Depression is a direct cause of type 2 diabetes and not just because people may be overweight, new research suggests.
Experts funded by the charity Diabetes UK have found a causal relationship and shared genetics suggesting depression may actually cause type 2 diabetes, which affects around 4.5m people in the UK.
Until now, researchers have known that people with type 2 diabetes are around twice as likely to suffer depression compared with those without diabetes and that people with depression have a higher risk of developing type 2.
However, it has been unclear whether depression caused type 2, or vice versa, or whether other factors are at play.
In the latest study, researchers used a statistical method called Mendelian randomisation to analyse genetic and health information.
They found, for the first time, that depression directly causes an increased risk of developing type 2 diabetes, but not that diabetes causes depression.
The study found that only 36.5% of the effect of depression on type 2 diabetes could be explained by obesity.
Obese people are significantly more likely to have type 2 diabetes than those of normal weight.
The researchers also noted seven genetic variants that contribute to both type 2 diabetes and depression.
These shared genes play a role in insulin secretion or inflammation in the brain, pancreas or fat tissue, with changes in these biological processes potentially explaining how depression increases type 2, they suggested.
While a direct cause was not found for diabetes causing depression, experts still believe that the burden of living with type 2 diabetes can be a factor in developing depression.
Dr Elizabeth Robertson, director of research at Diabetes UK, said: “This hugely important study gives us new insights into the links between genetics, type 2 diabetes and depression, indicating that depression can contribute to the development of type 2 diabetes.
“Type 2 diabetes is complex, with multiple risk factors – and previous research has shown that the condition is more common in people with depression.
“This study gives us greater insight into why and indicates that depression should now be considered a risk factor for type 2.
“This knowledge could help healthcare professionals to improve care and support for people with a history of depression and prevent more cases of type 2 diabetes.
“We strongly encourage anyone with depression to know their risk of type 2 diabetes by completing Diabetes UK’s free online know your risk tool, so they can get the right support to reduce their risk and avoid type 2 diabetes.”
Inga Prokopenko, professor e-One Health and head of statistical multi-omics at the University of Surrey, who led the study, said: “Our discovery illuminates depression as a contributing cause of type 2 diabetes and could help to improve prevention efforts.
“The findings are important for both individuals living with the conditions and healthcare providers, who should consider implementing additional examinations to help prevent type 2 diabetes onset in people suffering from depression.”
The study, published in the journal Diabetes Care, used data from hundreds of thousands of people in the UK and Finland, including 19,344 people with type 2 diabetes, more than 5,000 people diagnosed with depression and 153,079 who self-reported depression.
The researchers concluded: “Our results highlight the importance to prevent type 2 diabetes at the onset of depressive symptoms, and the need to maintain a healthy weight in the context of its effect on depression and type 2 diabetes co-morbidity.”