Study finds depression is not likely caused by a chemical imbalance or low serotonin levels

ATLANTA - When it comes to understanding what causes major depressive disorder, Emory psychiatrist Dr. Boadie Dunlop says the answer is complicated.

"We're talking about the most complex organ, or thing, in the entire universe, the human brain," Dunlop says. "It goes wrong in all sorts of ways. We're trying to understand it."

About 50 years ago, Dunlop says, doctors locked onto the idea depression might be caused by a chemical imbalance in the brain, theorizing low levels of serotonin could be driving the disorder.

Today, millions of Americans take serotonin-based antidepressants, such as Prozac, Celexa and Lexapro.

But a recent new study from University College London, in which researchers looked at serotonin levels in the blood, finds no link between low serotonin, or a chemical imbalance, and depression.

Dr. Dunlop says the findings back up what experts have been thinking for years: that the cause of major depressive disorder is more complex than a single chemical imbalance.

"By reducing depression to serotonin, we neglect the thousands of other neurochemicals and neurocircuitry factors that are involved in the experience of a depressed state," he says. "So, I don't, I think it's very complex, and that's why we haven't cracked it yet. We do not know the ultimate biological cause. But we have treatments that are effective even if they aren't getting at the true cause of the illness. This is what needs to be understood in the wake of this paper."

Dr. Dunlop says many with mild to moderate depression, who are not experiencing suicidal thoughts, can be helped by psychotherapy or medication or a combination of the two.

"So, the treatments are still the same treatments," he says. "The fact that the serotonin hypothesis has fallen out of favor has not impacted the fact that we still use, when we use medicines, serotonin drugs, because there is a massive amount of evidence about how effective they are, not just against depression, also for anxiety, impulsivity and suicidality. They help a variety of different things in adults."