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The ability to digest lactose in adults is inherited as a dominant Mendelian trait. This means that it is caused mainly by your genetic makeup with little or no influence from the environment.

Having lactose intolerance is not the same as being allergic to milk.

Unlike an allergy, lactose intolerance does not involve your immune system and does not necessarily mean that you will have to completely avoid dairy products.

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 and 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 Complete 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.

risk factors

• Age: Lactose intolerance can begin at different times in life. In people of European ancestry, it usually starts to affect children older than 5 years. In African-Americans it can occur as early as age 2. Infants born prematurely may have reduced levels of lactase, since production of this enzyme starts late in the last trimester.
• Ethnicity: Lactose intolerance is highly dependent on ethnicity: 95 percent of Asians, 60 to 80 percent of African Americans and Ashkenazi Jews, 80 to 100 percent of American Indians, and 50 to 80 percent of Hispanics have lactose intolerance. It is least common in people of North-European origin (2-5%).
• Genetics: The ability to digest lactose in adults is inherited as a dominant Mendelian trait. This means that it is caused mainly by your genetic makeup with little or no influence from the environment.

  A sequence variant (SNP) near the lactase gene has been identified that determines whether people are “lactase persistent” or “lactose intolerant”, that is whether the expression of the gene that controls lactase production is turned on or off during adulthood. This means that your genotype will, with high probability, predict whether you are lactose intolerant or not.

  Note however, that even if you turn out to have the variant that enables you to digest lactose during adulthood, you could still have other variants that make you sensitive or even allergic to milk.

  It should also be noted that the variant reported here does not account for every case of lactase persistence, particularly in people that trace their ancestry to sub-Saharan Africa. In this part of the world it is thought that other, presently unidentified, variants are also responsible for cases of lactase persistence.

prevention

Being lactose intolerant or lactase persistent during adulthood depends completely on your body’s ability to produce lactase, which is genetically determined. There is no scientific evidence to indicate that diet or lifestyle has an impact on your ability to produce lactase as an adult. For example, avoiding milk completely for long periods does not lead to a change in lactase production.

However, removing milk products from the diet usually improves the symptoms of lactose intolerance. Milk products that are soured or otherwise treated (like yogurts and solid cheeses) contain relatively low levels of lactose and the soured products may even contain the lactase-producing Lactobacillus Acidophilus. As a result, these milk products cause fewer problems for lactose intolerant people compared to other milk products. Lactose intolerant people who avoid milk products should supplement their diet with calcium-rich food and vitamin D to build up and maintain a healthy bone mass.

Note that having lactose intolerance is not the same as being allergic to milk. Unlike an allergy, lactose intolerance does not involve your immune system and does not necessarily mean that you will have to completely avoid dairy products.

more information

You can find out more information about lactose intolerance by talking to your doctor and visiting these Web sites:

• National Digestive Diseases Information Clearinghouse (NDDIC) on lactose intolerance
• Mayo Clinic information on lactose intolerance
• MedlinePlus article on lactose intolerance
• Wikipedia on lactose intolerance

scientific references

1. Beja-Pereira, A., Luikart, G., England, P.R., Bradley, D.G., Jann, O.C., Bertorelle, G., Chamberlain, A.T., Nunes, T.P., Metodiev, S., Ferrand, N., & Erhardt, G. (2003). Gene-culture coevolution betweencattle milk protein genes and human lactase genes. Nature Genetics, 35(4), 311-313.
2. Bersaglieri, T., Sabeti, P.C., Patterson, N., Vanderploeg, T., Schaffner, S.F., Drake, J.A., Rhodes, M., Reich, D.E., & Hirschhorn, J.N. (2004). Genetic signatures of strong recent positive selection at the lactase gene. American Journal of Human Genetics, 74(6), 1111-1120.
3. Enattah, N.S., Sahi, T., Savilahti, E., Terwilliger, J.D., Peltonen, L. & Jarvela, I. (2002). Identification of a variant associated with adult-type hypolactasia. Nature Genetics, 30(2), 233-237.
4. Jobling, M.A., Hurles, M.E. and Tyler-Smith, C. (2004) Human Evolutionary Genetics: origins, peoples and disease. London/New York.
5. Lomer, M.C.E., Parkes, G.C., & Sanderson, J.D. (2008). Review article: lactose intolerance in clinical practice – myths and realities. Alimentary Pharmacology & Therapeutics, 27 (2), 93-103.

This content was last reviewed on February 08, 2010.

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