Most of us in the industrialized world don’t get enough fiber–and average of just 15 grams a 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, said Justin Sonnenburg, PhD, associate professor of microbiology and immunology and senior author of a study published Jan. 13 in Nature.
The study raises concerns that the lower-fiber diets typical in industrialized societies may produce internal deficiencies that get passed along to future generations.
Conducted on mice, the study found that low-fiber diets not only deplete the microbiota of the gut, but can cause loss of bacterial diversity within those ecosystems in as few as three or four generations.
Once an entire population has experienced the extinction of key bacterial species, simply “eating right” may no longer be enough to restore these lost species to the guts of individuals in that population, the study suggests. Those of us who live in advanced industrial societies may already be heading down that path.
Thousands of distinct bacterial species inhabit every healthy individual’s large intestine. “We would have difficulty living without them,” he said. “They fend off pathogens, train our immune systems and even guide the development of our tissues.” While we pick up these microscopic passengers in the course of routine exposures throughout our lifetimes, one of the most significant sources of our intestinal bacterial populations is our immediate family, especially our mothers during childbirth and infancy.
“Numerous factors including widespread antibiotic use, more-frequent cesarean sections and less-frequent breastfeeding have been proposed for why we see this depletion in industrialized populations,” said the study’s lead author, Erica Sonnenburg, PhD, a senior research scientist at Stanford (she and Justin Sonnenburg are married). “We asked ourselves whether the huge difference in dietary fiber intake between traditional and modern populations could, alone, account for it.”
The Stanford researchers employed young laboratory mice that had been specially bred and raised in aseptic environments so that, unlike ordinary mice (and ordinary humans), their intestines were devoid of any microbial inhabitants. After populating the mice’s guts with microbes from a human donor, the scientists divided them into two groups. One group was fed a diet rich in plant-derived fiber. The other group’s diet, equivalent to the first with respect to protein, fat and calories, was practically devoid of fiber content.
During the experimentation that followed, the researchers analyzed fecal samples from the animals. The two groups’ gut-bacteria profiles were initially indistinguishable but soon diverged. “Within a couple of weeks, we saw a massive change,” said Justin Sonnenburg. “The low-fiber-intake mice harbored fewer bacterial species in their gut.” More than half of these bacterial species’ numbers had dwindled by over 75 percent, and many species seemed to have disappeared altogether.
After seven weeks, the mice that had consumed a low-fiber diet were switched back to a high-fiber diet for four weeks. The mice’s gut-bacteria profiles partly recovered — probably due to an uptick in abundance of some bacteria whose ranks had declined to undetectable levels during the low-fiber-intake period. Still, this restoration was only partial: One-third of the original species never fully recovered despite their return to a high-fiber diet.
No such changes were seen in the control mice consistently fed a high-fiber diet.
The real surprise came after mice had been bred and maintained on low-fiber diets for a few generations. In their experimental confines, these mice were exposed to microbes only through contact with their parents. Each successive generation’s gut-bacterial ecosystem declined in diversity. By generation four, the depletion had reached a point where nearly three-quarters of the bacterial species resident in their great-grandparents’ guts appeared absent in their own. Even after these mice were put back on a high-fiber diet, more than two-thirds of the bacterial species identified in the guts of their first-generation ancestors proved irretrievable, indicating extinction of those species by the fourth generation of fiber deprivation.
On the other hand, a somewhat more aggressive measure — fecal transplantation — did result in these lost species’ retrieval, the study found. Introducing fecal contents of fourth-generation high-fiber-diet mice into the intestines of fourth-generation low-fiber mice, together with putting them on the high-fiber diet for two weeks, fully restored their bacterial profiles. Within 10 days of the procedure, the composition and diversity of the bacteria in the intestines of this group were indistinguishable from those of control mice.
These findings hold major implications for humans, said Erica Sonnenburg. “There are very few ecosystems where low species diversity is a good thing. There’s no reason to think our gut is any exception,” she said.