Western, industrialized diets are seriously lacking in fiber, and it may be fundamentally changing the way our guts digest tough plant matter.
Even though fruit and vegetables are a key part of the human diet, scientists are just beginning to understand how our bodies break down the most abundant organic compound on Earth: cellulose, the tough material that lines the cell walls of plants.
Now, a new study by an international team of researchers has discovered previously unknown microbes hiding in the human gut that are capable of breaking down cellulose.
For decades, it was assumed that cellulose could not be broken down by the human body like it could be in the guts of cows, horses, sheep, or other mammals. Only in 2003 did scientists discover human gut bacteria that could digest these fibers, after all.
The recent study relied on the genes of that same bacterium to search for others like it. The exhaustive analysis used fecal samples to test the gut microbiome of humans from different times and regions. The findings suggest we have more in common with farm animals than we once thought.
Our guts, as it turns out, possess several species of cellulose-munching microbes that have evaded our notice until now. One species is strongly associated with ungulate mammals that chew cud, another with primates, and another with humans.
All three belong to the genus Ruminococcus – known to already have representatives in healthy (and unhealthy) human guts – and possess genes involved in the digestion of cellulose.
In fecal samples from hunter-gatherers, rural populations, and ancient humans living between 1,000 and 2,000 years ago, the three types of microbe were abundant.
Yet in populations from modern, industrialized societies, the same gut microbes were “conspicuously rare”.
“These findings collectively imply a decline of these species in the human gut, likely influenced by the shift toward westernized lifestyles,” write the study authors, led by microbiologist Sarah Moraïs from the Ben-Gurion University of the Negev in Israel.
It’s possible, the researchers explain, that if Ruminococcus microbes are deprived of plant fiber, their numbers in the gut decrease. The fear is that these missing species are somehow contributing to poor metabolic health among modern, urbanized people.
That possibility still needs to be investigated, but the authors of the current study think there “may be potential for intentional reintroduction or enrichment of these species in the human gut” through dietary supplements or specialized probiotics.
Today, some studies suggest that current fiber intake guidelines are too low in industrialized societies, and that human health may be suffering as a result.
Initial research shows that cellulose supplements, like plant fibers, can result in multiple and varied health benefits, including changes in gut microbes, immune responses, and gene expression.
But this avenue of research as a whole is “rather unexplored“. While the associations appear promising, the underlying mechanisms are largely a mystery.
The current study is an important step forward, as it brings to light previously unknown gut bacteria that may be important players in the health of the human gut, historically.
The evolutionary analysis “strongly suggests” that the human-associated strain of Ruminococcus bacteria was originally transferred to us from the gut of ruminants, possibly during domestication.
Living with animals, therefore, may have improved our ability to digest plants. Since taking up shop in our guts, these Ruminococcus microbes have made the human body their own.
Compared to Ruminococcus species in the guts of other mammals and non-human primates, the ones in us appear to have adapted to their new ecosystem and acquired genes from neighboring gut microbes.
After thousands of years, however, this feat of ‘colonization’ may be under threat. In some parts of the world, the human gut may no longer provide a suitable home for these microbes.
What this is doing to our health is currently unknown.
The study was published in Science.
