Climate Health Medical

Urban dwellers losing their ability to digest plant cellulose

Food crisis

HQ Team

March 22, 2024: A recent study titled “Cryptic diversity of cellulose-degrading gut bacteria in industrialized humans” says that urban dwellers are slowly losing the ability to break down cellulose found  in plants.

Cellulose  is the primary component of the cell walls of plants and of dietary fibre. It is tough to break down and requires a strong digestive system and gut bacteria to process. In 2003, it was found that humans also played host to bacteria that can break down the plant cellulose.

The  study found that we inherited this ability though a mix of ancestry and via our domestication of herbivores. But urban living has caused the number of these bacteria to shrink dramatically compared to animals and even the rural population.

Genome sequencing

The scientists wanted to get a complete picture of the bacteria in our guts, hence they  obtained gut samples from humans and ruminants. The bacteria found there were analysed for DNA sequencing. Those genomes were then compared to those of known cellulose-digesting bacteria . They were able to identify 25 genomes from ruminants and 22 from humans. Further analysis enabled them to categorize four distinct groups of cellulose-digesting species in humans

The researchers collected nearly 2000 more gut samples, as well as some vintage poop samples that humans left behind over 1,000 years ago. They discovered that some had long lineages in our ancestors, and others in  primates like macaques, baboons, gorillas, and chimpanzees), but surprisingly not in ruminants.

Diet influence on gut bacteria

The researchers also found that their prevalence was changing with the changes in human diet  In non-human primates, the frequency of these bacterial strains was in the 30–40 percent range. This was similar to what was found in samples of feces of our ancestors but higher than that seen in present humans. Here, there was a strong division.  Those living in a rural environment, both of whom eat very high fiber diets still had about 20 percent prevalence of these cellulose-digesting species. By contrast, those in industrialized countries had a prevalence under 5 percent.

In general, the more fiber in the diet of a culture, the more diverse their cellulose-digesting bacteria were. So, their diversity in humans has been going down as more of our population has shifted into urban living.

The researchers found some other strains in the human samples that they attributed  to those that are present in ruminants. “Our evolutionary analysis strongly suggests that [these strains] likely originated in the ruminant gut and later transferred to humans, possibly during domestication,” the researchers conclude.

The process of horizontal gene transfer between species appears to be a key feature of the adaptation of cellulose-digesters to their hosts. The digesting process through enzymes that is happening in the human gut is a lot dependent on our diets. And these enzyme for the cellulose breakdown purpose have to be exported outside the cells. So a part of the digestive products end up being  absorbed by the human digestive system. In addition . the bacteria in our gut use some of the food to produce chemicals that are useful to us.

This is why high-fibre diet is promoted.

The full report can be accessed here.

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