Lactose Intolerance in South Asia: A Genetic and Cultural Perspective

In South Asia, particularly in India, Pakistan, and Bangladesh, a significant portion of the adult population remains lactose intolerant despite the region's status as a leading dairy producer. Research led by Priya Moorjani, Ph.D., at the University of California, Berkeley, indicates that a genetic mutation enabling adults to digest lactose is less prevalent in these countries compared to Europe. This mutation, which allows for the continued production of lactase—the enzyme necessary for lactose digestion—shows a distinct north-to-south gradient across the region. For instance, communities such as the Toda in southern India and the Gujjar in northern Pakistan exhibit a high prevalence of this trait, with nearly 90% of adults carrying the milk-digestion gene.

The genetic variant associated with lactase persistence is believed to have originated from ancient herding populations of the Eurasian Steppe, who migrated southward with their livestock. Historical samples from the Swat Valley in northern Pakistan suggest that this gene appeared during the historic and medieval periods. However, the spread of this trait has been limited in much of South Asia due to various factors, including regional natural selection and cultural practices that influence marriage patterns and food consumption.

Despite the high levels of dairy production, many South Asians have adapted their diets to accommodate lactose intolerance. Traditional practices often involve fermenting milk into products like yogurt and cheese, which contain lower levels of lactose. The Food and Agriculture Organization ranks India as the world's largest milk producer, yet the consumption of fermented dairy products has become a safer choice for many families over generations. This cultural adaptation reflects a broader understanding of food processing as a means to mitigate digestive issues related to lactose.

While some researchers emphasize the role of genetics and natural selection in the prevalence of lactase persistence, others argue that cultural practices and dietary habits have played a more significant role in shaping lactose tolerance in South Asia. The tight-knit marriage circles within many communities may limit the genetic diversity necessary for the spread of beneficial traits, complicating the narrative of natural selection.

Moorjani's research highlights the complexity of lactase persistence, stating, “Our findings reveal that the evolution of lactase persistence is not a single narrative of selection.” This statement underscores the need for a nuanced understanding of how genetics, migration, and cultural practices intersect in shaping dietary habits.

Future studies aim to explore the historical consumption of fresh milk in South Asia and investigate whether other factors contributed to the persistence of the lactase gene. Understanding these dynamics may provide deeper insights into the relationship between genetics and dietary practices in the region.

• “The strength of selection acting on this allele may have been higher in the South Asian pastoralist populations than in Northern Europeans,” — Priya Moorjani, Ph.D., UC Berkeley

• “Our findings reveal that the evolution of lactase persistence is not a single narrative of selection,” — Priya Moorjani, Ph.D., UC Berkeley

This exploration of lactose intolerance in South Asia illustrates the intricate interplay between genetics, culture, and dietary practices, revealing a complex narrative that challenges simplistic assumptions about food and digestion.