Serotonin is a neurotransmitter that helps regulate sleep and appetite, mediate moods, and inhibit pain. Since about 95% of your serotonin is produced in your gastrointestinal tract, and your gastrointestinal tract is lined with a hundred million nerve cells, or neurons, it makes sense that the inner workings of your digestive system don’t just help you digest food, but also guide your emotions.

What’s more, the function of these neurons — and the production of neurotransmitters like serotonin — is highly influenced by the billions of "good" bacteria that make up your intestinal microbiome. These bacteria play an essential role in your health.

They protect the lining of your intestines and ensure they provide a strong barrier against toxins and "bad" bacteria; they limit inflammation; they improve how well you absorb nutrients from your food; and they activate neural pathways that travel directly between the gut and the brain.

Source - health.harvard.edu