90% of serotonin is made in your gut - Marty Makary Pt 3

1 Dec 2025 · 1 min

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A short transcript excerpt from a YouTube video (Tiny Health) where Marty Makary discusses the gut microbiome’s role in health — producing vitamins, clotting factors, and roughly 90% of the body’s serotonin — and links early-life antibiotic exposure to higher rates of chronic conditions (e.g., allergic rhinitis, obesity). He emphasizes that many two-year-olds have had multiple antibiotic courses and calls for improved antibiotic stewardship.

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We also know that the microbiome is producing things central to health. It makes vitamins, it makes clotting factors, it makes serotonin, which is involved in mood. 90% of your body's serotonin is made in the microbiome. What's happening when we alter the microbiome in the modern world, antibiotics can carpet bomb parts of the microbiome, resulting in overgrowth of one type of bacteria. And a study out of the Mayo Clinic that was recently published showed that kids who took antibiotics in the first two years of life had a higher rate of chronic diseases, from allergic rhinitis to obesity, compared to kids who did not have exposure in the first two years. Why is that? Well, it may be because their microbiome has been altered. Now, the average two year old has already received over two courses of antibiotics in their brief two years, sometimes in the first day of life, just as a precaution. And we're now trying to have better antibiotic stewardship.