Here’s another reason to eat your vegetables. Trillions of microbes in the human large intestine—known as the microbiome—depend on dietary fiber to thrive and give us energy. As fiber intake declines, so, too, does the range of bacteria that can survive in the gut. Now, a new study of multiple generations of mice fed a low-fiber diet indicates that this diversity plummets further with each generation, a hint of what might be happening in the human gut as we continue eating a contemporary diet of refined foods. The work might also help explain rises in many Western diseases, such as inflammatory bowel disease and obesity.
"This is a seminal study," says microbial ecologist Jens Walter, of the University of Alberta in Canada. "The magnitude by which the low-[fiber] diet depletes the microbiome in the mouse experiments is startling."
For much of human history in hunter-gatherer and early agrarian times, daily fiber intake was likely at least three or four times the officially recommended amounts today (something like 100 grams versus 25 grams)—and several times greater than average U.S. consumption now (about 15 grams). The trend has led many researchers, including microbiologist Erica Sonnenburg of Stanford University in Palo Alto, California, to suspect that the well-documented low diversity of gut microbes among people in developed countries—some 30% less diverse than in modern hunter-gatherers—is, in part, a product of drastically reduced fiber intake.
The new study confirms this relationship in rodents. Sonnenburg and her colleagues raised mice in a germ-free environment and then fed them human feces, giving them human gut bacteria. When fed a low-fiber diet (about 30% less fiber than the control high-fiber chow), the animals experienced a substantial dip in gut microbial diversity (with about 60% of the microbes losing at least half their populations). Mice then kept on low-fiber food and allowed to breed produced offspring with an even lower diversity, the team reports online today in Nature. And subsequent generations of low-fiber-fed mice continued to lose whole groups of microbes as the bacteria reached such reduced numbers in the parents that they could not be passed on via birth, nursing, or even mice's tendency to eat each other's feces.
By the fourth generation, the mouse microbiotas seemed to have reached a new normal, a stable low-diversity microbiota, harboring only just more than a quarter of the diversity enjoyed by the first generation. Notably, none of the low-fiber generations could be "rescued" by reintroducing high-fiber food. To reach the bacterial variety of their great-grandparents, the mice needed a fecal transplant from a high-fiber group and a high-fiber diet.
The findings raise questions of how our own diet affects our offspring. "While we pass on relatively few changes in our human DNA for each generation, this study indicates that we are potentially passing on huge changes in our gut microbiome," Sonnenburg says. Because the mice in this study started with microbes from a Westerner, whose microbiome was already low in diversity, the study also suggests that, "it may be possible for the Western microbiota to lose additional diversity," she adds. Microbiologist Eric Martens of the University of Michigan Medical School in Ann Arbor, marvels at the extent of the loss. "The surprise is that a proportion of organisms can actually be driven to extinction," he says.
Researchers have yet to prove that the same rapid decrease in microbe diversity is occurring over human generations—and if it is, just what it means for health. "In these complex ecosystems it is very difficult to know the exact outcome of biodiversity loss," Sonnenburg says. But, she notes, "it is likely these extinctions within the microbiota would have big effects." For example, other research has found that obese people are more likely to have lower microbial diversity in their guts than lean peers. And our studies of more macroenvironments teach us that a diverse ecosystem is heartier and quicker to rebound than a less diverse one.
In the meantime, as Martens suggests in an accompanying commentary in Nature, "you might just consider choosing a salad … or an extra serving of beans."