…And sugar makes me blue. Isn’t that one of the bitter lessons of childhood? The seductions of candy are irresistible, yet too much often turns to tears. And most of us learned that lesson again with alcohol. In another round of science catching up with conventional wisdom, the American Journal of Psychiatry this month published an article about how the early signs of diabetes, fondly called “sugar” by some southern folk, predict the later development of depression, fondly called the “blues” by many of us.
Listen to this blog on SoundCloud.
This study is worth celebrating because in the circles of mind-body medicine it has been rare for a longitudinal population study to collect both biological and psychological measures on community samples and then follow them long enough to be able to determine cause and effect relationships when a new illness develops. For the past fifteen years we have known that depression and heart disease have a circular relationship—one increases the risks for the other—but we had not closed that circle for depression and diabetes. Does the process of developing diabetes contribute to the process of developing depression?
The Netherlands Study of Depression and Anxiety
Brenda Penninx, MD, PhD, and her colleagues give us good reason to say yes, very likely. They began collecting their sample for the Netherlands Study of Depression and Anxiety in the late 1990s and early 2000s. They have followed this group of over 1500 adults for over 20 years, and for this report they selected 601 of their participants who had never had a depressive or anxiety disorder at the time of enrollment. Early in each person’s enrollment they collected three measures of insulin resistance, which is a common pathway for developing type II diabetes, and then they looked at who developed depression over the next nine years.
They found that all three measures of insulin resistance (high triglyceride-HDL ratio, high fasting blood glucose, and high waist circumference) predicted the new onset of major depression. This is fascinating because it suggests that even before an adult fully develops type II diabetes the process of developing of depression may have also begun. How could this be?
Pathways that Cross
The authors guess that among the likely possibilities deserving further study is the pathway of “neuroinflammation,” by which the diminishing effects of insulin lead to low grade inflammation in the central nervous system, a set up for fatigue, apathy, and social withdrawal. Inflammation also reduces serotonin levels and neurogenesis in the emotional brain centers, both of which contribute to depression. Insulin resistance is associated with the stress hormone cortisol, high levels of which can lead to depression. And genes may be at the root of all this. It’s possible that depression, diabetes, and heart disease represent three outcomes of troubled stress response systems that began with genes that are vulnerable to toxic stress.
Multimorbidity
Sound grim? What’s to celebrate? Here’s science spelling out how one disease process lays the pathway for another over many years. We have a big problem in this country with “multimorbidity,” or the number of people with many chronic conditions. Over 40% of adults in the US have two or more chronic conditions, and 12% have five or more. Most of our health care dollar is spent on people who have multiple chronic conditions.
Understanding how these pathways are related may be a key to prevention. That is, when programs like the National Diabetes Prevention Program succeed in slowing down the insulin resistance process for people with pre-diabetes, they may also be slowing down the process of developing depression. And, if confirmed by other studies, this kind of finding is likely to redefine what primary care clinicians and psychiatrists do to assess and treat people at risk for depression. A psychiatric evaluation may soon include measures of insulin resistance. But until more studies put this one in perspective, it’s enough for us to appreciate that one pathway to depression may cross with a pathway to diabetes. This study gives us another light shined on the mystery of how the stress response system works in health and illness. That’s worth celebrating.