Why Humans Beings Suffer from Lactose Intolerance

Adult Lactase Persistence

Approximately 65 percent of the world’s population is lactose intolerant. Most human beings cannot digest lactose, the sugar found in milk, beyond childhood. Virtually all human beings are lactose tolerant from birth to childhood. Only 35 percent, mostly Northern and Central Europeans, of humans continue to produce lactase, the enzyme that breaks down lactose, into adulthood. While scientists are not entirely sure why these populations evolved and developed adult lactase persistence, it is hypothesized that milk’s nutritious value, along with environmental factors played a role in the development of adult lactase persistence genes in specific human populations.

Populations of Northern and Central European descent have the highest percentage of individuals who can digest lactose into adulthood. The latest research shows that the gene mutation that caused adult lactase persistence came about sometime between 2,000 and 20,000 years ago. Prior to that, most humans could not digest lactose after childhood. For the gene to present itself and rapidly spread among the population tells scientists that there was extreme evolutionary pressures that caused the rapid development of this gene.

Milk contains a host of nutritious micronutrients, along with ample amounts of protein, fat, calcium, and carbohydrates. It stands to reason that those individuals in the population, who possessed the gene for adult lactase persistence, could continue to drink nutritious milk through adulthood would therefore have a fitness advantage over those that did not possess the gene. Additionally, those that did not possess the gene would suffer from diarrhea and other digestive ailments associated with lactose intolerance after drinking milk. Back before modern water sources and pedialyte, many of those individuals lacking the lactase persistence gene could have perished from dehydration caused by the diarrhea. This would only further expedite the proliferation of the adult lactase persistence gene through a population.

It is no coincidence that areas with long histories of dairy animal domestication are correlated with high adult lactase persistence in their respective populations. The European populations in particular were descendants of those humans who migrated from the Fertile Crescent, where they brought with them crops like wheat and barley. These crops were difficult to grow in the short growing season of Northern Europe. Milk, on the other hand, can be produced year around. During the cold winter months, milk may have been a valuable source of nutrition and only those adults with the adult lactase persistence gene could take advantage of it.

While there was likely a rapid development of the gene that allowed adults to breakdown lactose into adulthood, it remains a mystery as to how exactly it occurred. Despite this, there is enough evidence to suggest there were environmental factors that contributed to the proliferation of the adult lactase persistence gene. Milk would have been a nutritious powerhouse thousands of years ago and may have contributed to the survival of some human populations.