There’s big money to be had in gut bacteria.

Funding going toward companies related to the microbiome—the trillions of bacteria that live primarily in our digestive system—increased more than 400% between 2011 and 2015 and was up this year despite overall venture funding being down. Those were the stats Fortune senior writer Michal Lev-Ram cited to kick off a session on the so-called second brain.

What’s in part driving the investment is a broader movement within health care as “people are taking much more control of their health,” said Jessica Richman, co-founder and CEO of startup uBiome at Fortune’s Brainstorm Health summit on Wednesday.

Richman’s company sequences the microbiome of customers who send in samples swabbed from their nose, mouth, skin, gut, and genitals. The sequencing of this bacteria can help identify specific infections or health problems. Users can then take those results and treat their condition through changes to their diet, for example.

Richman noted that research has started to show that the microbiome is linked to mental health issues, such as depression and anxiety.

Julie Smolyansky, president and CEO of Lifeway Foods, said that research is only now confirming the long-held assumptions that good bacteria lead to good health and longevity. “Right now the science is catching up to the folklore,” she said. “Our gut is controlling our entire body. We’re actually starting to understand we have a second brain in the gut.”

Lifeway makes a fermented yogurt-like drink called kefir that contains probiotics—bacteria this is thought to promote good gut health. Smolyansky said her company is rolling out a probiotic in pill form as well.