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Stanford University School of Medicine

To debug your gut (and maybe your brain, too), make nice to the bugs that live inside it

Each of us is carrying about 100 trillion microbes in our gut. They're tiny and squirmy, but if you could line them all up end to end and convince them to stand still, they'd reach to the moon.

What are all those bugs doing in our gut? What happens when they aren't doing it? What should we be doing, or not doing, to keep them doing it?

I learned a little about that earlier this month, when I moderated a panel of three experts on our gut microbes' impact on our immune systems, our brains and the rest of us.

The discussion took place at a bookstore about 50 miles north of Stanford. Featured were the Stanford gut-microbiologist husband-and-wife team Justin Sonnenburg, PhD, and Erica Sonnenburg, PhD, authors of the book The Good Gut: Taking Control of Your Weight, Your Mood, and Your Long-Term Health; and Emeran Mayer, MD, PhD, a practicing gastroenterologist, UCLA professor of medicine, psychiatry and physiology, and author of  The Mind-Gut Connection: How the Hidden Conversation within Our Bodies Impacts Our Mood, Our Choices, and Our Overall Health.

To an enthusiastic audience of between 80 and 100 people, most of whom had their hands up half the time, the panel explained that the gut has its own nervous system, composed of something like 50 million to 100 nerve cells. That's roughly the size of an adult mouse's brain, and until relatively recently in evolution was the biggest concentration of nerve cells in any animal's body. Our gut microbes help train our immune systems, make vitamins, and a whole lot more.

We really don't know what the optimal gut-microbial composition is (and no two people's gut ecosystems are exactly the same), but we do know how best to achieve it. To work right, eat right. Our gut microbes feast on fiber, loosely defined as all the complex carbohydrates we can't digest but they can. When they don't get enough, they start munching on the mucus lining of our intestine. Not okay to chew the mucus. It's a protective barrier that keeps microbes from getting out of the gut and into the bloodstream, where they emphatically don't belong.

Modern inhabitants of industrialized societies average 15-20 grams of fiber intake a day. Members of hunter-gatherer tribes, whose diets presumably closely resemble what our ancestors ingested for 99.9 percent of our species' evolution, eat 100 grams -- the size of a hamburger -- daily.

Before you slap a 3.5-ounce fiberburger into that toasted bun, some advice: Don't. It'll make you sick. We've already permanently lost a lot of the bug strains that can digest this stuff. Merely gobbling fiber supplements won't replace them, either. Too much of any one type of fiber favors microbial strains that can digest it at the expense of the majority that can't. The result would be reduced microbial diversity, the opposite of what's desirable. Instead, eat lots of different fruits and vegetables. (Cooking them's fine. We've been doing that for hundreds of thousands of years).

The panelists snagged beaucoup de book sales, and I got a free sandwich and my choice of side orders. I wanted potato chips, but I picked the salad.

Previously: The die-off within us: Are our low-fiber diets ruining our descendants' lives?, Getting to the good gut: How to go about itCivilization and its dietary (dis)contents: Do modern diets starve our gut-microbial community? and Joyride: Brief post-antibiotic sugar spike gives pathogens a lift

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