Any Westerners who have thought to themselves, gee, Japanese people sure seem to get a lot more out of sushi than I do, are more right than they probably guessed. A new study finds that bacteria living in the intestines of Japanese but not North American people harbor genes for digesting red algae (a.k.a. seaweed, or nori) of the kind typically wrapped around sushi. The genes digest carbohydrates (or to be perfectly precise, polysaccharides) in the seaweed. As a result, Japanese people can extract energy out of the seaweed, but Americans derive no nutritional benefit.
The human gut is teeming with trillions of bacteria collectively called the microbiota, as NEWSWEEK reported in a 2007 story. These microbes synthesize vitamins for us and secrete enzymes that break down starch molecules found in, especially, fruits and vegetables, including seaweed. As the NEWSWEEK story said, “The stomach and intestines secrete 99 different enzymes for breaking these down into usable 6-carbon sugars, but the humble gut-dwelling Bacterioides theta produces almost 250, substantially increasing the energy we can extract from a given meal.” The more energy extracted from starch, the more calories we get from food.
In a paper in the April 8 issue of Nature, scientists led by Jan-Hendrik Hehemann and Mirjam Czjzek of the Université Pierre et Marie Curie in Paris and the Centre National de la Recherche Scientifique describe how they identified a new kind of enzyme, which, it turns out, digests a starch called porphyran that is abundant in red Porphyra algae. These algae include the kind that are used as nori for sushi. The only bacteria in the human gut that make this enzyme are Bacteroides plebeius, and the only people found to have these bacteria are Japanese—something Czjzek describes to me as “statistically surprising.”
And where did the Japanese get these nori-digesting bacteria? As best the scientists can figure, people who eat a lot of seaweed also consume a lot of marine bacteria that dine on the seaweed, too. Bacteria are notorious for being genetically promiscuous; that is, they pass DNA to strangers if the conditions are right. Marine bacteria that eat seaweed thus wound up in the guts of people who ate the seaweed (that is, Japanese people), transferred their seaweed-digesting genes to bacteria already living in the gut, and presto: the microbiota in Japanese guts now include bacteria able to digest nori.
When I asked Czjzek whether Americans or other non-Japanese who consume a lot of nori could acquire the gene just as people in Japan did, she explained that the special conditions that allow such gene transfer “are probably not often encountered.” And since nori sheets “are sterilized and roasted, in contrast to traditional sushi,” in which nori was untreated and therefore contained living marine bacteria, making “such an event even less probable.”
That actually deepens the mystery, since humans are born without gut bacteria but start acquiring them during infancy. The exact composition of the microbiota depends on what we eat and which gut bacteria are in the people around us. So while “Japanese are not born with Bacteroides plebeius,” Czjzek explains, “certain Japanese individuals are a ‘favorite playground’ for Bacteroides plebeius to inhabit ... Lateral gene transfer [from a marine bacterium to Bacteroides plebeius in the human gut] occurred once.” But once established in one or a few individuals, it spread to enough others that it’s essentially a stable, population-wide trait.