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Schizophrenia may cause type 2 diabetes, new study finds

Diabetic checking blood sugar levels. Piotr Adamowicz

People with schizophrenia tend to die up to 30 years earlier than the general population. Many of these untimely deaths are due to physical disorders, including heart attacks and stroke, for which diabetes is a major risk factor.

Antipsychotic drugs are known to increase the risk of type 2 diabetes, but there are other things that make schizophrenics particularly susceptible to the disorder, including poor diet and a lack of exercise. However, our latest study found that the risk of developing diabetes in people with schizophrenia remains high even when we take these factors into account.

People with long-term schizophrenia are three times more likely than the general population to have diabetes. The link between schizophrenia and diabetes was first made back in the 19th century. This was long before the use of antipsychotics, and in an era when diets were less likely to cause diabetes. This could suggest that there is a causative link between schizophrenia and diabetes.

Our study examined whether diabetes risk is already raised in people at the onset of schizophrenia – before they’ve started taking antipsychotic drugs or when they’ve only just started taking them.

We pooled data from multiple studies that examined evidence of diabetic risk in blood samples from people with early schizophrenia prescribed little or no antipsychotic medication. Diabetes is characterised by elevated blood glucose. The higher the level of glucose in the blood, the higher the risk of diabetes.

We demonstrated that compared with healthy individuals, people with schizophrenia had higher levels of glucose in the blood. We also looked at levels of insulin. Insulin is a hormone that triggers the movement of glucose from blood into tissue. Raised insulin levels are seen in type 2 diabetes. We demonstrated higher levels of insulin, and increased levels of insulin resistance in individuals with early schizophrenia.

Hints of a direct role of schizophrenia in diabetes

These results remained statistically significant even when we restricted our analysis to studies where people with schizophrenia were matched to healthy controls with regards their diet, the amount of exercise they engaged in and their ethnic background. This suggests that our results were not wholly driven by differences in lifestyle factors or ethnicity between the two groups, and may therefore point towards a direct role for schizophrenia in increasing risk of diabetes.

There are several factors that could increase the likelihood of developing both conditions. These include a shared genetic risk, as well as shared developmental risk factors. For example, premature birth and low birth weight are recognised as risk factors for the development of both schizophrenia and diabetes later in life. Raised levels of the stress hormone cortisol is also a risk factor for diabetes. It is possible that the stress associated with developing schizophrenia, which sees levels of cortisol rise, may also contribute to higher diabetic risk.

These findings are a wake-up call that we need to rethink the link between diabetes and schizophrenia and start prevention right from the onset of schizophrenia. It is a case of treating the mind and the body right from the start.

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