Packed with calcium, vitamin D, and protein, milk is a staple of American and European diets, but cultures in the East tend to avoid it due to lactose intolerance. Researchers from the Evolutionary Biology Centre at Uppsala University found in a study that calcium absorption has little to do with a person of European descent’s ability to handle milk consumption, also known as lactase persistence.

The calcium assimilation hypothesis suggests that early Europeans developed lactase persistence out of necessity to avoid calcium deficiency. With areas of low sunlight and diets based mainly off of cereal, some Northern Europeans undoubtedly had trouble maintaining a healthy balance of the two essential nutrients: calcium and vitamin D. Before this study, many scientists believed early Europeans added milk to their diet to facilitate calcium absorption.

"The evolution of lactase persistence is one of the best known and most dramatic examples of recent human evolution,” lead researcher Oddný Sverrisdóttir said in a statement. “One of the ironies of working in this area is that we know it happened, but we still don't fully know why.” Sverrisdóttir also pointed out that lactase persistence is highly concentrated in people from southern Sweden and Ireland.

The majority of scientists and health care professionals accept that the enzyme, lactase, prevents our lactose intolerance, but where this biological trait comes from remains a mystery. Research does show that lactase persistence is a result of natural selection within the past ten thousand years. Sverrisdóttir and her colleagues set out to determine why people of European decent are able to consume milk without fear of lactose intolerant side effects such as bloating, cramps, gas, and diarrhea.

Researchers tested DNA samples from the bones of early farmers in Spain, where sunlight is abundant and many people are lactase persistent. After comparing DNA from early Spanish farmers, researchers did not locate the gene mutation that causes lactase persistence in Europeans. In fact, computer simulations showed that “a lot!” of natural selection would be needed to drive lactase persistence up to what it is today in Spain.

"If natural selection is driving lactase persistence evolution in a place where people have no problems making vitamin D in their skin, then clearly the vitamin D and calcium explanation isn't cutting it,” Sverrisdóttir added. “So while the calcium assimilation hypothesis may have some relevance in Northern Europe it's clearly not the whole story.”

The Uppsala University research team concluded with their own hypothesis:

Although most early European farmers would not have been lactase persistent, they would still have been able to consume fermented milk products such as yoghurt and cheese, because fermentation converts much of the lactose into fats. But in famine conditions, such as when crops fail, they are likely to have eaten all the fermented milk foods, leaving only the more high-lactose products. This would have caused the usual lactose intolerance symptoms such as diarrhea. Diarrhea in in healthy people is not usually life-threatening, but in severely malnourished individuals it certainly can be. So famine could have led to episodes of very strong natural selection favoring lactase persistence.