How to Foster a Healthy Gut Flora

What’s more important: probiotics or prebiotics? And where can we best get them?

“Virtually every day we are all confronted with the activity of our intestine, and it is no surprise that at least some of us have developed a fascination for our intestinal condition and its relation to health and disease.”

“Over the last years the intestinal microbiota [our gut flora] has been identified as a fascinating ‘new organ’” with all sorts of functions. Well, if the bacteria in our gut make up an entire, separate organ inside our body, what about doing an organ transplant? I discuss this in my video How to Become a Fecal Transplant Super Donor.

What would happen if you transferred intestinal bacteria from lean donors into obese subjects? Researchers figured that rebalancing the obesity-causing bacteria with an infusion of gut bacteria from a lean donor might help. They had wanted the study to be placebo-controlled, which, for drugs is easy, because the control subjects can just be given a sugar pill. But, when you’re inserting a tube down people’s throats and transplanting feces, what do you use as the placebo—or poocebo, if you will? Both the donors and the subjects brought in fresh stools, and the subjects were randomized to either get a donor’s stool or their own collected feces. So, the placebo was simply getting their own stool back.

What happened? As you can see at 1:32 in my video, the insulin sensitivity of the skinny donors was up around 50, which is a good thing. High insulin sensitivity means a low level of insulin resistance, which is the cause of both type 2 diabetes and prediabetes. The obese subjects started out around 20 and, after an infusion of their own feces, stayed around 20. The group of obese donors getting the skinny fecal infusion similarly started out low but then shot up near to where the slim folks were.

It’s interesting that not all lean donors’ stools conveyed the same effect on insulin sensitivity. Some donors, the so-called super-fecal donors, had very significant effects, whereas others had little or no effect, as you can see at 2:02 in my video. It turns out this super-donor effect is most probably conveyed by the amounts of short-chain fatty acid-producing intestinal bacteria in their feces. These are the food bacteria that thrive off of the fiber we eat. The short-chain fatty acids produced by fiber-eating bacteria may contribute to the release of gut hormones that may be the cause of this beneficial, improved insulin sensitivity.

“The use of fecal transplantation has recently attracted considerable attention because of its success in treatments as well as its capacity to provide cause–effect relations,” that is, cause-and-effect evidence that the bacteria we have in our gut can affect our metabolism. Within a few months, however, the bacterial composition returned back to baseline, so the effects on the obese subjects were temporary.

We can get similar benefits by just feeding what few good gut bacteria we may already have. If you have a house full of rabbits and feed them pork rinds, all the bunnies will die. Yes, you can repopulate your house by infusing new bunnies, but if you keep feeding them pork rinds, they’ll eventually die off as well. Instead, even if you start off with just a few rabbits but if you feed them what they’re meant to eat, they’ll grow and multiply, and your house will soon be full of fiber-eating bunnies. Fecal transplants and probiotics are only temporary fixes if we keep putting the wrong fuel into our guts. But, by eating prebiotics, such as fiber, which means “increasing whole plant food consumption,” we may select for—and foster the growth of—our own good bacteria.

However, such effects may abate once the high-fiber intake ceases. Therefore, our dietary habits should include a continuous consumption of large quantities of high-fiber foods to improve our health. Otherwise, we may be starving our microbial selves.


The microbiome is one of the most exciting research areas in medicine these days. For more information, see, for example:

For more on health sources of prebiotics, check out:

In health,

Michael Greger, M.D.

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