Prebiotics: Tending Our Inner Garden

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Why our immune system confuses unhealthy diets with dysbiosis—an overrun of bad bacteria in our colon.


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Prebiotics: Tending Our Inner Garden

The total surface area of our gut is maybe 3,000 square feet, counting all the little folds, larger than a tennis court, yet only a single layer of cells separates our inner core from the outer chaos. The primary fuel that keeps this critical cell layer alive is a short-chain fatty acid called butyrate, which our good bacteria make from the fiber we eat.We feed the good bacteria in our gut, and they feed us right back. 

They take the prebiotics we eat, like fiber, and in return provide the vital fuel source that feeds the cells that line our colon, a prototypical example of the kind of symbiosis between us and our gut flora. How important are these compounds our good bacteria derive from fiber? There is a condition known as diversion colitis that frequently develops in segments of the colon or rectum after surgical diversion of the fecal stream, meaning if you skip a segment of the bowel, like with an ileostomy, so that food no longer passes through that section, it becomes inflamed and can start bleeding, breaking down, closing off.

How frequently does this happen? Up to 100% of the time — but the inflammation uniformly disappears after you reattach it to the fecal flow. We didn't know what caused it — maybe some kind of bacterial over-growth or bad bacteria? Or was it a nutritional deficiency of the lining of the colon due

to the absence of the fiber needed to create the short-chain fatty acids? We didn't know until this study where they cured the inflammation by bathing the lining in what it so desperately needed.Severe inflammation gone in just a few weeks. We feed the good bacteria in our gut, and they feed us right back.It makes sense that we have good bacteria in our gut that feed us, tries to keep us healthy. They got a pretty good thing going.It's warm and moist and food just keeps magically coming down the pipe, but if we die, they lose out on all that. If we die, they die, so it's in their best evolutionary interest to keep our colon happy. But there are bad bugs too, like cholera, that cause diarrhea.

They have a different strategy.The sicker they can make us, the more explosive the diarrhea, the better their chances of spreading to other people, into other colons. They don't care if we die because they don't intend on going down with the ship. So how does the body keep the good bacteria around while getting rid of the bad? Think about it.

We have literally trillions of bacteria in our gut and so our immune system must constantly maintain a balance between tolerance to good bacteria while attacking bad bacteria. If we mess up this fine balance and start attacking harmless bacteria, it could lead to inflammatory bowel disease, where we're in constant red alert attack mode. The mechanisms by which the immune system maintains this critical balance remained largely undefined until now. If you think about it, there's got to be a way for our good bacteria to signal to our immune system that they're the good guys. And that signal is butyrate. Butyrate suppresses the inflammatory reaction, tells our immune system to stand down.

So butyrate may behave as a microbial signal to inform our immune system that the relative levels of good bacteria are within the desired range. Butyrate calms the immune system down, saying, in effect, all's well; you got the good guys on board, ultimately rendering the intestinal immune system hyporesponsive, meaning accommodating to the beneficial bacteria. But in the absence of the calming effect of butyrate, our immune system is back in full force,attacking the bacteria within our gut because they're obviously not the right ones since butyrate levels are so low.

So we evolved to have butyrate suppress our immune reaction, so should our good bacteria ever get wiped out and bad bacteria take over, our immune system would be able to sense this and go on a rampage and destroy the invaders, and continue rampaging until there were only good bacteria creating butyrate to put the immune system back to sleep.

OK, here's the critical piece. Here's why this all matters. What if we don't eat enough fiber? Remember, our good bacteria use fiber to create butyrate. So if we don't eat enough fiber, then we can't make enough butyrate. We could have lots of good bacteria, but if we don't feed them fiber they can't make butyrate. Sensing such low levels of butyrate, our body mistakenly thinks our gut must be filled with bad bacteria and reacts accordingly. Our body can mistake low fiber intake for having a population of bad bacteria in our gut.

Our body doesn't know about processed food. It evolved over millions of years getting massive fiber intake. Even during the Paleolithic period, 100 grams of fiber a day.So, on fiber-deficient Western diets, eating our Spam on Wonderbread, when our body detects low butyrate levels in the gut it doesn't think low fiber — as far as our body's concerned there's no such thing as low fiber — it thinks bad bacteria. Because for or millions of years, low butyrate means bad bacteria, so that's the signal for our body to go on the inflammatory offensive. So that's one reason why fiber can be so anti-inflammatory, one of the reasons fiber intake is critical for optimal health.Not fiber supplements, but whole plant foods. Fiber supplementation with something like Metamucil may not replicate the results seen with a diet naturally high in fiber.