Milking Our Genes: Lactose Intolerance Explained

Lactose intolerance is still common throughout the world but the development of dairy farming in Northern Europe led to the evolution of individuals who can digest milk.

Most people cannot drink milk as adults, despite its importance as a cheap and nutritious food source. Scientists have shown that the gene which enables adults to digest milk appeared, and became predominant in Northern Europe, as dairy farming took off. The rapid spread of the lactase gene is an example of the power of natural selection to select for a gene with strong evolutionary advantages.

Symptoms of Lactose Intolerance

Symptoms usually appear 30 minutes to 2 hours after consumption of milk or milk-rich products. These can vary quite significantly in their intensity; commonly, symptoms include nausea, cramps, bloating, diarrhoea and flatulence.

Lactose intolerance is not the same as milk allergies; these are an immune response to milk proteins.

There is no known treatment for lactose intolerance. Unlike milk allergies, intolerant individuals cannot become tolerant by exposure to small, then increasing quantities of lactose. However, lactose intolerance is easily managed by avoiding lactose, particularly as lactose-free milk is available in most supermarkets.

Many of the symptoms are dose-related so avoiding dairy products completely is not usually necessary. Lactose intolerant people can still eat yoghurts and small amounts of hard cheeses to ensure regular consumption of calcium and other essential nutrients.

What Causes Lactose Intolerance?

Milk is a rich source of protein and the carbohydrate lactose, a major source of energy for babies and young children. Lactose cannot be absorbed intact in the gut: the enzyme lactase breaks it down into its component sugars, glucose and galactose, which are easily transported across the gut wall.

Most lactose intolerance in adults is caused by an absence of the lactase enzyme. This is a developmental adaptation. All mammals, including humans, have high levels of expression of lactase in early life when their sole source of food is milk. As weaning takes place, the gene for lactase is gradually switched off and lactase expression decreases. This begins at 2-3 years of age and is generally complete by 5-10 years old. This inability to produce lactase to digest milk is often termed lactase non-persistence.

Lactase non-persistence is seen in the majority of the world’s population, including most of Asia and Africa. For example, in non-pastoral communities, such as the Chinese, only 1% produces lactase into adulthood. Conversely in Northern Europe, particularly Scandinavia, and in some nomadic tribes of the Middle East and Africa, lactase persistence is much more common.

Genes Involved in Milk Digestion

The genetic ability to digest milk can be seen as an example of evolution in action. Domestication of dairy animals and the increasing availability of a nutritious and clean food source led to the selection of the genes for lactase persistence. This gave some early Europeans a huge survival advantage.

Analysis of DNA from skeletons of Neolithic Europeans (between 5800 BC and 5000BC) revealed that the lactase gene was absent. Despite the fact that these were some of the earliest farming communities in Europe, these Europeans would not have been tolerant to milk. Scientists believe that the lactase gene, allowing the digestion of milk, emerged after dairy farming really took hold, such that over 90% of northern Europeans now have the lactase gene.

As the human genome project revealed the complexity of human genetic inheritance and evolution, scientists are studying the genetic variants of the lactase gene. New variants have been discovered in pastoral communities in East Africa which may have evolved independently from and more recently than the European lactase gene.

Attention is now focused on whether persistence of the lactase gene is associated with rising obesity and associated diabetes in Western countries or whether in some cases it is protective. The jury is still out on the long term effects of lactase persistence but, with increasing use of large scale genomic studies, the answer is likely to emerge soon.