It seems like a straightforward topic. How does obesity affect the brain? The associations are complicated, chicken and egg sorts of problems. Does being depressed make you unable to care for yourself and exercise, so you pack on the pounds? Does being anxious send you straight for the Doritos? Or does being obese and the social stigma lead to increased stress, depression, and anxiety? Does a third underlying inflammatory mechanism cause all three conditions? Is it all of the above?
The prevalence of all three disorders is extraordinarily high these days. Lifetime prevalence of depression is up to 21% of the general population, anxiety 31% (1), and obesity 32.2% in the United States, and between 7 and 27% in Europe (2).
Depression is certainly found more often in people with obesity, as is anxiety. Depression and anxiety also share common risk factors with obesity, including increased rates of cardiovascular disease and type II diabetes. An inactive lifestyle is found more often among depressed, anxious, and obese individuals (3). An educated understanding of obesity makes one realize that the underlying metabolic derangements leading to obesity precede sloth, not the other way around. It is easier to see the basics of how being depressed would keep you from exercising, and then lack of exercising would exacerbate the depression. Depression and anxiety can lead to social isolation - does social isolation leave you nothing to do but stay at home watching TV and eating?
And then, of course, there is the dysregulation of the hypothalamic-pituitary-adrenal axis. Yes, hormones, cortisol, and stress. It's pretty clear to everyone that the HPA axis is screwed up in depression and especially in depression with anxiety (4). There are also correlates between excess cortisol and abdominal obesity, abdominal obesity and insulin resistance, insulin resistance and depression...
But what (besides stress) can cause the dysregulation of the HPA axis? I still have some more papers to read, but it's funny how the speculation in most research articles stops right there. We get to the HPA axis and stress and, voila, the answer to Life, Depression, Obesity, and Everything. You all know what I think about modern stress, right? Sure, life is stressful - but it can't just be stress. We were stressed in the 1900s, the 1920s, the 1940s... and we sure have a heck of a lot more obesity, depression, and anxiety now than we had then.
What else is different? We have a lot more electronics (maybe). And reality television. Oh, and, certainly, we eat a lot differently than we used to. I've found two major possible links between diet and changes in the HPA axis over the past few months. We are undoubtedly low on magnesium as a population, and magnesium plays a role in cooling off the cortisol/stress side of the HPA axis. Also, we don't consume nearly as many phospholipids as we used to, especially since we were all told to go low fat and low cholesterol, and some studies show a link between phospholipid consumption and the reactivity of the HPA axis.
There's more to understanding depression and obesity, and I hope to use the next post to delve into the neurotransmitters and biochemistry a bit more. The truth is, of course, that HPA axis issues are just one step in the chain to our portly, sad population. Serotonin, dopamine, and norepinephrine are involved too. And anyone who can explain how obesity is related to depression and anxiety also have to explain how depression and anxiety can also lead to weight loss.
There's a lot of explaining to do.