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(The Conversation) — Every Thanksgiving, the myths of tryptophan’s almost magical powers re-emerge. There is the myth of turkey and drowsiness: eating a lot of turkey meat supposedly makes people feel tired because it contains an amino acid called tryptophan.
This molecule travels to the brain, where it is converted into a neurotransmitter called serotonin, which in turn is converted into a hormone called melatonin. And ready! Drowsiness arrives.
But science and the internet agree: it’s not the turkey’s tryptophan that is to blame for the post-feast nap. All protein sources, and even vegetables, contain some tryptophan; turkey is nothing special in this regard.
So the myth of turkey sleepiness may be fading, but other legends about tryptophan’s effects on the brain are taking hold. Some people are considering tryptophan supplements as an unconventional treatment for depression. Others are curious whether eating foods high or low in tryptophan might be helpful in influencing mood. Recently, some scientists have even proposed that gut bacteria are causing changes in emotions by producing or breaking down tryptophan.
This connection between tryptophan and mood is an area of ongoing research. And while some are enthralled by tryptophan’s potential, it’s unclear if the excitement is justified.
For example, in 2000 researchers found that when people ate an isolated protein high in tryptophan, they felt less stress when solving math problems.
However, placebo-controlled clinical trials, in general, have not shown much connection. Some studies have found that pure tryptophan supplements provided little to no benefit for people with depression. Some studies have even looked at what happens when tryptophan is removed from people’s diets, but they also found little to no effect.
So what do these mixed results explain?
Serotonin itself still holds mysteries
In addition to human studies, the biology of tryptophan has been well studied in rodents. Research conducted in the early 1970s showed that taking tryptophan supplements can increase serotonin, a neurotransmitter that has historically been associated with feelings of well-being and happiness.
Since then, scientists have learned a lot of interesting facts about serotonin. For example, there are 14 separate receptors for serotonin and they are found throughout the brain.
Researchers have learned how to affect this system with drugs, but not very precisely. For example, drugs such as selective serotonin reuptake inhibitor antidepressants – better known as SSRIs – do not target individual receptors and are not limited to specific regions of the brain. Instead, SSRIs – the best known is Prozac – bluntly increase serotonin everywhere.
This non-specificity is why, in my opinion, it is hard to believe that SSRIs work at all. Here’s an analogy: Let’s say you’re Jeff Bezos and you want to increase Amazon’s revenue by speeding up your deliveries. So you decide to increase the speed on all delivery vehicles. From now on, each truck will increase its speed by 5%. It may be logistical genius, or perhaps, more likely, it ends up in chaos. Like increasing serotonin throughout the brain, this direct approach might not be ideal.
Analogies aside, whether SSRIs affect people’s moods is an experimental question, and some research has supported the idea that these drugs work. However, especially in recent times, its effectiveness has come under intense scrutiny. Some recent reviews cite 30 years of study and question the clinical value of SSRIs, while others argue that these drugs improve symptoms of depression.
It’s complicated and there are still some disagreements, but most psychiatrists agree that SSRIs are not effective for everyone. These drugs are not a psychiatric panacea.
More chemical adjustment for mood
In light of all this, I often wonder if psychiatric researchers need 73 studies looking at whether tryptophan depletion has an impact on mood.
When it comes to understanding the connections between gut bacteria and the brain, or the biggest challenge of understanding and treating mental illness, should researchers really keep thinking about tryptophan?
It seems true that, like SSRIs, increasing tryptophan has a huge impact on serotonin. It is definitely possible that increasing serotonin could influence mood and therefore increasing tryptophan could do the same. But it’s also possible that manipulating something as complicated as human emotion requires a little more nuance.
Psychiatric research has long moved away from the idea that your brain is a bag of chemicals; modern neuroscientists ask for a little more specificity. From this perspective, I am skeptical of the idea that tryptophan is the remedy for depression that psychiatry needs. Not only has experimental research found rather weak results, but the theory itself is not very convincing.
Serotonin, seemingly full of psychiatric possibilities, has long fascinated psychiatric researchers. But what the last half century seems to have shown is that the neuroscience of human emotions is not simple. To promote lasting changes in mental health, scientists may need a little more reverence for the complex emotional beings that we all are.
So no, a big turkey dinner, as packed with delicious tryptophan as it could be, probably won’t be the neurochemical booster to your Thanksgiving mood.