Why are adults in certain societies unable to digest milk products as adults?

December 22, 2025 •

4 min read

Approximately 65% of the world’s population has a reduced ability to digest lactose after infancy, a condition known as lactose nonpersistence. This seemingly common digestive quirk is not a random occurrence but a product of fascinating evolutionary and cultural processes that explain why adults in certain societies are unable to digest milk products. The variation in lactose tolerance across human populations is a clear example of how our biology is shaped by environmental and historical factors.

Quick Summary

This article delves into the complex reasons behind regional variations in adult lactose digestion. It explains the genetic basis of lactase persistence and how its presence or absence relates to a population's historical and cultural practices, particularly the adoption of dairy farming and milk consumption patterns. The content covers the science of lactase deficiency, the evolutionary timelines of different gene mutations, and global prevalence maps illustrating these fascinating human adaptations.

Key Points

Lactase Nonpersistence: The normal adult human condition is to stop producing the lactase enzyme after infancy, leading to an inability to digest lactose, the sugar in milk.
Genetic Mutations: A genetic mutation, known as lactase persistence, allows some individuals to continue producing lactase into adulthood, enabling them to digest milk products.
Evolutionary Advantage: This mutation became more prevalent in populations with a history of dairy farming, as it provided a nutritional advantage, especially in times of scarcity.
Global Variation: The prevalence of lactose intolerance varies drastically by region; it is lowest in Northern Europe and highest in East Asia, reflecting different cultural and historical relationships with dairy.
Cultural Adaptation: Societies with high rates of intolerance developed culinary methods, like fermenting milk into yogurt and cheese, which reduce lactose content and allow for dairy consumption.
Not an Allergy: Lactose intolerance is a digestive issue caused by a lack of lactase, not an immune system reaction like a milk allergy.

The Genetic Basis of Lactose Intolerance

At the core of the adult inability to digest milk is a genetic trait known as lactase nonpersistence, or hypolactasia. All infant mammals, including humans, produce the enzyme lactase to break down lactose, the sugar found in milk. This is a crucial adaptation for survival during the nursing period. However, in most of the world's population, the LCT gene, which provides instructions for making lactase, is naturally downregulated after weaning. The production of the lactase enzyme decreases significantly, or even stops, causing undigested lactose to pass into the large intestine.

When undigested lactose reaches the large intestine, gut bacteria ferment it, producing gases and fluids that lead to the uncomfortable symptoms of lactose intolerance, such as bloating, gas, abdominal pain, and diarrhea. In contrast, some human populations have a genetic mutation that keeps the LCT gene active throughout adulthood. This trait is called lactase persistence and allows for the continued digestion of milk products. Several different genetic mutations in a regulatory region of a nearby gene called MCM6 have been identified, which shows that this trait evolved independently in different parts of the world.

The Role of Natural Selection and Dairying

The geographic distribution of lactase persistence is not random; it is a powerful example of gene-culture coevolution. The spread of this trait is closely tied to the history of dairy farming and livestock domestication, which occurred independently in multiple regions over the last 10,000 years. For populations that traditionally did not domesticate animals for milk or fermented it heavily, there was no selective pressure to maintain lactase production beyond infancy. Thus, lactase nonpersistence remained the norm in these groups.

Conversely, for pastoralist and dairying societies, being able to digest milk as an adult offered a significant survival advantage. Milk is a nutrient-rich and relatively pathogen-free food source, especially valuable in times of famine or when clean water was scarce. This gave individuals with the lactase persistence mutation a greater chance of survival and reproduction, allowing the trait to spread rapidly through natural selection.

Regional Differences in Lactose Tolerance

The stark differences in lactose tolerance are visible when comparing different global populations.

High Lactase Persistence (Low Intolerance): Northern European populations have the highest rates of lactase persistence, with as few as 5% of adults being lactose intolerant. This is attributed to a long history of intensive dairy farming. Some pastoralist groups in parts of Africa, the Middle East, and Central Asia also have high rates of lactase persistence, stemming from independently evolved mutations.
High Lactase Nonpersistence (High Intolerance): Lactose nonpersistence is the global default and is particularly prevalent in East Asian populations, where rates of lactose intolerance can be as high as 70-100%. It is also common in people of West African, Southern European (Greek, Italian), Middle Eastern, Hispanic/Latino, and Native American descent.

Cultural Adaptations to Lactose Intolerance

Societies with low rates of lactase persistence have developed cultural and culinary practices that allow them to consume dairy without the negative side effects of undigested lactose. These traditional foodways bypass the issue by either fermenting the lactose or consuming dairy products with naturally lower lactose content.

Here are some common cultural strategies:

Fermentation: The bacteria used to make yogurt, cheese, and kefir break down much of the lactose, making these products easier to digest. This is why many lactose-intolerant individuals can enjoy fermented dairy. For example, the nomadic peoples of Mongolia, who have low rates of lactase persistence, traditionally consume fermented mare's milk (airag), which has very little lactose.
Aging: Hard cheeses, like cheddar and parmesan, are aged for longer periods, which naturally reduces their lactose content to very low levels.
Milk as a flavoring: Many cultures with high intolerance rates use milk sparingly as an ingredient rather than as a primary beverage, further minimizing lactose exposure.

Comparison of Lactose Tolerance by Region and Dairy Consumption


Feature	Northern Europe	East Asia	East Africa (Pastoralists)
Genetic Trait	High lactase persistence	High lactase nonpersistence	High lactase persistence (independently evolved)
Prevalence of Intolerance	Very low (5-15%)	Very high (70-100%)	Variable, but some groups very low
Historical Dairy Consumption	Extensive, with consumption of fresh milk	Traditionally low, minimal fresh milk consumption	Extensive reliance on fresh milk from livestock
Culinary Adaptations	Minimal need for adaptations	Extensive use of dairy alternatives and minimal dairy consumption	Consumption of fermented products like amasi and fresh milk

Conclusion

The inability of adults in certain societies to digest milk is not an anomaly but a reflection of humanity's diverse evolutionary journey. The distribution of lactose tolerance is directly linked to the historical development of dairying practices, with natural selection favoring lactase persistence in populations that relied on milk from domesticated animals. While the majority of the world's population remains lactose nonpersistent, the cultural innovations of cheesemaking, fermentation, and other dairy processing techniques have allowed low-tolerance societies to incorporate dairy into their diets. This fascinating case study of gene-culture coevolution highlights how our dietary history has literally shaped our DNA. Understanding these deep historical roots not only explains a common digestive issue but also sheds light on the interplay between biology, culture, and human evolution. For more insight into this process, the National Institutes of Health provides extensive resources.

Frequently Asked Questions

The core reason is a genetic trait called lactase nonpersistence. After weaning, most human adults naturally produce less of the enzyme lactase, which is needed to break down lactose, the sugar in milk.

Lactose intolerance is actually the ancestral norm for all mammals, including humans, once they are weaned from mothers' milk. The ability to digest lactose into adulthood (lactase persistence) is a relatively recent evolutionary adaptation.

The ability to digest milk as an adult is linked to a population's history of dairy farming. Societies that relied on domesticated animals for milk experienced strong natural selection for lactase persistence, while those without a history of dairying retained the ancestral, intolerant state.

No, lactose intolerance is not a food allergy. It is a digestive issue caused by an enzyme deficiency, whereas a food allergy involves an immune system response to a protein in the food, such as milk protein.

Lactose intolerance is very common in East Asian populations because their ancestors had no historical selective pressure to evolve lactase persistence. Dairy farming was not a traditional part of their diet, so the normal decline of lactase production after infancy persisted.

Many cultures with high intolerance use fermented dairy products like yogurt, cheese, or kefir, where bacteria have already broken down much of the lactose. Small amounts of lactose are often tolerated, and some hard cheeses contain very little lactose.

Yes, primary lactose intolerance typically begins to develop after weaning and gradually increases as lactase production naturally declines. The onset often occurs in late childhood or early adulthood.