Lactase tolerance or lactase persistence - the ability of adults to digest lactose in milk – has changed our dietary habits for years. However, it remains a mystery how and when that ability became established or occurred. An international team of scientists recently suggest that lactase persistence spread throughout Central Europe in only a few thousand years, a transformation that happened quickly compared to most evolutionary changes observed in humans.
Genomic data of lactase persistence (LP)
To come up with such findings, Joachim Burger from the Institute of Organismic and Molecular Evolution in the Johannes Gutenberg University of Mainz and team tested the genetic material of the bones of 14 warriors from the Tollense Bronze Age battlefield in northern Germany. In short, these are bones of warriors who died during the Bronze Age battle around 1,200 BC. The battle happened on the banks of the Tollense river in present-day Germany.
Despite being more than 3,000 years old, Burger and the team were still able to sequence the DNA from the bone fragments they recovered from the excavation site. Co-author Krishna Veeramah, Ph.D. from the Department of Ecology and Evolution Stony Brook University led part of the study that analyzed how the genetic ancestry of said warriors compared to other ancient and modern populations. Then, they compared the frequency of lactase-persistent (LP) allele to other ancient and modern populations, such as the genotypes of 37 individuals from the Pontic-Caspian Steppe region and Eastern Europe and 18 people from the Bronze Age site Mokrin in Serbia.
They found out that the introduction of agriculture in the continent would have involved, in part, in the consumption of dairy from early sheep, goats, and cattle domesticates. Yet, despite the battle occurring for more than 4,000 years, only one in eight of the warriors examined had a genetic variant that allowed them to break down lactose, as published by Phys.org.
Lactase persistence frequency
Veeramah explained that when they looked at other European genetic information from the early Medieval period, which is less than 2,000 years later, more than 60% of people already could drink milk as adults. Such finding is close to what they previously observed in modern Central European nations that range from 70 to 90%.
The authors said that it is an “incredibly fast rate” of change for the gene that controls the digestion of milk. They also found that by possessing such a genetic change, Europeans in the past with lactase persistence ability had a 6% greater chance of producing kids. They refer to it as the “selection coefficient.” In population genetics, it is the measure of differences in relative fitness.
But why did it provide such a big evolutionary advantage if a person can digest the sugar in milk? The lead author said there is no definitive answer to such a question yet. Some studies in evolutionary genetics show that those who expressed LP phenotypes would have had a significant advantage in terms of the nutritional acquisition, especially in societies where pastoralism and domestication of milk-producing animals became a way of life.
Veeramah and colleagues also mentioned some factors for the evolutionary advantage of lactase persistent individuals. These include improved calcium absorption, suppression of malaria symptoms through a reduction of an organic compound called p-aminobenzoic acid, avoidance of diarrhea during famine conditions, improvements in gut health through galacto-oligosaccharides and galactose reshaping the colonic microbiome, and the increased economic efficiency of calorie production.
It’s no surprise since milk is nutritious. It contains micronutrients, carbohydrates, and calcium. Most babies can digest milk without getting an upset stomach because of enzyme lactase. However, until several thousand years ago, the enzyme turned off the moment a person grew into adulthood. This means that most adults have become lactase nonpersistent or lactose intolerant.
Mark Thomas, an evolutionary geneticist from the University College of London, told US news platform NPR that a combination of two reasons could be the reason why there was a lactase mutation. First is that farmers that settled in Northern Europe brought crops native to that region, such as barley and wheat. Yet, because of the shorter growing season in the area, these crops were more likely to fail and it caused famine. "If you're a farmer in Southern Europe, and you milk a cow in the morning and you leave the milk out, it will be yogurt by noon,” Thomas said. He added that if a healthy but lactose intolerant individual drank that milk, they would get diarrhea but if they are malnourished, there’s a possibility of them dying.
Thomas, who is not involved in the study, explained that people with lactase mutation would have survived and passed on their genes. The combination of longer processing time for milk and famine is like a “double whammy,” he said.
The global prevalence of lactose intolerance by country
Today, approximately 65% of the human population is lactose intolerant. This is most prevalent in East Asian descent, according to Genetics Home Reference.
Milk ProCon.org also published the countries with the highest prevalence of lactose intolerance. This includes Ghana, Malawi, South Korea, Yemen (all 100%), Solomon Islands (99%), Armenia, Vietnam, Zambia, (98%), Azerbaijan, Oman (96%), the Democratic Republic of the Congo, Mozambique, and Syria (95%). On the other hand, countries with the lowest percentage of the population with lactose intolerance are Denmark, Ireland (4), Sweden (7%), United Kingdom (8%), New Zealand (10%), Netherlands, Norway (12%), Niger (13%), Belgium (15%), Cyprus, Germany (16%), and Finland (19%).
The prevalence of lactose intolerance is lowest in countries with a long history of dependence on unfermented milk products as their food source.
Food ingredients that often contain lactose include milk powder, milk protein, milk solids, nonfat dry milk, whey, and whey solids or protein. Some people with lactose intolerance due to an underlying condition may restore their body’s ability to digest lactose but the process may take months. For their treatment, they are usually advised to include only small servings of dairy products in their regular meals.
LP varies widely in frequency across populations. The recent study shows that the ingestion of milk may have provided greater chances of survival when supplies of drinking water have been contaminated or during food shortages.