Lactose intolerance is the natural order of things for mammals such as ourselves. This seems an anathema to most people living in Europe, and in those countries to which Europeans migrated and settled (USA, Canada, Australia, South Africa and so on). Most of us are so used to drinking milk or eating dairy products without an adverse reaction that we consider this to be the norm. Yet it is not. Those who are lactose intolerant are in fact normal, while the rest of us are, quite simply, “mutants”.
Infant mammals, such as ourselves, nurse on their mothers to drink milk, which is rich in lactose. A gene known as LCT provides instructions for making the enzyme lactase (β-D-galactosidase), which is secreted in our intestines to digest lactose. A specific DNA sequence in another gene (the MCM6 gene) helps control whether the LCT gene is turned on or off. With the LCT gene “on”, the enzyme cleaves the lactose molecule into its two subunits, the simple sugars glucose and galactose, which can be absorbed by the infant to fuel growth. Since lactose occurs mostly in milk, in most mammals, the production of lactase gradually decreases with maturity due to a lack of continuing consumption after weaning.
So, having gained the developmental benefits of their mother’s milk, most mammals normally cease to produce lactase. At least several thousand years ago, some humans developed a mutation in the MCM6 gene that keeps the LCT gene turned on even after breast feeding is stopped. People who are lactose intolerant do not have this mutation, and lacking lactase, consuming milk or other dairy products may cause an adverse reaction.
Many people with ancestry in Europe, West Asia, South Asia, the Sahel belt in West Africa, East Africa and a few other parts of Central Africa maintain lactase production into adulthood. This is known as lactase persistence. In many of these areas, milk from cattle, goats, and sheep is used as a substantial source of food. It was in these regions, therefore, that genes for lifelong lactase production first evolved, albeit relatively recently in the last 10,000 years.
Genetic analysis shows lactase persistence (lactose tolerance if you prefer) has developed several times in different places, independently. As an example of convergent evolution, where similar features evolve in species of different lineages, this has led to more than 70% of western Europeans able to drink milk as adults, compared with less than 30% of people from areas of Africa, eastern and south-eastern Asia and Oceania.
Although its symptoms were described by the Greek physician and the "Father of Medicine", Hippocrates (460-370 BC), recognition of the extent and genetic basis of lactose intolerance is (understandably) relatively recent. Until the 1960s, the prevailing assumption of western medicine was that tolerance was the norm and that intolerance was either the result of milk allergy, an intestinal pathogen, or else was psychosomatic (it being recognised that some cultures did not practice dairying, and people from those cultures often reacted badly to consuming milk). There are two reasons for this perception:
· Firstly, most western countries have a predominantly European heritage, so have low frequencies of lactose intolerance, and have an extensive cultural history of dairying. Tolerance therefore was the norm in most of the societies investigated by medical researchers at that point.
· Secondly, within even these societies, lactose intolerance tends to be under-reported: genetically lactase non-persistent individuals can tolerate varying quantities of lactose before showing symptoms, and their symptoms differ in severity. Most can digest a small quantity of milk, for example in tea or coffee, without suffering any adverse effects.