India, Aug. 25 -- The most important dinner guests at your table tonight won't pull up a chair or engage in chitchat. They are your gut microbes and there are trillions of them eating with you.

A few decades ago, "microbiome" drew blank stares outside research labs. Now the word pops up in startup pitches, with everyone from wellness influencers to venture capitalists convinced that gut bacteria hold the keys to health, happiness, and maybe immortality. Some of it is hype that is not backed by science but rather by an insatiable need to sell probiotics and supplements.

But behind the hype, science has been assembling real evidence on how gut microbes really do influence human health.

A sweeping review published in Nature Reviews Gastro...