A Common Misunderstanding About the Evolution of Lactase Persistence

Are you able to consume lactose-containing foods such as cow’s milk and ice cream without experiencing acute gastrointestinal problems like bloating and diarrhea? If so, then you may have heard that it’s perfectly safe and healthy for you to drink milk. This idea, that people who are capable of digesting the milk sugar lactose even after they are weaned can safely include milk and other dairy foods into their diet, is widespread. That’s worrying, seeing as it’s fallacious…

Lactase persistence explained

We’re all capable of digesting lactose when we’re very young; however, most of us lose that ability when we grow up and are no longer breast fed. Only those who carry a gene that confers a lactase persistence phenotype and/or harbor a gut microbiome that is adapted to break down lactose are able to continue consuming lactose-containing foods post infancy.

In the time that has passed since we humans first domesticated cattle, the frequency of lactase persistence alleles in the human gene pool has gradually increased. These alleles allow for the expression of lactase – the enzyme responsible for breaking down lactose – post infancy. In other words, they confer their bearers with the ability to consume cow’s milk and other lactose-containing foods in adulthood without ending up in the bathroom feeling bloated and miserable.

Different alleles have taken hold in different populations. Moreover, the number of people who are able to digest lactose varies greatly from country to country, in part because some countries have a longer history of dairying than others. In some parts of the world, such as Northern Europe and the U.S., the vast majority of people are able to drink milk. In other parts however, such as South Africa, lactose intolerance is the norm.

The fact that a significant percentage of contemporary humans are capable of breaking down lactose even long after they are weaned has led some people to think that cow’s milk and other dairy foods can safely be included into the diets of a fairly large number of people. Some imply that it’s not only unproblematic, but healthy, for people with a lactase persistence genotype to consume lactose-containing dairy foods. What these individuals fail to recognize is that lactase persistence alleles didn’t spread within human populations because they made people age gracefully and steer clear of chronic degenerative disease, but rather because they enhanced people’s reproductive success.

The health and fitness implications of milk consumption

As alluded to above, the only thing that the institution of lactase persistence really tells us is that there must have been a marked fitness benefit to continued lactase expression under the relevant conditions of life, implying that it was beneficial, from a Darwinian point of view, to be able to consume lactose-containing foods post infancy. If there wasn’t, lactase persistence wouldn’t have spread. That’s not the same as saying that it was all rainbows and butterflies to drink milk though.

To illustrate this, let’s imagine that we’re observing a Neolithic population of humans who rely on just one or a few types of grains for the bulk of their caloric intake. Famine is an issue and so is malnutrition. Under such circumstances, it would naturally be beneficial, from a fitness perspective, to have access to and be able to utilize, in a nutritional sense, the milk of another animal, as it would give one a caloric boost and help protect against several micronutrient deficiencies, something that would ultimately help one survive and reproduce.

That’s not the same as to say that it’s the ideal solution though. Under such messy circumstances, it’s overall effect would likely be positive, as it would help buffer against much disease and malnutrition; however, when viewed in isolation, it would likely be revealed that it has several adverse effects on the organism, some of which might not fully manifest themselves until late in life. The reason why this is to be expected is that milk has an unusual nutritional profile featuring a number of compounds besides lactose that have not historically been a part of people’s diets, including miRNAs and casein.

Most likely, a much better solution, both from a health and fitness perspective, would be to eat a variety of fruits, vegetables, and high-quality animal source foods, which we humans have plenty of evolutionary experience with, and hence, are well adapted to eating. But if that’s not a viable option for the farmers in question, it would certainly be better to drink milk than to do nothing. As long as the net fitness result of having the capability to engage in such a practise is positive, it would spread.

To bring this point home, let’s imagine that you lived 6000 years ago in a part of the world where cows had been domesticated. If you were able to drink the milk of those cows without experiencing any gastrointestinal issues, you would likely have had an edge in the struggle for existence when compared with someone whose gut always turned runny when he drank milk.

Basically, you would have been able to make use of a significant source of protein, fat, thiamin, calcium, magnesium, and various other nutrients that the lactose-intolerant individual wouldn’t have been able to tap into. Those nutrients could prove to be very valuable, particularly in times of food scarcity, and would likely have helped you (as well as any offspring you might have passed the lactase persistence trait to) survive and reproduce. That doesn’t exclude the possibility that all the milk you were taking in was causing some health issues though. It might for example have put you at an increased risk of developing Alzheimer’s disease.

But, seeing as Alzheimer’s disease usually develops very late in the life span (often long after the typical age of death back in Neolithic times), natural selection wouldn’t have paid much attention to that disease, nor its connection with milk consumption. In other words, from a Darwinian point of view, the positive effects that were conferred by the trait that allowed you to digest lactose would have outweighed the negative ones.

One would assume that those people who experienced the least amount of negative health effects from consuming milk would have had an edge in the evolutionary competition in the parts of the world where milk has long been a part of people’s diet, but again, seeing as many of the negative health effects of milk consumption are of such a nature that they don’t have a major impact on survival and reproduction, natural selection doesn’t pay that much attention to them.

Could other factors have been responsible for the evolution of lactase persistence?

It’s generally believed that the reason why lactase persistence alleles have spread so rapidly within many human populations over the most recent millennia is that they are beneficial from a purely nutritional point of view. What’s important to point out though, is that certain other explanatory theories for this phenomenon have been proposed. These theories don’t replace the conventional one; rather, they complement it.

Irrespective of which combination of theories that best reflect the truth, the fact remains: the reason why lactase persistence alleles have become a much more dominant part of the human gene pool over the past ~ 10.000 years is not that they make people who carry them age gracefully and avoid degenerative disease per se, but rather because they enhance our ability to survive and reproduce in certain environmental contexts.

Key takeaway points:

• Lactase persistence is a fairly recent human adaptation that allows for continued expression of the enzyme lactase after weaning and into adulthood, something that makes it possible to consume lactose-containing foods (e.g., cow’s milk) throughout the life span without experiencing gastrointestinal distress. A somewhat similar capability can also be obtained via gut microbiota adaptation.
• Lactase persistence evolved because it conferred significant survival and reproductive advantages under certain environmental settings. This doesn’t necessarily mean that it’s unequivocally healthy to drink milk.
• Context matters in evolution. The pros/cons equation of a particular practise, such as drinking milk, may vary depending on environmental (including dietary) circumstances. If the person or population in question is eating a poor, nutrient-deficient diet, the benefits of drinking milk may outweigh any potential costs, as milk is rich in many nutrients.
• Milk (e.g., cow’s milk) has a configuration that differs markedly from that of the foods we’re evolutionarily accustomed to consuming. Besides lactose, it contains a number of other special compounds, including miRNAs and whey and casein proteins, which have not surprisingly, from a Darwinian point of view, been shown to be troublesome. Of note, digestion only represents one step in the nutritional utilization process, meaning that even if a particular compound, such as lactose, is enzymatically cleaved/digested, it may still be problematic. All of this is to say that lactase persistence only represents partial adaptation to milk.
• The reason why lactase persistence has been under such strong selection is presumably that the gastrointestinal distress that accompanies the consumption of lactose in the absence of lactase activity has acute fitness consequences and would discourage milk-drinking altogether. Any potentially more subtle adverse impacts, such as effects related to long-term bone health or cognitive function, would not be expected to arouse natural selection to the same extent.

Picture: Food photo created by jcomp – www.freepik.com