On the occasion of World Mental Health Day, we have relevant news.
A new study, published last month in The American Journal of Psychiatry, demonstrated a link between insulin resistance and the risk of developing depression.
Further, the researchers discovered that the evidence for a link between depression and new-onset cases of insulin resistance was strongest for participants who developed prediabetes.
Insulin resistance is a common metabolic condition where your body struggles to use insulin effectively. At first, this leads to chronically elevated levels of insulin, and eventually — as your cells become “immune” to the constant presence of insulin — even these high levels of insulin fail to control blood sugar, leading to the high blood sugar levels that result in a prediabetes diagnosis.
Depression, also called major depressive disorder, is a widespread mood disorder serious enough to affect people’s ability to function and enjoy their life.
In the study, conducted in the Netherlands, researchers reviewed a larger data set, and identified health records from 601 adults with no history of depression at onset. They followed the participants’ records for nine years.
The scientists looked at three metrics associated with insulin resistance: high triglyceride to high-density lipoprotein (HDL) ratio; prediabetes, assessed by fasting blood sugar level; and high central adiposity, assessed by waist circumference.
During the nine-year follow-up period, about 14% of participants developed depression. All three markers of insulin resistance were associated with greater risk of depression.
Higher triglyceride to HDL ratio showed the strongest link with depression, with a hazard ratio of 1.89. Higher fasting plasma glucose level was also linked, with a hazard ratio of 1.37. And higher waist circumference showed the weakest link, with a hazard ratio of just 1.11.
Next, the researchers studied whether new onset of any of these conditions during the first two years of the study was predictive of a bout with depression during the remaining seven years of follow-up.
They discovered that those who developed prediabetes in the two-year period were 2.66 times more likely to experience depression than those with normal blood sugar levels. For the other two metrics, no association was found.
This is an observational study and all of the usual cautions apply: correlation does not equal causation, and confounding variables could certainly exist.
But this study’s design makes it stronger than many other observational studies.
First, the biomarkers measured provide cleaner and more objective variables than data pulled from from subjective measures like food frequency questionnaires.
Also, the chronological tracking shows new-onset cases of prediabetes clearly coming before the onset of depression. This makes the direction of a potential causal relationship more obvious.
Finally, the hazard ratio of 2.66 is above 2.0, the level where our policy for grading scientific evidence gives more weight to observational results.
For more on how what you eat can affect your mental health, please check out our mental health guides: