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New research finds a connection between insulin resistance and a higher risk of major depressive disorder. Johner Images/Getty Images
  • Insulin resistance occurs when the body does not respond correctly to insulin and cannot easily take up glucose from the blood. This dysfunction causes the pancreas to make more insulin to compensate.
  • Major depressive disorder is a mood disorder that can have a profound effect on people’s overall well-being.
  • A new study suggests that people with insulin resistance have a higher risk of developing major depressive disorder.

Insulin resistance can lead to the development of type 2 diabetes, which, without proper management, can lead to severe health problems.

Major depressive disorder is a common and severe mental health problem. Its prominence means that researchers are striving to identify factors that put people at risk of developing depression.

A new study by researchers at Stanford University School of Medicine, CA, found that participants with insulin resistance had a higher risk of developing depression than those who did not. The findings appear in The American Journal of Psychiatry.

As the Centers for Disease Control and Prevention (CDC) note, the chronic condition of diabetes centers around the hormone insulin, which the pancreas produces. When the body releases insulin, this hormone allows the body’s cells to absorb sugar from the food the person eats.

Without enough insulin, the sugar from food builds up in the bloodstream, depleting the body’s energy stores. Excessively high levels of blood sugar can lead to serious long-term health complications. Currently, 34.2 million people have diabetes in the United States, where it is the seventh leading cause of death.

In addition to the many people with diabetes, about 88 million adults in the U.S. have what is called prediabetes. This occurs when blood sugar levels are high but not high enough for the person to receive a diabetes diagnosis.

The American Diabetes Association notes that insulin resistance, also called impaired insulin sensitivity, occurs when the body stops responding to insulin the way it should. The insulin that is available becomes less effective, and the pancreas produces more insulin to try to compensate.

As the resistance gets worse, the pancreatic cells that make insulin can wear out. Eventually, the pancreas fails to produce enough insulin, and blood sugar levels rise. Insulin resistance can then develop into prediabetes or type 2 diabetes.

The exact cause of insulin resistance remains unclear. However, both genetic and lifestyle factors may contribute to its development.

Major depressive disorder, which people more commonly just call depression, is a highly prevalent mental health condition. The CDC estimates that 1 in 6 adults will experience depression at some point in their life.

The National Institute of Mental Health describes depression as a mood disorder that influences how people think and feel. It can also have a major influence on a person’s ability to function in their day-to-day life. A doctor may diagnose someone with depression after specific symptoms have been present for 2 weeks or more.

The possible symptoms of depression include:

  • feelings of anxiety or sadness that are persistent
  • feelings of hopelessness, guilt, or irritability
  • difficulty sleeping
  • changes in appetite
  • loss of interest in activities or hobbies
  • pain, such as a headache, that has no apparent cause or does not get better with treatment
  • suicidal thoughts or attempting suicide

Not everyone with depression experiences every symptom of the condition, but it can take a serious toll on people’s well-being and their daily activities. It can also increase the risk of suicide.

Due to the significance of depression, researchers are continuing to look into risk factors that can help detect and treat the condition at an earlier stage.

The study in question collected data from participants who were part of the Netherlands Study of Depression and Anxiety (NESDA). The researchers followed the study participants over a 9-year period.

They measured three factors that indicate insulin resistance:

  1. the ratio of triglycerides to high-density lipoprotein cholesterol, which the researchers say “has been well-correlated with the gold standard for insulin resistance and is often used in a clinical context”
  2. blood sugar level, in the form of fasting plasma glucose level
  3. waist circumference

The 601 participants that the team included in the data analysis were those who had no previous history of clinical depression or anxiety. The researchers screened the participants for depression and other psychiatric disorders, took lab specimens, and checked the participants’ physical measurements at their initial evaluation and again after 2 years.

The participants then underwent psychiatric evaluations 4, 6, and 9 years after initial evaluation. The team based the criteria for a clinical diagnosis of incidents of depression on the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders, a standardized classification system for the diagnosis of mental health conditions.

The researchers accounted for several covariates, including the participants’ age, sex, education level, level of physical activity, smoking status, and alcohol use.

Their main analysis found that all three indicators of insulin resistance were associated with an increased risk of depression.

The researchers further examined the specific subgroup of people who did not have indicators of insulin resistance when the study started but developed prediabetes — based on plasma glucose levels and other indicators of insulin resistance — by the 2-year follow-up.

They found that those who developed prediabetes during the first 2 years of the study were more than two times as likely to be experiencing major depression at the 9-year follow-up than those who had normal plasma glucose levels at the 2-year point.

When Medical News Today asked her to comment on the study, Dr. Faye Riley, senior research communications officer at Diabetes UK, said, “This research offers important clues into the two-way links between these complex conditions [diabetes and depression] and the impact of factors such as blood cholesterol and sugar levels.”

The study did have limitations. For example, the researchers were unable to utilize the euglycemic clamp technique, which they describe as the gold standard, to assess insulin resistance.

Also, the authors note that “this analysis was not powered to measure the development of metabolic pathology in a 2-year follow-up period.” Due to this, they recommend that future studies replicate the findings and continue to research the link between the onset of prediabetes and depression.

Study author Dr. Katie Watson also pointed out that more research is necessary to determine the direction of the relationship between insulin resistance and depression. She explained to MNT:

“It came as a bit of a surprise that being insulin resistant was linked to a significantly higher rate of depression during a 9-year follow-up period. We were not sure of the directionality of the relationship between the two health conditions. Now, it seems plausible that there is a bidirectional relationship between insulin resistance and depression; we have to do more research to figure that out.”

She further explained how the information that the team gleaned from the study may influence further research and clinical practice. She told MNT: “We are also exploring the idea of the metabolic subtype of depression. It is possible that different treatments or approaches help manage depression in people with insulin resistance. These tools may inform clinical practices in the future.”

Dr. Riley was also enthusiastic about further research in this area. She told MNT, “We look forward to larger studies that help further pinpoint the biological links between diabetes and depression, helping uncover the most effective ways to reduce the risk of depression for those with or at risk of type 2 diabetes, and vice versa.”

“We also hope this will lead to better ways for healthcare professionals to identify people who are at risk of developing depression, ensuring they get the treatment and care they need early on,” Dr. Riley added.