The link between type 2 diabetes and psychiatric disorders

Living with mental disorders is challenging enough without adding physical ailments. But recent research suggests that people with psychiatric disorders are also more likely to have type 2 diabetes, and the combination could be devastating.

“When people with pre-existing mental illnesses develop diabetes, their outcomes are much worse,” said Anne Doherty, an associate professor of psychiatry at University College Dublin. Compared to people with type 2 diabetes who do not have mental illness, “they are more likely to have complications and are significantly more likely to die younger.” The relationship goes both ways; people with diabetes are also more likely to have psychiatric disorders and have worse outcomes than people without diabetes.
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As doctors and researchers strive to untangle the mechanisms underlying these links, they are beginning to integrate the treatment of these disparate diseases. “There are some studies that show that by treating depression aggressively and assertively, you can actually improve people’s diabetes control — and their lives in general — so it’s really quite exciting,” Doherty says.

The association between diabetes and psychiatric disorders highlights the close links between mental and physical health. A better understanding of these associations could give us a better chance of preventing or treating such conditions.

Researchers have known for some time about the link between certain serious psychiatric illnesses, such as depression or schizophrenia, and higher rates of type 2 diabetes. Until recently, it was unclear to what extent this association extended to other psychiatric disorders.

In a recent study, Nanna Lindekilde, a Ph.D. student at the University of Southern Denmark, examined the link between type 2 diabetes and a wide variety of psychiatric disorders. She and her colleagues analyzed 32 systematic reviews of the subject, which were based on 245 different primary studies conducted between 1980 and 2020.

“In general, there is an increased risk of type 2 diabetes in people with a psychiatric disorder,” Lindekilde says. “Most research has previously focused on people with depression or schizophrenia, but we have shown an increased risk in a wide variety of psychiatric disorders.”

Type 2 diabetes affects an estimated 6% to 9% of the world’s population and 10.5% of the US population. Lindekilde found much higher rates of type 2 diabetes in people with psychiatric disorders, including 39.7% type 2 diabetes in people with a sleep disorder and 20.7% in those with binge eating disorder. Other disorders with a high rate of type 2 diabetes were substance use disorders (15%), anxiety disorder (13%), bipolar disorder (11%) and psychosis (11%).

Moreover, the association between type 2 diabetes and many psychiatric disorders is two-way. For example, studies have shown that people with depression are more likely to develop diabetes, and people with diabetes are more likely to have depression.

The mechanisms underlying this two-way relationship are a mystery, but researchers are exploring intriguing hypotheses.

Deciphering complex underlying mechanisms

Both psychiatric disorders and chronic illnesses like diabetes are known to take a psychological toll on patients, which can add to the risk of each.

“People with psychiatric illness tend to smoke more, may have poor diet and less physical activity, so you get these knock-on effects on your physical health,” said Seena Fazel, a professor of forensic psychiatry at the University of Oxford. For example, depression can make it more difficult to exercise, eat healthy, or stick to a medication schedule, all of which can increase diabetes risk.

Sleep is also known to be crucial for proper metabolic function. Mental disorders can disrupt sleep and affect a person’s metabolism, leading to an increased risk of diabetes. In addition, certain psychiatric medications can lead to weight gain and difficulty controlling blood sugar, leading to a higher risk of type 2 diabetes. “In some cases, treating mental illness can actually exacerbate underlying metabolic problems,” Fazel says.

On the other hand, chronic diseases such as diabetes can contribute to poorer mental health. “Living with a chronic condition can lower your mood and sometimes cause people to self-medicate with alcohol and drugs,” Fazel says. “People with diabetes are more likely to have depression because of the psychological effects of the disease, such as the limitation of certain things you can do and the kind of prognosis you think is hopeless.”

Researchers have also recently collected tantalizing hints about common biological pathways that may underlie both diabetes and psychiatric disorders.

Shared genetic pathways may be a possible culprit, but existing studies are too few and too small to draw firm conclusions. Researchers are particularly interested in the possible genetic overlap between schizophrenia and type 2 diabetes as a way to explain higher rates of diabetes in people with schizophrenia. “While it is highly likely that there are overlapping genetic mechanisms, our understanding of this architecture remains limited,” said Amir Sariaslan, senior research fellow in psychiatric epidemiology at the University of Oxford. Inflammation is another likely suspect underlying both diabetes and psychiatric disorders. Chronic inflammatory responses are associated with a higher risk of developing type 2 diabetes, while certain markers of inflammation are found at higher levels in people with psychiatric disorders such as depression. Scientists are investigating various inflammatory pathways and molecules to see if they may underlie both diabetes and psychiatric disorders. “It certainly draws a lot of research interest,” Fazel says.

Scientists are also focusing on the role of the roughly 100 trillion microbes in our gut and associated genomes, collectively known as the gut microbiome. There is some evidence that changes in the gut microbiome may affect the brain and metabolism, theoretically affecting diabetes and psychiatric disorders.

But what we know about these connections is still very preliminary, especially in humans. “Much of the research is taking place at the lab level, and there’s probably a long way to go before it’s fully applied to human populations,” Doherty says. “I think in five years we will learn a lot more about this area.”

It is challenging to conduct large studies on people with psychiatric conditions, let alone when these patients also have diabetes, but large population studies could provide hints about some of the mechanisms underlying both conditions. Lindekilde is conducting a study of 250,000 Danish individuals to explore possible mechanisms that could explain the link between a wide variety of psychiatric disorders and type 2 diabetes.

Learning more about which mediating mechanisms play the greatest role and whether the mechanisms differ between psychiatric disorders is important and could aid future initiatives to prevent or manage these disorders, she says.

Worse when they are together

Mental disorders and diabetes are all independently associated with dying at a younger age. “On average, people with severe mental illness die 17 years younger than everyone else, which is a huge mortality gap,” Doherty says. And people with diabetes generally live 7.5 years shorter than people without the disease, with even greater effects in people who get diabetes at a younger age. But having both can be particularly harmful.

“When people with pre-existing mental illnesses develop diabetes, their outcomes are much worse,” Doherty says. She points to a study of patients in a London clinic that treats diabetes complications. When researchers evaluated all clinic patients for depression, they found that those with major depression were more than three times more likely to die at 18 months than those without major depression.

In a recent population study of more than 250,000 people in Sweden, Fazel found a similar increase in mortality in patients with psychiatric disorders who also had chronic conditions such as respiratory problems, cardiovascular disease and diabetes. He found that the risk of dying within five years increased dramatically for people with diabetes if they also had a psychiatric disorder, such as depression.

To adjust for background factors such as ethnicity, family environment, and socioeconomic status, Fazel compared mortality among siblings with diabetes, one with a psychiatric disorder and one without. “The interesting thing is that there is still an increased mortality risk even when you take these sibling models into account,” Fazel says.

Fazel and others are still investigating why the combination of diabetes and psychiatric disorders leads to increased mortality. Some possible hypotheses are that psychiatric disorders can lead to delays in seeking diabetes treatment or difficulties in following diabetes treatments.

“I think these mechanisms are really important, because that’s where you can actually address and prevent the increased mortality,” says Fazel. “The mortality risks are very high, and that really signals that this is an important challenge for health care systems to address.”

An integrated approach to treatment

Researchers are increasingly considering integrated approaches to the treatment of diabetes and psychiatric disorders. “Both conditions should be managed together if possible,” Doherty says.

That starts with finding out when these conditions occur together, by screening diabetes patients for psychiatric disorders and vice versa. Some countries, such as the UK, have started to include mental health screening in diabetes care, but it is still far from common.

General practitioners and general practitioners can play an important role in early detection. Fazel says, “If primary care physicians identify and treat these psychiatric comorbidities early, we know it can improve outcomes.”

Integrating mental and physical health treatments can help reduce mortality. “Mental health care should be a big part of diabetes care,” says Doherty. “Another thing is to embed good physical monitoring in mental health clinics.”

Such integrated approaches have had some success in the US and UK. Doherty participated in a program in the UK called Three Dimensions for Diabetes that integrated mental health, social care and diabetes treatment, and provided mental health care to patients with diabetes. “It was actually extraordinary, the number of people that we found had significant mental health issues that had not been diagnosed before, and the only marker was the fact that they were just unable to control their diabetes,” Doherty says.

The integrated approach turned out to be very effective. “We found a significant improvement in diabetes control,” she says.

Future studies will continue to examine the link between type 2 diabetes and a wide variety of psychiatric conditions, aiming to find common approaches to treat or treat both. The link between these two seemingly unrelated conditions just goes to show how our mental and physical health are inexorably intertwined. “It argues against this division that you have mental or physical health problems because they actually overlap,” Fazel says.

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