Researchers at Cornell University have shown in a recent study that lactose intolerance is largely a factor of cultural evolution. That is, members of ethnic groups that emerged in regions where raising cattle was common often are more genetically predisposed to digest milk products. Meanwhile, people whose ancestors came from regions where extreme temperatures, short growing seasons and dangerous animal-borne diseases made animal husbandry expensive and difficult often feel cramped and nauseous after eating dairy products.
In cheese-happy Denmark, for instance, only 2 percent of the population studied was lactose intolerant. In Zambia, near the equator, 100 percent of the individuals studied were lactose intolerant.
"The implication is that harsh climates and dangerous diseases negatively impact dairy herding and geographically restrict the availability of milk, and that humans have physiologically adapted to that," evolutionary biologist Paul Sherman, a professor of neurobiology and behavior at Cornell, said in a statement.
The key to lactose intolerance is the enzyme lactase, which is required to digest milk. Although infants around the world produce the enzyme, more than half the people in the world, particularly those of Asian and African descent, stop producing it as they mature. People of northern European descent tend to continue to produce the enzyme because of a genetic mutation, according to Sherman. Thus, they can drink milk throughout life.
Sherman's work is part of the emerging field of Darwinian medicine, which seeks to examine how genetic history impacts health. The popularity of spices in hot climates can be explained in part by antimicrobial compounds, according to Sherman, while morning sickness in pregnant women may be a way to protect embryos against pathogens ingested by the mother that emerged in evolution.
Sherman and student Gabrielle Bloom studied data on lactose intolerance from 270 different ethnic groups from 39 nations ranging from Greenland to southern Africa. Overall, the statistics revealed that lactose intolerance decreases with increasing latitude (moving toward the poles) and increases with rising temperature, particularly when climate changes heighten the difficulty in maintaining dairy herds.
Around 61 percent of the people studied worldwide were lactose intolerant, Sherman's study found.
The data also allowed Sherman to propose an answer to the puzzle of the existence of 13 lactose tolerant groups in the Middle East and Africa. These groups are, or were, nomads and thus regularly lived in regions more favorable to dairy farming.
Historians have theorized that this digestive culture clash contributed to hostilities between Vikings and indigenous tribes in Greenland 1,000 years ago. The Vikings, some believe, offered the lactose-intolerant natives milk.
Their findings will be published in a forthcoming issue of Evolution and Human Behavior.