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Lactose Intolerance
If you like milk and other dairy products, then consider yourself lucky if you have the mutation in the LCT (Lactase) gene that allows you to enjoy them without bloating, cramping and diarrhea! These are the symptoms of lactose intolerance.
It may surprise you to learn that lactose tolerance (sometimes also called “lactase persistence”, that is the ability of human adults to digest milk products without experiencing the aforementioned symptoms), is unique among mammals and a relatively new trait among humans.
The history of lactose tolerance is fascinating, as it involves a genetic variant that spread within and among human populations due to positive natural selection , because of the survival and reproductive advantages that it conferred on those who carried it.
Lactose is a natural sugar found in milk and most dairy products that is broken down by the enzyme lactase, produced by cells in the digestive tract. At birth, all mammals produce the lactase enzyme and can therefore drink their mother’s milk without experiencing bloating, cramping or diarrhea. After weaning however, mammal infants stop producing lactase and prepare for an adult diet of raw meats, grass or other delicacies!! Originally, this was also the case for all humans. However, a few thousand years ago a mutation occurred in the lactase gene of one human ancestor that allowed him or her to continue to digest the lactose in dairy products into adulthood. This is why the trait is sometimes called “lactase persistence”. We do not know who this ancestor was, but it is likely that this person lived somewhere in Europe and belonged to a group that kept milk-producing animals.
The continued production of lactase into adulthood turned out to be highly advantageous, probably because it provided a rich and constant source of nutrition and fluid in groups that kept dairy animals. Individuals in such groups that did not carry the mutation seem to have been at a relative disadvantage, particularly at times when dairy products were the only nourishment available. As a result, the underlying mutation quickly spread within Northern Europe and to some other parts of the world and rose, through positive natural selection , to high frequency in many populations that used domesticated animals such as cattle and goats.
Lactose intolerance ranges in frequency from 2-5% in Northern Europe and up to nearly 100% in Asia, South-Africa and Latin-America, with intermediate rates in North-America and North-Africa. An estimated 30 to 50 million American adults are thought to be lactose intolerant.
The deCODEme Genetic Scan identifies a variant SNP close to the lactase gene (LCT) on chromosome 2 and gives an interpretation of the associated genetic risk for lactose intolerance.
- Jobling, M.A., Hurles, M.E. and Tyler-Smith, C. (2004) Human Evolutionary Genetics: origins, peoples and disease. London/New York.
- Beja-Pereira A et al. Nat Genet. 2003 35(4):311-3. Gene-culture coevolution between cattle milk protein genes and human lactase genes.
This content was last reviewed on February 11, 2010.