Depression and Serotonin: What the New Review Actually Says

Some quite strong feelings and very broad conclusions have come following the recent publication of a review study published in the reputable journal Molecular Psychiatry that found little evidence that low levels of the brain neurotransmitter serotonin are related to the development of depression. The study has received strong media coverage and has spurred intense exchanges on social media. Some see the study as a scientific earthquake and total vindication for those who have been skeptical of the “biological” theory of depression from the start, while others view it as the penultimate dead horse beating that has absolutely no bearing on current practices for understanding or treating depression, now the world’s #1 cause for disability.

The study is what is called an “umbrella” review, which means that no new data are presented and the authors are reviewing and summarizing studies that themselves reviewed and summarized individual research studies. They focus on studies that have used various lines of investigation to link depression with low serotonin levels. This includes research (in people only) that compare levels of serotonin in the blood or cerebrospinal fluid between people who are depressed and not-depressed, studies of how well certain protein receptors are able to bind serotonin when depressed, and studies examining the role of a single but very famous gene, the serotonin transporter. The bottom line is that they find little to no evidence from the types of studies they examined that low serotonin levels or activity play a significant role in the development of depression.

These kinds of studies often put people to sleep, but in this case the reaction has been intense and personal. Much of this has to do with the authors pulling in the term “chemical imbalance” as implications of their work. This poorly-chosen term is actually one of psychiatry’s creation and now it’s being thrown back in our face imbued with even broader meaning.

While originally employed as a shortcut term for the monoamine hypothesis (briefly, the idea that some kind of deficiency in a few brain neurotransmitters, including serotonin, was a key contributor to depression), it was quickly incorporated by the pharmaceutical industry as a catch phrase for marketing purposes to depict depression as a biological illness requiring biological treatments.

The monoamine hypothesis faded as a dominant model with further research and was supplanted decades ago by the “biopsychosocial” model of psychiatric disorders which continues to prevail today. Nevertheless, the old chemical imbalance lingo remains a lightening rod to critics of psychiatry. More recently, the term has begun to be the punching bag for people who don’t believe that biological factors (also a squishy term) of any sort play a role in causing depression.

Unfortunately, the current review makes little distinction between the narrower serotonin deficit theory of depression they actually address and this ever expanding but still ill-defined “chemical imbalance” view of mental illness. This has predictably pushed the door wide open for those who want to take this review as proof that neurobiology doesn’t matter at all when it comes to depression, a claim which isn’t supported by this review or wider research whatsoever.

There is some middle ground here. While many of us in psychiatry are a little embarrassed by what now looks like some over simplistic and naïve ideas about the development of depression, there’s no denying that many of these individual studies supporting a straightforward role of serotonin and depression created quite a lot of enthusiasm in their day among the psychiatric community when first published.

These notions were then imparted to students and patients in an attempt to explain what depression was. I remember some of my own Powerpoint slides I previously used in teaching related to a very influential study that the onset of depression was related to the combination of having a particular version of the serotonin transporter gene combined with the presence of an adverse environment. There’s also no denying that selected studies supporting serotonin’s role were heavily leveraged by the pharmaceutical industry to market more antidepressants. For most of us, however, the attractiveness of these simple theories wasn’t in their advertising value but in their ability to help patients see their struggles as something that wasn’t their fault at a time when feelings of guilt and worthlessness were already sky high. Overall, then, this study is an uncomfortable reminder that we did indeed learn and repeat ideas that today look a little foolish.

At the same time, it is important not to let people take this extremely limited study to wild and sweeping conclusions and to prevent the portrayal of the psychiatric community in archaic and stereotypical forms. Depression experts have well moved on from the low serotonin theory years if not decades ago, and although they could have announced this shift better, there certainly is no organized effort to suppress this information. Over two years ago I published a post here on Psychology Today called The Rise and Fall of the Depression Gene which cited some of the same research as this review. Personally, I don’t think I’ve used the term “chemical imbalance” to explain depression in 20 years and current textbooks and information sources provide much more nuance and balance (and vagueness) when describing the origin of depression. Yes, you can still hear people occasionally drop the chemical imbalance term when trying to turn complicated processes into quick soundbites, but that’s a long way from it being an organized and accepted theory promoted by the proverbial psychiatry establishment.

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The review also has a number of real problems, which is a little ironic for a study which is one of the few not to have a “limitations” section as part of the manuscript. I’ll blame the editorial staff of the journal for that one, as well as for letting the authors use studies that examine simple depression versus controls differences in serotonin levels to conclude that serotonin, let alone all biological factors, have no role in depression at all.

Reading this study, one would never know that there are animal studies, neuroimaging studies, twin and adoption studies, inflammation research, and many other lines of evidence suggesting that depression is a very complex condition that people can arrive at from multiple pathways. Interestingly, if you look directly at some of the source studies for this review, you will see some of this evidence. For example, the meta-analysis by Ogawa cited in the review did indeed find no evidence of a link between serotonin and depression but did find evidence of a link between dopamine and depression. This statement should not be interpreted as a pitch to trade one overly simplistic view of depression for another but to point out the hazards of making conclusions that far overstep your data.

Another poor choice in this review is that while the authors don’t quite tell readers to stop taking antidepressants, they walk right up to that edge with their claim that the old chemical imbalance theory is one of the primary justifications for why people take them (rather than something like wanting to feel better). This, in my view, is careless and problematic for people who take antidepressants and for those who care for them. Just as many rightly point out that the fact that antidepressants' help shouldn’t be used as evidence of a serotonin deficit in depression, the lack of a clear serotonin deficit in depression shouldn’t be used as evidence to abandon the use of these important medications, any more than (as been said previously) a lack of an “acetaminophen deficit” should be used as evidence not to use Tylenol when you have a headache. Admittedly, we don’t know very well how antidepressants work, but for millions of people, they do.

In the end, it seems best to welcome this study for what it does say while being quite clear about what it doesn’t. Depression is complicated. Different people get there from different paths and find their way out through different means. Ascribing all depression as due to low serotonin, or poor diet, or trauma, or smartphones, or poverty will just end with a study like this.