Serotonin and the Gut-Brain Axis: Can Food Affect Your Mood?

Feeling good: Where does it begin?

Serotonin is often referred to as the "feel-good" neurotransmitter because it helps to promote feelings of well-being, happiness, and relaxation. It plays an important role in regulating mood, appetite, sleep, and other functions.

Something you may not know, however, is that most serotonin is produced in the gut. There are certain gut cells that take up the amino acid tryptophan, which can be obtained through the diet, and use it to make serotonin.

Does this mean that we can boost serotonin production by eating foods high in tryptophan? Keep reading to learn more. 

Serotonin: It's not just in your brain, it's in your gut too

While some serotonin is produced in the brain, most of it is produced in the gut by cells known as enterochromaffin cells. That’s right, your gut is a serotonin-producing machine, creating over 90% of the body’s total serotonin [1].

In healthy people, gut-derived serotonin can't reach the brain because of a protective wall called the blood-brain barrier. So we actually have two separate pools of serotonin: gut-derived serotonin and brain-derived serotonin.

Now, you might be thinking, "Well, if gut serotonin can't reach the brain, then what's the point of producing that much serotonin in the gut?" Most of us have heard of serotonin in relation to its brain functions and effects on mood and happiness.

But in fact, serotonin has many important functions throughout the body:

• Regulating gut movements
• Supporting the creation of new blood cells in the bone marrow
• Modulating immune responses
• Participating in glucose metabolism [2]

Do gut microbes contribute to serotonin production?

Scientists think that gut microbes may influence the levels of serotonin in the gut in different ways. Some gut bacteria may boost serotonin levels by stimulating chromaffin cells to produce serotonin, possibly via short-chain fatty acids [3], [4], [5]. And in laboratory experiments, it appears some gut bacteria use tryptophan to produce serotonin themselves [6], [7].

Other gut bacteria may instead reduce serotonin levels by absorbing it [8], or by digesting tryptophan. While not a common threat, some gut bacteria have the ability to break down tryptophan to make up other compounds [9].

Serotonin and the gut-brain axis

It is well established that gut-derived serotonin can’t cross the blood brain barrier. But we know that there is a two-way communication pathway known as the gut-brain axis that connects the gut with the brain via the vagus nerve [10].

Gut microbes can send signals from the gut to the brain via this bidirectional pathway, something that scientists have named the microbiota-gut-brain axis [1].

Is it possible that gut serotonin has an influence on the brain via the gut-brain axis? Can your gut microbes impact serotonin levels in the brain?

Emerging new research suggests that the interplay between gut serotonin and the gut microbiome may impact the brain’s reception to serotonin. One study in mice found that gut bacteria that increased serotonin production in the gut also altered the expression of serotonin receptors in the brain [4].

Plus, some scientists believe that by stimulating the vagus nerve, gut-derived serotonin can have an impact on the gut-brain axis [11]. In other words, it’s possible that your gut’s serotonin can still influence your brain, even if it's indirectly.

Also, experiments in mice have shown that supplementation with probiotic Lactobacillus helveticus increases the production of serotonin in the brain [12]. Scientists are not sure how this happens, it could be that these bacteria promote tryptophan availability for serotonin production (tryptophan can cross the blood brain barrier) or that certain signals are sent via the vagus nerve. We know that L. helveticus is able to produce serotonin in laboratory settings [7].

Are there foods that boost serotonin production?

The only nutrient that our body needs to make serotonin is the amino acid tryptophan, which is obtained from the diet [13]. Foods with the highest content of tryptophan include:

• Turkey, chicken, tuna, eggs
• Milk, cheese
• Peanuts, cashew nuts, sunflower seeds
• Beans, lentils
• Oats
• Avocado, bananas [14]

While in theory, eating more tryptophan-rich foods could modify your mood by increasing serotonin production, there aren’t any human studies that have proven this.

However, a recent small study found that adults following a psychobiotic diet (a diet that’s low on processed foods and high on fermented and plant-based, fiber-rich foods) felt less stressed and had differences in tryptophan metabolism [15].

So while it seems that a healthy diet can impact tryptophan levels in your body and make you feel less stressed out, we can’t say for sure this is due to increased production of serotonin.

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Thank you for joining us for The Science of Happiness

Thank you for joining us during our May Mental Health Awareness Month blog series, The Science of Happiness! In case you missed the other posts in the series, check them out here:

• From Belly to Brain: The Gut-Brain Axis
• Tiny Tummies, Big Brains: The Gut Microbiome and Cognitive Development in Infants
• Gut Check: How Short-Chain Fatty Acids Can Keep Your Brain in Shape

Curious about your baby’s gut? We now know healthier guts = healthier babies. Tiny Health’s revolutionary mess-free Baby Gut Health Test takes less than 5 minutes, and you’ll get a personalized report with actionable results so you can start taking steps towards optimal health. 

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