prevented or delayed if these factors were eliminated.

The Diabetes Prevention Program demonstrated that lifestyle changes can improve metabolic outcomes. Among people at increased risk of diabetes, an intensive programme combining dietary changes, physical activity and weight loss reduced the incidence of type 2 diabetes by 58% compared with placebo over an average of 2.8 years.

Metabolic health deserves attention while we still feel healthy. Identifying changes early creates an opportunity to work with a healthcare professional and take appropriate action.