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.
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