Serotonin: Definition, Function, & Boosters

By Charlie Huntington, M.A., Ph. D. Candidate
​Reviewed by Tchiki Davis, M.A., Ph.D.
Read this article to learn the definition and function of serotonin as well as how to increase your well-being by boosting your serotonin levels.
*This page may include affiliate links; that means we earn from qualifying purchases of products.
Serotonin is one of the most important neurotransmitters in our bodies – important enough that you’ve probably heard of it before, probably in the context of discussions of mental health concerns such as depression or anxiety. But you may not be aware of just what serotonin is and how essential it is to the healthy functioning of your body.​
With topics like this, it is easy to get lost in an onslaught of complicated terminology, the kind of vocabulary you might need a medical degree to truly understand. I hope that in reading this article, you come to understand not just more about serotonin, but also how to talk about it in terms that your friends and family can easily understand too.

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What Is Serotonin? (A Definition)

First of all, serotonin is a neurotransmitter, which means it is a chemical that moves back and forth in the space (called the synapse) between two nerve cells, or neurons. When one neuron is ready to send a message to another, it sends an electrical signal to its end, which causes a neurotransmitter such as serotonin to communicate that message to another cell. Once that message has been communicated, the neurotransmitter can stay with the new cell, return to the old cell, or hang out in the space between them.
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Certain neurons are programmed to interact with certain neurotransmitters, and neurons that communicate with other cells via serotonin are found throughout your central nervous system – in other words, throughout your entire body (Fuller & Wong, 1990). Neurons that use serotonin are also found in especially high amounts in your gastrointestinal tract – what we commonly call the “gut” (Furness & Costa, 1982).

Serotonin Function

Since the role of a neurotransmitter is to help neurons communicate with each other, having just the right amount of serotonin where it’s needed is key to the healthy functioning of any organism, including humans. Serotonin facilitates communication throughout our central nervous system, helping to regulate many different body functions. While you might already be aware that serotonin levels affect our mood and emotions, serotonin is also involved in the regulation of our sleep, memory, appetite, digestion, body temperature, and how awake and alert we are (Jacobs & Azmitia, 1992). Part of why it serves this function is that many serotonin receptors are located in the brainstem, a primitive part of your nervous system that sits between the brain itself and the rest of your central nervous system.
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Your body’s goal in regulating itself is to reach homeostasis, or a balanced, “just right” place for each body function. Serotonin plays a critical role in trying to find that balance, and when the balance isn’t there, problems result. If there is too little or too much serotonin reaching the part of the brain called the hypothalamus, for example, it may be harder to keep your body temperature regulated. And if serotonin levels are off in your hippocampus, you may focus too much or too little on certain negative emotions or thoughts (Yabut et al., 2019). 

Serotonin in the Gut

Perhaps you have read some of the new research describing how closely related our brains and our guts are. Serotonin is no exception to this finding! Fully 95% of the serotonin in your body develops and is stored in your gut (Furness & Costa, 1982). Your gastrointestinal tract creates serotonin by converting tryptophan, an amino acid in the foods that you eat, into serotonin.
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In addition to impacting how much serotonin the rest of your body gets, the serotonin in your gut is important for healthy gut functioning (Gershon & Tack, 2007). In experiments with animals, changing the amount of serotonin in their intestines has caused changes in their gut microbiome and caused inflammation in the gut (Ghia et al., 2009).

Serotonin vs Dopamine

Serotonin and dopamine are both very important neurotransmitters, but their roles and functions in the brain and body are different. For the brain, dopamine is a “reward” chemical, involved in telling us what to pay attention to and work toward (Conio et al., 2020). Having more dopamine in your system generally means you are going to easily concentrate on what would be rewarding to you.

By contrast, serotonin’s role in the brain seems to be related to the regulation of negative thoughts and emotions (Dayan & Huys, 2008). Serotonin’s role may be to help us shift away from negative patterns – without enough serotonin in key regions of our brain, we may have a hard time saying to ourselves, “I’m really stuck in a spiral of negative feelings. I should go talk to a friend or do something to distract myself.”
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In summary, you can think of dopamine and serotonin as the “happy” neurotransmitters. Just keep in mind that one of them – dopamine – enables us to feel pleasure, while the other – serotonin – helps us avoid negative thoughts and feelings.

Serotonin and Depression

You’ve probably heard something about how serotonin and depression are related to each other, and for good reason. The “serotonin hypothesis” of depression is one of the oldest theories about why people get depressed. In its simplest form, it states that a lack of activity in the serotonin pathways in the brain causes depression (Cowen & Browning, 2015).
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Scientists have determined this connection through several methods. First, they have observed that when people’s brains are deprived of tryptophan, the amino acid that becomes serotonin, many of them develop depression within a few hours (Lam et al., 1996). Second, they have observed abnormal levels of serotonin production in many people who are depressed (Jann & Slade, 2007). Finally, they have observed that providing the brain with more serotonin or tryptophan decreases depression symptoms (Turner et al., 2006).

Serotonin Deficiency

Where does this serotonin deficiency come from? As you might have expected, there is some evidence that suggests that our genes play a role. People whose genetic code features less effective versions of certain serotonin-related genes may have a harder time generating, maintaining, and moving around serotonin (Zhang et al., 2005). If you have heard that depression runs in families, well, that’s right – less effective serotonin processing can be hereditary.

Serotonin deficiency can also result from an invalidating, stressful, or consistently negative environment (Cowen & Browning, 2015). And as with almost all biological and psychological topics, serotonin deficiency can grow through epigenetic pathways.

What does this epigenetic pathway look like? Imagine you are born with a risk of low serotonin because of your genes. You find it hard to transition out of negative thought patterns and feelings. This makes it harder to have positive and meaningful experiences over time. Without those positive experiences, your serotonin levels are unlikely to go up – in fact, they may even go down further.
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Unfortunately, this pattern is experienced by many people with depression. Knowing this can help us understand how brain chemistry makes it so hard for people with mental health disorders to heal. Nobody can force themselves, just of their own will, to have more serotonin where they need it. Thankfully, there are many tools available to people who might be experiencing abnormal levels of serotonin.

Serotonin Reuptake Inhibitors

Psychiatric drugs that increase the amount of available serotonin are the best available form of medication for addressing depression and anxiety (Hoffman & Mathew, 2008). Of these, selective serotonin reuptake inhibitors (SSRIs) are the most commonly prescribed and used. SSRIs get their name from their function: they stop the reuptake of only serotonin, and not other neurotransmitters.
 
What does reuptake mean? Remember how neurotransmitters can either stay with the new neuron, return to the old neuron, or hang out in between them? SSRIs make sure that reuptake – going back to the original neuron – happens less often, and that serotonin stays in the space between the neurons instead. This makes it easier for communication between the neurons to keep happening.
 
SSRIs were first introduced as a psychiatric drug in the 1980s, and since then, many research studies have attested to how effective they can be in treating depression and anxiety, even to the point of causing permanent, positive changes in the brain’s serotonin pathways (Sharp & Cowen, 2011). However, they are often less effective for people who are severely depressed or who have many genetic risk factors for low serotonin (Uher et al., 2014).
 
For a visual demonstration of how SSRIs work, watch the following video:

Video: Selective Serotonin Reuptake Inhibitors (SSRIs)

Serotonin Boosting Foods

Since our bodies cannot naturally produce tryptophan – the amino acid that is converted into serotonin – we rely on getting tryptophan from our food. The foods we eat impact how much serotonin is in our brains and where specifically it is found (Haleem & Mahmoud, 2021). Just as important, research tells us that eating foods high in tryptophan can improve our moods and decrease our anxiety (Lindseth et al., 2015).
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Dairy
Cow milk provides large amounts of tryptophan, with whole milk having the highest concentration. Since cheese comes from the same source, it’s no surprise that it also has tryptophan, although at a lower concentration than milk.
 
Meat
You might have heard before – correctly – that turkey provides plenty of tryptophan. But chicken also contains high amounts of tryptophan, as do chicken eggs.
 
Fish
You can also get tryptophan from canned tuna, but you’ll get a huge boost from eating salmon.
 
Oats
If you want to start your day off with a healthy dose of tryptophan, a bowl of oatmeal is an effective place to start.
 
Nuts and Seeds
Peanuts provide a good supply of tryptophan, as do walnuts and pumpkin seeds.
 
Dark Green Leafy Vegetables
Kale and spinach are also good sources of tryptophan.
 
Soy Products
Finally, vegetarians and vegans who’ve been worrying about their tryptophan options will be happy to know that tofu, soy milk, and soy sauce also provide tryptophan.
 

It is important to bear in mind that having a healthy gut overall will help your stomach make the most of the tryptophan you ingest. This means you can help yourself by limiting your intake of foods that disrupt your gut bacteria, such as processed sugars, trans fats, and artificial sweeteners. Instead, you could focus on eating foods with lots of prebiotics and probiotics, such as yogurt.

Serotonin Supplements

Scientific research has shown that at least one serotonin supplement, 5-Hydroxytryptophan (5-HTP), has positive effects. In clinical trials, people who are given a pill containing 5-HTP have experienced less appetite, better sleep, and decreases in their anxiety and depression (Halford et al., 2011; Maffei, 2020).

Serotonin Overdose

Outside of consuming psychiatric medications, it is extremely unlikely for a person to have too much serotonin in their system. But people who are taking SSRIs can experience what is called “serotonin syndrome”, which typically involves symptoms such as shivering and diarrhea, but can also include fever, tightening of the muscles, and even seizures.
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However, serotonin syndrome occurs mostly when individuals increase their SSRI dosage or are prescribed other drugs to take at the same time as their SSRIs, such as during a hospital visit (Isbister et al., 2007). In other words, a serotonin overdose won’t be a concern for most people, and even people taking SSRIs should be fine, as long as they follow their doctor’s orders and don’t take drugs (legal or illegal) that they haven’t been prescribed.

Articles Related to Serotonin

Want to learn more? Here are some related articles that might be helpful.

Books Related to Serotonin

To keep learning, here are some books to explore:​​

Final Thoughts on Serotonin

The more we learn about serotonin, the clearer its vital role in regulating our minds and bodies becomes. If you think you might be experiencing the symptoms of somebody with low serotonin levels, I encourage you to talk to your primary care physician. If you are curious about serotonin supplementation, know that the risks appear to be minimal, and the upsides considerable.
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Either way, talk to a healthcare professional you trust. Serotonin is far from the only factor in depression, anxiety, gut health, and the other health concerns discussed in this article. Only a professional can best determine whether your needs are best met by addressing your serotonin levels or by some other intervention.

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