Mental and Physical Illness: What's the Link?

Tomorrow, I am to have my gallbladder removed because of some serious pains I've had over the past two or three months. I'm not looking forward to it -- let's face it, who looks forward to surgery? And in light of the surgery, I've been thinking about the link between mental and physical health.

A happy little green gallbladder

Doing a little research on the internet, it looks as though quite a few studies have been conducted on the topic of physical health in patients with schizophrenia. This article concludes that schizophrenic patients have more physical illness problems than those without schizophrenia.  It's not clear from the article, though, if schizophrenia causes the physical illness, or the other way around.

This web page from the UK talks about the link between depression and physical illness, discussing how physical illness can cause mental illness.  There's an entire book which is a collection of essays about the link between the two, called Depression and Physical Illness (Amazon.com has it listed at $108, so I wouldn't recommend picking up a copy).

Anybody that watches TV knows that there's also a link between depression, anxiety, and fibromyalgia.  Doctors consider depression and anxiety to be a symptom of fibromyalgia, And this study discusses a link between rheumatoid arthritis and depression.

I guess my question is, which comes first?  Is mental illness (depression, bipolar disorder, schizophrenia) more likely to cause physical problems, or vice versa?  With depression, it seems like it could be obvious:  you feel like crap physically, so mentally you start to feel like crap. In my post about World Mental Health Day, I mention that doctors estimate upwards of 50% of cancer patients suffer from depression.  Well, I'd be depressed, too, if I had cancer.  

Of course, physical illness isn't the only reason people suffer from depression (and sometimes, there is no reason -- it's an illness all to itself).  But is is possible for chronic mental illness to cause chronic physical illness?

I know in my own experience, when I am in a bad phase, I definitely tend to neglect myself.  I don't eat right, I don't exercise, I sleep a lot, I avoid doctors (and pretty much everyone else).  Inevitably, this kind of neglect leads to physical complaints.  I usually get headaches, muscle aches, and stomach problems associated with my depressive episodes.

I wonder, though, can the mere fact of having a mental illness can be linked to physical illness?  Like, are people with chronic depression more sensitive to pain?  We always hear about neurons in the brain being blamed for depression -- is it possible these neurons that are "faulty" could be causing physical pain, too?  Or, if I'm bipolar, could my weird chemical shifts that cause it also cause shifts in my physical feelings as well? 

Admittedly, I could probably do some deeper research to answer these kinds of questions right now.  And maybe later I'll look into it.  But I feel kind of lazy at the moment, so I'm just sort of reflecting. 

And, for the record, I don't think my anxiety or bipolar depression caused my gallbladder problems. 

I blame my kids.