Lactose intolerance is a common condition where the body has difficulty digesting lactose, the primary sugar found in milk. The prevalence of this difficulty varies widely across the global population based on ancestry and history. The question of whether Japanese people are lactose intolerant points toward a fascinating intersection of genetics, history, and diet.
The Biological Basis of Lactose Intolerance
Lactose is a disaccharide sugar made up of two smaller sugar molecules, glucose and galactose, chemically bonded together. For the small intestine to absorb lactose, this bond must first be broken by lactase, a digestive enzyme. Lactase is produced by cells lining the small intestine and is most active during infancy when milk is the primary source of nutrition.
In the majority of the world’s population, lactase production naturally decreases significantly after childhood weaning, a process called lactase non-persistence. When lactase levels are low, undigested lactose travels to the large intestine where gut bacteria ferment it. This bacterial activity produces gases and organic acids, which can lead to symptoms like bloating, gas, abdominal pain, and diarrhea.
Global Patterns of Lactase Persistence
The ability to maintain high lactase production into adulthood is known as lactase persistence, a relatively recent evolutionary adaptation in humans. For most of human history and in most mammal species, lactase activity declined dramatically after early childhood. Lactase persistence arose due to specific genetic changes that keep the lactase gene active beyond childhood.
This trait became widespread in populations that historically relied on consuming fresh milk from domesticated animals, such as in Northern Europe, Africa, and the Middle East. Drinking fresh milk provided a significant nutritional advantage in these regions, driving the selection for the lactase persistence trait. Conversely, East Asian populations, including the Japanese, lacked a long-standing cultural tradition of consuming large amounts of unfermented dairy products, meaning no evolutionary pressure existed for this genetic persistence to develop.
Prevalence in the Japanese Population
The vast majority of Japanese adults are genetically non-persistent, meaning they retain the ancestral trait of low lactase activity in adulthood. Estimates of lactase non-persistence in East Asian populations are generally very high, and the Japanese population shows a similar trend. Estimates for non-persistence in Japanese adults often fall in the range of 80% to over 90%.
Historically, the Japanese diet contained very little dairy, with consumption largely limited to medicinal or aristocratic use. Dairy consumption only began to increase significantly after World War II, largely due to Western influences and the introduction of milk into school lunch programs. This relatively recent change in diet has not been long enough to alter the underlying genetics of the population.
Although a high percentage of the population has low lactase levels, many non-persistent Japanese adults can still consume small amounts of milk without symptoms. The incidence of symptomatic milk intolerance may be lower than the genetic prevalence of non-persistence due to variations in individual sensitivity and the dose of lactose consumed. Furthermore, consuming fermented dairy products like yogurt and hard cheeses, which have lower lactose content, is a common way to minimize potential digestive discomfort.