You Can Have Your Cheesecake and Eat Sushi Too

If you love sushi as much as I do, you probably know that sushi rolls are wrapped in a kind of seaweed called nori. Humans lack an enzyme for breaking down porphyran, a polysaccharide found in nori, so when you eat sushi, porphyran is supposed to go right through your system. However, research on Japanese nationals has revealed that porphyran is broken down in their digestive systems. How do the Japanese pull off this extraordinary feat of sushi digestion? As it turns out, some seaweed-eating marine bacteria possess porphyranase, an enzyme that breaks down porphyran, and centuries of copious sushi consumption by the Japanese have resulted in the accidental consumption of these bacteria. Once in the large intestine, these marine bacteria swapped genes with the gut bacteria, which is how Japanese gut bacteria acquired the porphyranase gene.

Unfortunately, being Japanese also has a culinary downside – a whopping 73% of the Japanese population is lactose intolerant. Lactose is a disaccharide found in milk. Like all young mammals, human babies produce the lactase enzyme that breaks down lactose in their mothers’ milk. But while other young mammals stop producing lactase when it is rendered obsolete after weaning, some humans produce lactase throughout their lives. The cause of this disparity? About 10000 years ago, a number of human populations learned to domesticate and milk ruminants. Consequently, adults began consuming dairy products, and lactase persistence, the preservation of lactase production after adolescence, evolved (independently) in each of these populations. But what about the populations that stuck to the original dairy-free diet? They never evolved lactase persistence, which is why these populations tend to have extremely high lactose intolerance rates. Put simply, most of the adult members of these populations can’t produce lactase after eating a slice of cheesecake, and this often results in symptoms such as abdominal pain, diarrhea and gas.

The traditional Japanese diet does not include dairy, which explains why lactase persistence never evolved in the Japanese population. But although the Japanese have been consuming large quantities of dairy products since the 50s, most Japanese adults do not exhibit lactose intolerance symptoms! This remarkable finding can only be explained by the hypothesized presence of lactase-producing bacteria in the Japanese microbiome.