Waaay back in June I wrote up Depression 1, which referenced a few of the studies linking depressive disorders to metabolic syndrome and diabetes. Today a new study came out from some of our favorite epidemiologists, Frank Hu and crew. Dr. Hu actually wrote an editorial questioning the wisdom of population-wide nutritional advice that resulted in replacing saturated fat with refined carbohydrates. However, he also signed off on that monumentally silly paper about "low carb" meat eaters versus low carb veggie-lovers that everyone made fun of a few months back.
Today's paper is "Bidirectional Association Between Depression and Type 2 Diabetes Mellitus in Women" from the Archives of Internal Medicine. The researchers followed 65,381 women in the Nurses' Health Study from 1996 to 2006. Clinical depression was defined by having diagnosed depression or using antidepressants, and depressed mood was defined as having a high score on the 5 item Mental Health Index questionnaire. The nurses self-reported having type 2 diabetes which was validated by medical record review.
During the 10 years of follow up, there were 2844 new cases of type 2 diabetes. Those with poor mood had a higher chance of developing diabetes. 7415 women developed clinical depression, and women who were on insulin to control type 2 diabetes had a risk ratio of 1.53 for developing clinical depression. Overall, the data showed that diabetes increased the risk of depression, and depression increased the risk of diabetes, and that more severe depression and more severe diabetes increased the risk of the other illness even more. Countless covariates did not explain the bidirectional connection.
On a population level, 23.5 million American adults have diabetes (about 10%, though about 23% of those >=60). 6.7% (14.8 million) of the US adult population has depression in any given year, with a lifetime incidence in women of around 20%. A previous meta-analysis (1) found that the odds of depression in the diabetic group was twice that of the nondiabetic comparison group. In a couple of papers, diabetes risk was increased by 60% in patients who had diagnosed depression. Frank Hu's group paper from this week made some effort to figure out what comes first - depression or diabetes? Depression (atypical depression that is) can make you crave carbs and leave you unmotivated to perform exercise. Having diabetes is usually pretty stressful too, putting you at greater risk for depression, and "depression may result from the biochemical changes directly caused by diabetes."
That last quote is perilously close to my theory, that metabolic syndrome and leptin/insulin derangements probably underlie both disorders. That's more "big picture" evolutionary medicine style thinking than I'm used to in reading the discussions of these papers, but don't get too hopeful, it was only touched on in that one sentence. I discussed the underlying mechanisms of depression and metabolic syndrome and how they are connected in Stress is Metabolic Syndrome and Chronic Stress is Chronic Illness. These aren't really radical ideas. Oh well.