The ability to digest lactose, the sugar found in milk, is controlled by the LCT gene (Lactase gene). This gene encodes lactase-phlorizin hydrolase, an enzyme responsible for breaking down lactose into glucose and galactose in the small intestine. While all mammalian infants express lactase, its persistence into adulthood varies significantly across human populations due to genetic evolution and environmental factors.
Lactose and Its Digestion
Lactose is a disaccharide found in milk and dairy products. It must be broken down by lactase before being absorbed into the bloodstream. In individuals with lactase persistence, the enzyme continues to be produced into adulthood, allowing them to digest dairy products without issues. In contrast, lactase non-persistent individuals experience lactose intolerance when consuming dairy, leading to symptoms such as bloating, diarrhea, and stomach cramps.
LCT Gene Expression and Regulation
Lactase production occurs in the small intestine, specifically in the brush border of the intestinal epithelial cells. In most mammals, lactase activity declines after weaning. However, in some human populations, genetic mutations have enabled continued lactase production into adulthood, a trait known as lactase persistence.
This trait is controlled by regulatory elements upstream of the LCT gene, primarily within the MCM6 gene. Specific single nucleotide polymorphisms (SNPs) in this region, such as C/T-13910 and G/A-22018, have been linked to lactase persistence.
The Evolution of Lactase Persistence Across Populations
Lactase persistence has evolved independently in different human populations as a response to dairy farming and milk consumption. The trait is most commonly found in populations with a long history of dairy agriculture, including:
- Northern Europeans (~80-90% lactase persistence)
- Certain African pastoralist groups (such as the Maasai and Fulani)
- Middle Eastern dairy farming communities
In contrast, populations with little historical dairy consumption, such as East Asians and many Indigenous American groups, tend to have lower frequencies of lactase persistence (<20%).
LCT Genotypes and Their Impact
The genetic variation in LCT affects whether an individual can digest lactose efficiently. The key genotypes include:
- Homozygous dominant (TT at -13910 position) → High lactase production, lactose tolerant.
- Heterozygous (CT at -13910 position) → Intermediate lactase production, often tolerant.
- Homozygous recessive (CC at -13910 position) → Low lactase production, lactose intolerant.
The inheritance of LCT gene variants follows a dominant pattern, meaning that even one copy of the persistence allele can allow lactose digestion.
Symptoms of Lactose Intolerance
Individuals with lactase non-persistence may experience digestive discomfort upon consuming dairy products, including:
- Bloating
- Abdominal cramps
- Gas
- Diarrhea
- Nausea
These symptoms arise due to fermentation of undigested lactose in the colon, producing gases like hydrogen and methane.
Lactose in Different Dairy Products
The lactose content in dairy products varies significantly, influencing how well individuals with lactose intolerance can tolerate them:
- High lactose: Milk, soft cheeses, yogurt (though yogurt contains lactase-producing bacteria that aid digestion)
- Low lactose: Aged hard cheeses (such as Parmesan, Cheddar), butter, and lactose-free milk
Conclusion
The evolution of lactase persistence is a fascinating example of natural selection in human populations. While the LCT gene determines lactose digestion ability, dietary habits and adaptations continue to shape tolerance levels. Understanding these genetic differences allows individuals to manage their diets more effectively, avoiding discomfort while still enjoying dairy products when possible.