Japanese Milk Consumption: Asymptomatic Lactose Intolerance Following a Recent Cultural Diffusion

Like all mammals, humans typically lose the ability to digest milk in adolescence, thereafter experiencing gastrointestinal discomfort when exposed to the sugar lactose, found only in milk. Some individuals of European and African descent posses mutations that preserve lactose digestion into adulthood, the classic example of gene-culture coevolution. For lack of the genetic mutations that arose in dairying cultures, adult milk consumption is correspondingly rare in Africa, Asia, the Americas. Here we present evidence that greatly complicates this story, focusing on the recent adoption of government-sponsored milk-drinking in Japan. The Japanese have no known mutations for lactose tolerance, and only began producing and consuming milk in large quantities after the end of the American Occupation in 1952. Nevertheless, and despite the standard gene-culture story, the Japanese milk industry is thriving and reports less than a fifth of the population claim to be lactose intolerant. To substantiate this claim, we conducted 41 lactose intolerance tests on a sample of adults from across the Japanese archipelago, administering 25g of pure lactose in water. By standard diagnostic criterion of +20 ppm of H2 on a breath hydrogen test, we found 77% do not produce lactase enzymes. This result came as a surprise to our subjects, as most described no discomfort during the test, and regularly consume milk. We speculate that an alternative solution involves a lactose-friendly microbiome.