Who doesn’t love a scoop of ice cream on a hot day, a cheesy slice of pizza, or a tall, refreshing glass of milk with some cookies? If you are like most people, milk products are some of your favorite foods, and yet the milk sugar, lactose—a disaccharide made from glucose and galactose—can cause serious digestive issues for a lot of the world’s population. Bloating, cramps, and diarrhea….those "Got Milk?" commercials definitely made a smart move leaving those symptoms out of their marketing campaign. Got Bloating?
What exactly is lactose intolerance, and what causes it? First of all, it’s important to realize that your small intestine, which absorbs most of your nutrients, cannot readily absorb lactose. Ideally, your intestinal walls should house an enzyme called lactase that breaks down the lactose into glucose and galactose. These monosaccharides can then be absorbed into your bloodstream. Most mammals, including people, have plenty of lactase when they are little babies. This makes sense because mammals initially rely on milk from their mothers. As baby mammals get older, they produce less and less lactase, until, in adulthood, they can no longer digest lactose.2 When lactose doesn’t get broken down and absorbed, it instead passes into the large intestine, where it causes all sorts of trouble. Bacteria living in the gut love lactose. It’s like a feast, and the result is that they produce quite a bit of gas. That gas means bloating, cramping, and flatulence for you. On top of that, having all that undigested sugar in the gut causes water to rush in—water always likes to move into areas where there are lots of solutes, or molecules to be dissolved—which can cause diarrhea.3 Sweet cheeses, how delicious and cruel you are!
If you have lactose intolerance, you know it is no fun, but look on the bright side: you at least have a lot of people that can commiserate with you. In fact, over 60% of the people on Earth suffer from lactose intolerance.4 This number is much higher in certain populations – over 95% of people in some East Asian, African, and Native American populations.2 In the United States, the incidence of lactose intolerance is over 75% in African Americans and over 90% in Asian Americans.4
European populations tend to have much lower incidences of lactose intolerance.2 Members of these populations continue producing lactase as adults, and this ability is associated with a single genetic change.5 These folks can eat all the ice cream, butter, yogurt, cheese, and triple chocolate pudding they want, and they digest it fine. How’d they get so lucky? Why do they have this capability, when other populations don’t?
It seems that lactose tolerance probably evolved relatively recently – within the last 10,000 years, after humans began to keep domestic animals for agriculture. Archaeological evidence suggests that the first Europeans to store milk lived in Central Europe about 7,500 years ago, and they may have traveled from there to other parts of Europe. A similar genetic change may have arisen completely independently in African and Asian populations that are also lactose-tolerant. We don’t really know the reason why lactose tolerance evolved, but there are lots of possible reasons why this trait may have been beneficial to ancient people, and therefore, favored by natural selection (read: the process by which beneficial characteristics are selected to pass from one generation to the next).
Suddenly, evolving the ability to digest milk would provide a huge survival advantage – people would have a new source of calories, protein, calcium, and vitamin D. Milk would be a fresh beverage for them to drink if water were scarce or contaminated. Milk would also be a pretty steady and predictable source of nutrition and energy5. What's more, people would have milk moustaches, and we all know how cool those look.
As for why some populations do not show this ability, one explanation is that, in areas where people could not keep dairy animals for one reason or another, lactose tolerance did not provide a particular survival advantage. If there was no advantage to digesting lactose, the trait might not have become as widespread as it did in the populations where having lactose tolerance was extremely beneficial4.
If you are one of those carefree people who can chow down on a hot fudge sundae or blow milk bubbles with a straw (come on, admit it – it’s a little more than awesome), just remember to thank your lucky lactase for helping you out with the digestive process.