The Link Between Type 2 Diabetes and Psychiatric Disorders

Living with mental-health disorders is challenging enough without adding physical ailments to the mix. But recent research suggests that people with psychiatric disorders also have higher rates of Type 2 diabetes, and the combination can be devastating.

“When people who have pre-existing mental illnesses develop diabetes, their outcomes are much worse,” says Anne Doherty, an associate professor of psychiatry at University College Dublin. Compared to people with Type 2 diabetes who don’t have mental illnesses, “they are more likely to develop complications, and they’re significantly more likely to die younger.” The relationship goes both ways; people with diabetes also tend to have higher rates of psychiatric disorders and face worse outcomes than people without diabetes.

As doctors and researchers strive to untangle the mechanisms underlying these links, they’re starting 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, overall, their lives—so it’s really quite exciting,” Doherty says.

The association between diabetes and psychiatric disorders highlights the close connections between mental and physical health. An improved understanding of these connections could give us a better shot at preventing or treating such conditions.

Researchers have known for a while about the link between certain severe psychiatric illnesses, such as depression or schizophrenia, and higher rates of Type 2 diabetes. Until recently, it was unclear how much this association extended to other psychiatric disorders.

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In a recent study, Nanna Lindekilde, a Ph.D. student at the University of Southern Denmark, investigated the links between Type 2 diabetes and a broad range of psychiatric disorders. She and her colleagues analyzed 32 systematic reviews of the topic—which were based on 245 different primary studies conducted between 1980 and 2020.

“There, in general, 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 across a broad range of psychiatric disorders.”

Type 2 diabetes is thought to affect 6% to 9% of the world’s population and 10.5% of the U.S. population. Lindekilde found much higher Type 2 diabetes rates in people with psychiatric disorders, including a 39.7% rate of Type 2 diabetes among people with a sleep disorder and 20.7% rate among those with a binge-eating disorder. Other disorders with high rates of Type 2 diabetes included substance-use disorder (15%), anxiety disorder (13%), bipolar disorder (11%), and psychosis (11%).

What’s more, the association between Type 2 diabetes and many psychiatric disorders goes both ways. For example, studies have shown that people with depression are more likely to get diabetes, and people with diabetes are more likely to get 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 contribute to the risk of each.

“People with psychiatric illnesses often smoke more, they may have poor nutrition, lower physical activities, so you get these knock-on effects on your physical health,” says Seena Fazel, a professor of forensic psychiatry at the University of Oxford. For instance, depression can make it harder to exercise, eat healthy, or adhere to a medication regimen, all of which can increase diabetes risk.

Sleep is also known to be crucial to proper metabolic function. Psychiatric disorders can disrupt sleep and impair a person’s metabolism, leading to an increased risk of diabetes. Plus, certain psychiatric medications can cause weight gain and difficulty managing blood sugar, leading to a higher risk of Type 2 diabetes. “In some cases, the treatment for a mental illness can actually exacerbate underlying metabolic problems,” Fazel says.

On the other hand, chronic illnesses such as diabetes can contribute to worse mental health. “Living with a chronic condition can lower your mood and also sometimes lead people to self-medicate with alcohol and drugs,” Fazel says. “People with diabetes may be more likely to get depression because of the psychological effects of the illness, such as the restriction on certain things you can do and the sort of prognosis that you think may be hopeless.”

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

Shared genetic pathways could be a potential culprit, but existing studies are too few and too small to make any definitive conclusions. Researchers have been particularly interested in the potential 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,” says Amir Sariaslan, a 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, whereas 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 might underlie both diabetes and psychiatric disorders. “It’s definitely attracting a lot of research interest,” Fazel says.

Scientists are also focusing on the role of the 100 trillion or so microbes in our gut and their associated genomes, collectively known as the gut microbiome. There are hints that changes to the gut microbiome can affect the brain and metabolism, theoretically influencing diabetes and psychiatric disorders.

But what we know about these links is still very preliminary, particularly in humans. “A lot of the research is happening at a lab level, and there’s probably a ways to go before it’s fully applied to human populations,” Doherty says. “I think we’re going to know an awful lot more about this area in five years’ time.”

It’s challenging to run large trials of people with psychiatric illnesses, let alone when these patients also have diabetes, but large population studies could provide hints about some of the mechanisms underlying both disorders. Lindekilde is conducting a study of 250,000 Danish individuals to investigate potential mechanisms that might explain the association between a broad range of psychiatric disorders and Type 2 diabetes.

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

Worse when they’re together

Psychiatric 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 everybody else, which is a massive mortality gap,” Doherty says. And people with diabetes generally live 7.5 years less than those without the disease, with even greater effects among those who get diabetes at a younger age. But having both can be particularly harmful.

“When people who have pre-existing mental illnesses develop diabetes, their outcomes are much worse,” Doherty says. She points to a study of patients at 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 as likely to die after 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 among 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 when 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 elevated mortality risk still exists even if you account for these sibling models,” Fazel says.

Fazel and others are still investigating why the combination of diabetes and psychiatric disorders leads to increased mortality. Some potential hypotheses are that psychiatric disorders may lead to delays in seeking diabetes treatment or difficulty adhering to diabetes treatments.

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

An integrated approach to treatment

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

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

Primary care physicians and general practitioners can play an important role in early detection. Fazel says, “If family physicians identify and treat these psychiatric comorbidities early on, we know that that can improve the outcomes.”

Integrating mental and physical health treatment could help reduce mortality. “A big part of diabetes care should be mental-health care,” Doherty says. “Another thing is to embed good physical monitoring into mental-health clinics.”

Such integrated approaches have shown some success in the U.S. and the U.K. Doherty took part in a U.K. program called Three Dimensions for Diabetes that integrated mental health, social care, and diabetes treatment, and provided mental-health care for diabetes patients. “It was actually extraordinary, the proportion of people that we found to have significant mental-health problems that hadn’t previously been diagnosed, and the one marker was the fact that they just weren’t able to manage their diabetes,” says Doherty.

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

Future studies will continue to study the link between Type 2 diabetes and a wide range of psychiatric conditions, with the goal of finding common approaches to managing or treating both. The connection between these two seemingly unrelated disorders just goes to show how our mental and physical health are inexorably intertwined. “It argues against this division that you’ve either got mental or physical health problems, because actually they do overlap,” Fazel says.