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The Microbiome, Probiotics, and Everyday Gut Health

Microbe counts, gut immunity, serotonin, probiotics and food targets. The microbiome numbers that became too easy to repeat.

Food, gut & metabolism
By Andre Heeg, MD, DDSCreated July 21, 2026
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Key Takeaways

Several popular microbiome claims are wrong. Bacterial cells are close to human cells in number, and gut serotonin does not enter the brain.

  • A revised estimate put bacterial cells near 3.8 x 10^13 and human cells near 3.0 x 10^13 for one reference adult male, with substantial variation. The older claim of 100 trillion microbes and a large bacterial majority is not supported. [1]
  • The gut has a major mucosal immune role, but 70% of all immune cells is not a stable count. Most serotonin is peripheral and does not cross the blood-brain barrier, so gut production does not establish brain availability or mood causation. [2] [3]
  • Small individual-response and post-antibiotic studies do not show that most probiotics backfire for most people. Effects are product, strain, indication, timing, and host specific. Reference 22 evaluated Saccharomyces boulardii, not LGG. [4] [5]
  • Thirty plant foods per week came from cross-sectional citizen science, not a randomized dose trial. The three-hour evening gap, twelve-hour fast, and simple fermented-food serving rule are optional personal routines, not established treatments or universal requirements. [6]

Common questions

Do bacterial cells outnumber human cells ten to one?

No. A revised estimate put bacterial cells near 3.8 x 10^13 and human cells near 3.0 x 10^13 in 1 reference adult male. Both numbers vary. The old 10-to-1 claim is not supported. [1]

Is 70% of the immune system in the gut, and is most serotonin made there?

The gut has a major immune role, but 70% is not a stable count of all immune cells. Most serotonin is made outside the brain and cannot cross the blood-brain barrier. Gut production alone says little about mood. [2] [3]

Can probiotics make gut health worse after antibiotics?

A few small studies found delayed microbiome recovery after certain probiotics. They do not show that most products harm most people. Strain, product, timing, antibiotic, indication and the host all matter. Reference 22 studied Saccharomyces boulardii, not LGG. [4] [5]

How much fiber and fermented food do I need for a healthy microbiome?

There is no universal target. Thirty plant foods a week came from cross-sectional citizen science, not a dose trial. A 3-hour evening gap, a 12-hour fast and a daily fermented-food serving are optional routines, not established treatments. [6]

Should I take a probiotic after every course of antibiotics?

No universal rule supports a probiotic after every antibiotic course. Small studies report different effects by strain, antibiotic, timing, and person. A useful clinical question names the product, strain, antibiotic, and reason. [4] [5]

References

  1. Sender, R., Fuchs, S., & Milo, R. (2016). Revised estimates for the number of human and bacteria cells in the body. PLOS Biology, 14(8), e1002533. https://doi.org/10.1371/journal.pbio.1002533
  2. Sender, R., Weiss, Y., Navon, Y., et al. (2023). The total mass, number, and distribution of immune cells in the human body. Proceedings of the National Academy of Sciences, 120(44), e2308511120. https://doi.org/10.1073/pnas.2308511120
  3. Jones, L. A., Sun, E. W., Martin, A. M., & Keating, D. J. (2020). The ever-changing roles of serotonin. International Journal of Biochemistry & Cell Biology, 125, 105776. https://doi.org/10.1016/j.biocel.2020.105776
  4. Suez, J., Zmora, N., Zilberman-Schapira, G., et al. (2018). Post-antibiotic gut mucosal microbiome reconstitution is impaired by probiotics and improved by autologous FMT. Cell, 174(6), 1406–1423.e16. https://pubmed.ncbi.nlm.nih.gov/30193113/
  5. Szajewska, H., & Kołodziej, M. (2015). Systematic review with meta-analysis: Saccharomyces boulardii in the prevention of antibiotic-associated diarrhoea. Alimentary Pharmacology & Therapeutics, 42(7), 793–801. https://doi.org/10.1111/apt.13344
  6. Wastyk, H. C., Fragiadakis, G. K., Perelman, D., et al. (2021). Gut-microbiota-targeted diets modulate human immune status. Cell, 184(16), 4137–4153.e14. https://doi.org/10.1016/j.cell.2021.06.019

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