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Can low-fiber diets’ damage to our gut-microbial ecosystems get passed down over generations?

fast food decisionsUh-oh.

A study conducted in mice raises suspicions that we humans may be halfway down the road to the permanent loss of friendly gut-dwelling bacteria who've been our constant companions for hundreds of millennia. That's probably not good.

Virtually all health experts agree that low-fiber diets are sub-optimal. One big reason: Fiber, which can’t be digested by human enzymes, is the main food source for the friendly bacteria that colonize our colons. Thousands of distinct bacterial species thrive within every healthy mammal’s large intestine. Far from being victimized by these colonic cohabitants, we'd be hard put to live without them. They fend off pathogens, train our immune systems, help us digest food we'd otherwise be unable to use and even guide the development of our tissues.

From a news release I wrote about the new study, which was spearheaded by Stanford microbiology/nutrition explorers  (and husband/wife team) Justin Sonnenburg, PhD, and Erica Sonnenburg, PhD, and published in Nature:

[Previous] surveys of humans’ gut-dwelling microbes have shown that the diversity of bacterial species inhabiting the intestines of individual members of hunter-gatherer and rural agrarian populations greatly exceeds that of individuals living in modern industrialized societies. ... In fact, these studies indicate the complete absence, throughout industrialized populations, of numerous bacterial species that are shared among many of the hunter-gatherer and rural agrarian populations surveyed, despite those groups’ being dispersed across vast geographic expanses ranging from Africa to South America to Papua New Guinea.

Another piece of information: The proliferation of nearly fiber-free, processed convenience foods since the mid-20th century has resulted in average-per-capita fiber consumption in industrialized societies of about 15 grams per day. That’s as little as one-tenth of the intake among the world’s dwindling hunter-gatherer and rural agrarian populations, whose living conditions and dietary intake presumably most closely resemble those of our common human ancestors.

Perhaps the most significant sources of our intestinal bacterial populations is our immediate family, especially our mothers during childbirth and infancy. So, if our low-fiber diets are depleting our intestinal ecosystems, could that depletion get passed down from one generation to the next?

To gauge low-fiber intake's intergenerational toll on gut-microbe diversity, the Sonnenburgs (who recently co-authored the popular book The Good Gut) raised several generations of mice (specially bred so their intestinal ecosystems were composed entirely of microorganisms transplanted from a human donor's gut) on a low-fiber diet, while keeping them isolated from contact with non-family members. By generation four, there was a profound, irreversible loss of bacterial species within the microbial ecosystems residing in the mice's intestines. Just putting the fourth-generation mice back on the high-fiber diets of their pre-experiment great-grandparents didn't come close to restoring those progenitors' gut-bug diversity. (It took a fecal transplant to accomplish that.) Meanwhile, none of this intergenerational gut-microbial species loss occurred in mice who were maintained on high-fiber diets all the way down the line.

Food for thought: As it's been about two full generations since most of us switched over to relatively fiber-free diets, we could be looking at only a few more decades before the only route back to a fully functioning gut-microbial ecosystem may be mass fecal transplants. But from whom? How you gonna get re-exposed to essential friendly bacteria if nobody around you is carrying them, either?

Previously: Getting to the good gut: How to go about it, Civilization and its dietary (dis)contents: Do modern diets starve our gut-microbial community?, The future of probiotics and Eat a germ, fight an allergy
Photo by Richard Rutter

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