Chapter 177
50-Food-Challenge
The Yanomami in the Amazon rainforest have the most diverse microbiome ever recorded. They had never had contact with the modern world, and I wonder how that conversation must have gone: We come in peace. Will you give us your poop?
Today’s low-fiber diet is considered one of the main causes of microbiome impoverishment. The loss of fiber from the modern diet is compared to the Chicxulub asteroid, the meteorite that wiped out the dinosaurs, because of the profound and possibly catastrophic change in our microbiome ecosystem that it causes. Why can’t we just take fiber as a supplement? In plant foods there are literally thousands of types of fiber, and each one feeds different bacteria in the gut. In contrast to whole foods such as brown rice or whole-grain barley, fiber supplements do not appear to contribute to greater diversity in the microbiome. In addition, brown rice and barley work better together than alone. That is the idea behind the “50-Food Challenge,” the recommendation to eat at least 50 different plant foods each week so that, by achieving diversity, a very broad spectrum of bacteria is nourished.
No wonder fiber supplements are a poor substitute. Some of them, such as psyllium husk or psyllium (brand name Metamucil), do not seem to be utilized by our microbiome at all. This arrogance reminds me of probiotic dietary supplements: Thousands of different bacterial species live in our gut, all of which may be interacting, but we are surprised that half a dozen out of those thousands, when we stuff them into a capsule, don’t accomplish more? No microbe exists on its own. The key bacilli that feed on starch, such as bifidobacteria, produce acetate, which feeds some of the most important producers of butyrate, and lactate, which acidifies the gut. This further stimulates the growth of the butyrate producers and suppresses the growth of bad bacteria—just like sauerkraut does. The best way to support this complex interplay is to eat plants—and not pills or powders.
Resistant starch
Fiber is not the only prebiotic. About 30 percent of the calorie content of human breast milk, for example, consists of “indigestible” oligosaccharides. We can’t digest them, but guess who can? Bifidobacterium infantis, good bacteria in the guts of infants. That’s how important the relationship between human and bacterium is—we were created as a symbiotic species.
Inulin, found concentrated in vegetables such as onions and garlic, can have an “enormous” bifidogenic effect. Ironically, some people with irritable bowel syndrome actively avoid inulin because it is a type of FODMAP (“fermentable oligo-, di- and monosaccharides and polyols”). Anyone who eats a low-FODMAP diet soon develops a bifidobacteria deficiency, which has led to the theory that such dietary patterns may impair gut health in the long term.
There is “resistant starch”—starches that are not digested in the small intestine, so they reach the large intestine, where, as a prebiotic, they feed our good bacteria, just like fiber does. I already mentioned the trick of cooling cooked starch, but the best source of resistant starch is legumes. Two servings of cooked chickpeas every day can reduce colonization with disease-causing and putrefaction-causing gut bacteria within three weeks. In study participants, levels of a bacterium that produces a lot of ammonia fell by about 50 percent. That may explain why even one serving of legumes a day is associated with a 20 percent lower risk of colorectal cancer. In rats fed black beans, the incidence of colon cancer caused by a carcinogen decreased by 75 percent.
As with fiber, you have to consume prebiotics and have bacteria that feed on prebiotics to achieve a benefit. People who harbor starch eaters like Ruminococcus can ferment almost all the resistant starch they eat, whereas others can use only 20 to 30 percent of the resistant starch. How can the growth of these good bacteria be promoted? Eat more foods that contain resistant starch! After subjects were randomized to a diet high in resistant starch, the amount of their starch eaters such as Ruminococcus quadrupled within ten days.
Whole grains
Bifidobacteria’s preferred prebiotic is starch. So how do we get more starch into the large intestine? By packaging it in fiber—that is, in whole grains and legumes. In the chapter “Blocking Calories” in How Not to Diet, I go into this in detail. When we chew and our stomach works, everything we eat is ground into particles smaller than two millimeters before it enters the intestine. That may sound tiny, but a two-millimeter wheat particle can contain about 10 000 plant cells full of starch, of which only about 3800 are broken open, meaning about 62 percent of the starch remains in that grain particle, protected by the indigestible plant cell walls—remains that mean a sumptuous meal for the microbiome.
Things look very different with ground whole-grain products. Ground flour particles can be 100 times smaller—even smaller than the cells themselves—so that virtually every one may be broken open, releasing its contents early and leaving the gut flora empty-handed. That is the reason we should “de-flour” our diets as much as possible. Whole grain is good, but whole kernels (groats) are even better. For the same reason, nuts can improve the microbiome by promoting the growth of good bacteria that produce short-chain fatty acids, but the same amount of nut butter does not appear to have a prebiotic effect.
Remember acarbose, the drug that turns regular starch into resistant starch? The average and maximum lifespan extension of mice by acarbose could be due to the release of a hormone called GLP-1 from specialized L cells that line the colon. That is the hormone mimicked by an expensive new class of weight-loss injections, for example Wegovy. With prebiotics you can achieve exactly the same thing without drugs. Researchers produced this effect in a Petri dish and in humans by giving study participants an SCFA enema or simply having them eat fiber—or, better yet, fiber-rich foods.
Polyphenol prebiotics
Another important class of prebiotics are the polyphenols in fruits and vegetables. Critics who question the effectiveness of polyphenols often point to studies showing their low bioavailability. For example, up to 85 percent of the polyphenol pigments that make blueberries blue are not absorbed and end up in the colon, but improved detection methods recently showed that a large proportion of the polyphenols are absorbed after all. And the magic happens in the colon.
When blueberry polyphenols are mixed with a fecal bacterial culture, beneficial bacteria such as bifidobacteria and Lactobacillus multiply within a few hours. When randomized subjects eat about a cup of wild blueberries, it leads to a significant increase in bifidobacteria in their stool. How do we know that comes from the polyphenols and not the fiber? Well, apples also promote bifidobacteria, but pectin, the fiber isolated from apples, does not. Bananas and berries have similar amounts of fiber, but bananas contain fewer polyphenols. Eating bananas does not significantly promote bifidobacteria, which is further evidence that polyphenols may play a special role.
In an intervention study in which older adults were randomized to eat berries and dark chocolate instead of low-polyphenol snacks, it led to a significant increase in the good (butyrate-producing) bacteria and to a strengthening of the gut barrier. But the best evidence probably comes from polyphenol-rich beverages. Tea leaves and coffee beans contain many polyphenols that make it into the drink, while the fiber remains completely behind. Both green tea and coffee are bifidogenic. Three cups of coffee a day can significantly increase the amount of bifidobacteria in the gut within three weeks.