Drug companies spend billions of dollars each year advertising their products. In fact, according to a brilliant expose of the pharmaceutical industry by Dr. Marcia Angell (former editor of The New England Journal of Medicine), drug companies spend about 3 times as much on marketing as they do on research and development!
And they've hit on a particularly effective marketing angle for their profitable line of antidepressant meds: it's all about 'chemical imbalance'. Just tell people they have a brain-related 'chemical imbalance', and most will assume it's a condition that can only be remedied by ingesting more chemicals (i.e., by taking expensive medications).
But this widespread assumption is flawed in its underlying logic. Simply put: medication is not the only way to change the depressed brain. Psychotherapy changes the brain. Pill placebo changes the brain. Exercise changes the brain.
Let's look at this last point in a little more detail. One of the most serious consequences of depression is the fact that it suppresses a key growth hormone in the brain (it's called BDNF, or brain-derived neurotrophin factor). Without adequate levels of this growth hormone, we can't form new connections between neurons, and those new connections are crucial to our ability to form new memories (this is the reason, in fact, that most depressed patients experience poor short-term memory). Over time, low levels of this growth hormone cause key regions of the brain to shrink. That's right: over time, depression causes brain damage.
But when we get aerobic exercise, this triggers a massive increase in the brain's production of neural growth hormone (BDNF). Not only does exercise help protect the depressed brain from damage, but it serves as a powerful antidepressant activity in its own right.
In a landmark study at Duke Medical Center, aerobic exercise (just 3 times per week for 30 minutes) was found to be as effective as Zoloft in the short term, and even more effective than Zoloft in the long-term treatment of depression.
Exercise is a potent treatment for depression. This is why the British Medical Association (a group much less influenced by the drug industry than our own American Medical Association) recently recommended exercise over antidepressant meds as a first-line treatment for depressive illness. The Brits now seem to understand what most Americans do not: chemical imbalance can be remedied by experience, not just medication.