June 20, 2014

By Medical Discovery News

Drinking milk might seem perfectly natural, but it’s actually anything but. Humans are the only species who retain the ability to digest milk after childhood, or at least some of us do. Up to half of adults worldwide don’t have the ability to break down lactose, the main sugar in milk, because their bodies stop producing the enzyme lactase after the age of five.

About 65-75 percent of the population has some degree of lactose intolerance, the most common cause for digestive issues with dairy. Lactase breaks down lactose into simpler forms of sugar that can be absorbed by the bloodstream. Without this enzyme, lactose is fermented by bacteria, causing symptoms like abdominal pain, bloating, flatulence, nausea, and diarrhea 30 minutes to two hours after eating. Populations that have long relied on unfermented milk have the lowest rate of lactose intolerance – only five percent among the Swiss.

Humans’ ability to drink milk actually began as a genetic mutation, like the superheroes of X-Men comics. According to the leading theory, 7,800 years ago humans began to move northward. Since the sun is not out as long in northern latitudes, they could not absorb enough vitamin D from sunlight and needed another source to thrive. Milk is high in vitamin D, which aids in calcium absorption. Humans adapted to this change in their diets and developed a variant of the lactase gene that allowed them to continue synthesizing the enzyme throughout their lifetimes. Since humans with the gene variant had the advantage of consuming more vitamin D, they were successful in passing that gene on to future generations.

But new research suggests that this theory is either wrong or other factors were involved. Scientists in northeastern Spain discovered well-preserved skeletons of people who lived 5,000 years ago. DNA testing revealed that none of these eight skeletons carried the genetic mutation for lactase production. Further testing also showed that these ancient humans are indeed related to modern Spaniards. Next, computer simulations determined that over 5,000 years, chance alone would not have allowed one-third their descendants to digest milk. Strong selection for this trait would have been necessary.

These scientists developed a theory that early farmers began eating fermented dairy products such as cheeses, which have lower levels of lactose. But when food was scarce, they ran out of fermented dairy products and began to consume unfermented milk as a food source. Then, those who acquired the mutated gene for lactose production would have thrived. Those without the mutation would have suffered from diarrhea, making their situation worse, perhaps even life-threatening if they were already starving.

While the need for vitamin D from milk may have been a factor in the spread of lactase persistence, these new findings show that other factors may have also been a part of the selection process that drove this mutation into the population. Now if we could only figure out a way to turn on lactase genes again during adulthood, everyone with lactose intolerance could enjoy a pain-free ice cream cone.

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