Lactose Intolerance and Lactase Persistence

Why 65% of Humans Can't Drink Milk

Cow and Calf: Middle Kingdom Egypt Sarcophagus
Sarcophagus, Middle Kingdom, Ancient Egyptian, c2040-1786 BC. Detail showing a cow being milked. Her calf is tethered to her leg. The calf behind her belongs to another cow. (Photo by Ann Ronan Pictures/Print Collector/Getty Images). Photo by Ann Ronan Pictures/Print Collector/Getty Images

A total of 65% of the human population today has lactose intolerance (LI): drinking animal milk makes them ill, with symptoms including cramps and bloating. That is the typical pattern for most mammals: they stop being able to digest animal milk once they have moved on to solid foods.

The other 35% of the human population can safely consume animal milk after weaning, that is to say they have lactase persistence (LP), and archaeologists believe that is a genetic trait that developed between 7,000–9,000 years ago among several dairying communities in places like northern Europe, eastern Africa, and northern India.

Evidence and Background

Lactase persistence, the ability to drink milk as an adult and the opposite of lactose intolerance, is a trait that arose in humans as a direct result of our domestication of other mammals. Lactose is the main carbohydrate (disaccharide sugar) in animal milk, including humans, cows, sheep, camels, horses, and dogs. In fact, if a being is a mammal, the mothers give milk, and mother's milk is the major energy source for human infants and all very young mammals.

Mammals cannot normally process lactose in its ordinary state, and so a natural enzyme called lactase (or lactase-phlorizin-hydrolase, LPH) is present in all mammals at birth. Lactase breaks down the lactose carbohydrate into usable parts (glucose and galactose). As the mammal matures and moves beyond mother's milk to other food types (is weaned), the production of lactase decreases: eventually, most adult mammals become lactose intolerant.

However, in about 35% of the human population, that enzyme continues to work past the point of weaning: people who have that working enzyme as adults can consume animal milk safely: the lactase persistence (LP) trait. The other 65% of the human population is lactose intolerant and cannot drink milk without ill effects: the undigested lactose sits in the small intestine and causes the differing severity of diarrhea, cramps, bloating, and chronic flatulence.

Frequency of LP Trait in Human Populations

While it is true that 35% of the world's population has the lactase persistence trait, the likelihood that you have it depends largely on geography, on where you and your ancestors lived. These are estimates, based on fairly small sample sizes.

  • Eastern and Southern Europe: 15–54% have LP enzyme
  • Central and Western Europe: 62–86%
  • British Isles and Scandinavia: 89–96%
  • Northern India: 63%
  • Southern India: 23%
  • East Asia, Native Americans: rare
  • Africa: patchy, with highest percentages associated with cattle pastoralists
  • Middle East: patchy, with highest percentages associated with camel pastoralists

The reason for the geographic variation in lactase persistence has to do with its origins. LP is believed to have arisen because of the domestication of mammals, and the subsequent introduction of dairying.

Dairying and Lactase Persistence

Dairying--raising cattle, sheep, goats, and camels for their milk and milk products--began with goats, about 10,000 years ago in what is today Turkey. Cheese, a reduced lactose dairy product, was first invented about 8,000 years ago, in that same neighborhood in western Asia--making cheese removes the lactose-rich whey from the curds. The table above shows that the highest percentage of people who can consume milk safely are from the British Isles and Scandinavia, not in western Asia where dairying was invented. Scholars believe that is because the ability to safely consume milk was a genetically selected advantage in response to milk consumption, developed over 2,000–3,000 years.

Genetic studies conducted by Yuval Itan and colleagues suggest that the European lactase persistence gene (named -13,910*T for its location on the lactase gene in Europeans) appears to have arisen about 9,000 years ago, consequent with the spread of dairying into Europe. -13.910:T is found in populations all over Europe and Asia, but not every lactase persistent person has the -13,910*T gene--in African pastoralists the lactase persistence gene is called -14,010*C. Other recently identified LP genes include -22.018:G>A in Finland; and -13.907:G and -14.009 in East Africa and so on: there are no doubt other as-yet unidentified gene variants. They all, however, likely arose as a result of a reliance on milk consumption by adults.

Calcium Assimilation Hypothesis

The calcium assimilation hypothesis suggests that lactase persistence might have gotten a boost in Scandinavia because in high-latitude regions reduced sunlight doesn't allow sufficient synthesis of vitamin D through the skin, and getting it from animal milk would have been a useful substitute for recent immigrants to the region.

On the other hand, studies of DNA sequences of African cattle pastoralists indicate that the mutation of -14,010*C occurred about 7,000 years ago, in a place where a lack of vitamin D was certainly not a problem.

TRB and PWC

The lactase/lactose set of theories test the larger debate over the arrival of agriculture in Scandinavia, a debate over two groups of people named by their ceramic styles, the Funnel Beaker culture (abbreviated TRB from its German name, Tricherrandbecher) and the Pitted Ware culture (PWC). By and large, scholars believe the PWC were hunter-gatherers who lived in Scandinavia about 5,500 years ago when the TRB agriculturalists from the Mediterranean region migrated into the north. The debate centers around whether the two cultures merged or the TRB replaced the PWC.

DNA studies (including the presence of the LP gene) on PWC burials in Sweden indicate that the PWC culture had a different genetic background from those of modern Scandinavian populations: modern Scandinavians have far higher percentages of the T allele (74 percent) compared to PWC (5 percent), supporting the TRB replacement hypothesis.

Khoisan Herders and Hunter-Gatherers

Two 2014 studies (Breton et al. and Macholdt et al.) investigated lactase persistence alleles among southern African Khoisan hunter-gatherer and pastoralist groups, part of a recent reassessment of the traditional concepts of the Khoisan and the broadening of applications for the appearance of LP. "Khoisan" is a collective term for people who speak non-Bantu languages with click consonants and includes both Khoe, known to have been cattle herders from about 2,000 years ago, and San often described as the prototypical (maybe even stereotypical) hunter-gatherers. Both groups are often assumed to have remained largely isolated throughout prehistory.

But the presence of LP alleles, along with other recently identified evidence such as shared elements of Bantu languages among Khoisan people and recent archaeological discoveries of sheep pastoralism at Leopard Cave in Namibia, has suggested to scholars that African Khoisan were not isolated, but instead were descended from multiple migrations of people from other parts of Africa. The work included a comprehensive study of LP alleles in modern southern African populations, descendants of hunter-gatherers, cattle and sheep pastoralists and agropastoralists; they found that Khoe (herding groups) carried the East African version of the LP allele (-14010*C ) in medium frequencies, indicating they are likely partly descended from pastoralists from Kenya and Tanzania. The LP allele is absent, or in very low frequencies, among Bantu-speakers in Angola and South Africa and among San hunter-gatherers.

The studies conclude that at least 2000 years ago, pastoralism was brought by a small group of eastern African migrants to southern Africa, where they were assimilated and their practices adopted by local Khoe groups.

Why Lactase Persistence?

The genetic variants that allow (some) people to consume mammal milk safely arose about 10,000 years ago as the domestic process was being undertaken. Those variations allowed populations with the gene to broaden their dietary repertoire, and incorporate more milk into their diet. That selection is among the strongest in the human genome, with a strong influence on human reproduction and survival.

However, under that hypothesis, it would seem logical that populations with higher levels of milk dependence (such as nomadic herders) should have higher LP frequencies: but that is not always true. Long-term herders in Asia have quite low frequencies (Mongols 12 percent; Kazakhs 14–30 percent). Sami reindeer hunters have a lower LP frequency than the rest of the Swedish population (40-75 percent versus 91 percent). That might be because different mammals have different concentrations of lactose, or there may be some as-yet-undetected health adaptation to milk.

In addition, some researchers have suggested that the gene arose only in times of ecological stress, when milk had to be a larger part of the diet, and it might have been more difficult for individuals to survive milk's ill effects under those circumstances.

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