Japanese sushi was one of the first cuisines to introduce seaweed to the Western palate. (provided by Pixabay)
However thoroughly we chew our food, we’re not able to digest everything we consume in full. In the case of plant fiber, 40 trillion bacteria living in our large intestine help us break down the substance into nutrients we can absorb.
Seaweed also contains fiber, but human gut bacteria aren’t capable of breaking it down, as its chemical structure is markedly different from that of plant fiber. That’s why it’s normal for humans to not be able to digest seaweed.
But seaweed has been consumed in Korea, Japan, and coastal areas of China for millennia. In Korea alone, over 500 species of seaweed live along the coast, among which 50 different types of seaweed are eaten as food, including gim, miyeok, dasima, parae, umutgasari, tot, gamtae, and maesaengi.
According to a study, East Asians are capable of digesting seaweed because their gut bacteria went through repeated “genetic upgrades” that enabled them to digest and absorb seaweed.
An international team of researchers including Jan-Hendrik Hehemann, a professor at the Max Planck Institute of Marine Biology in Germany, wrote in an article published in the most recent issue of the scholarly journal Cell Host & Microbe that “genes for edible seaweed digestion” were transferred into human gut bacteria in “at least four separate events.”
A seaweed cultivation site (Getty Images Bank)
As a colossal number of seaweeds grow in the ocean, many bacteria of oceanic origin are able to degrade sea-growing algae. In their article, Hehemann et al. argue that certain processes enabled genes that encode seaweed-degrading enzymes to be mobilized into human gut bacteria.
“Whether they came directly from an oceanic bacterium someone just happened to consume or through a more complex path into the human gut is still a mystery,” Eric Martens, a professor at the University of Michigan and a corresponding author of the study, said in a University of Michigan press release.
The researchers suggested the possibility of a “lateral gene-transfer” as the likely scenario: in other words, humans may have consumed seaweed-degrading bacteria together with seaweed, after which the genes of the bacteria may have transferred into that of human gut bacteria inside the large intestine. For such bacteria, the large intestine of humans consuming seaweed daily would have been the land of opportunity.
The researchers concluded that opportunities for such “genetic upgrades” would have been few and far between. That’s because seaweed-degrading bacteria clinging onto the surface of seaweed would have had to survive the process of manufacturing and cooking, after which they would have had to travel through the stomach and the small intestine alive in order to transfer its genes into that of gut bacteria. Having said that, once the lateral gene transfer was complete and human gut bacteria developed the ability to degrade seaweed, they would have easily passed on from mother to offspring generation after generation.
Hehemann and his colleagues previously published in 2010 an article in Nature announcing that they found the genes of marine Bacteroidetes capable of degrading gim in the gut microbiota of a Japanese person. Mirjam Czjzek, a researcher at the Station Biologique de Roscoff in France who participated in the study, described the discovery as a “funny coincidence.”
In the human large intestine, some 40 trillion bacteria help break down non-digestible fiber into absorbable material. Pictured is a highly enlarged group of colon bacillus. (provided by the US Department of Agriculture)
The 2010 study was often misinterpreted to signify that out of all humans, only Japanese people have seaweed-degrading genes in their gut microbiota. The 2022 study found genes that encode seaweed-degrading enzymes in Japanese as well as Chinese gut microbiota.
Though seaweed consumption has been traditionally high among Koreans, they were not included as subjects in the 2022 study. Other than in East Asia, the tradition of seaweed consumption has been observed in the coastal regions of North America as well as in Iceland.
Bacteroidetes make up a big part of human gut microbiota and are widely found in both terrestrial and marine environments. In the recent study, researchers found the genes of Bacteroidetes capable of degrading fiber in gim as well as other seaweeds in multiple human gut microbiota.
The study also reported that Firmicutes, another component of human gut microbiota, obtained genes that enabled them to degrade polysaccharide in seaweed. Martens said, “Firmicutes are known to live in fish intestines and the closest ancestors of the genes that appear to have jumped into human gut Firmicutes were ones found in fish.”
The researchers wrote in their article, “Our findings provide insight into both the adaptability of gut microbes to utilize new nutrients and novel tools to engineer them.”
Rich in vitamins, antioxidants, and minerals such as calcium, and boasting a high protein content of 47 percent in the case of gim, seaweed is increasingly emerging as a superfood that offers a wide array of health benefits. Researchers are looking into seaweed’s potential as feed for livestock, as the process of farming sea-growing algae is quick and doesn’t require land nor irrigation.
The discussed article can be found at Cell Host & Microbe, DOI: 10.1016/j.chom.2022.02.001
By Cho Hong-sup, environment correspondent
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