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Lactose intolerance is a common GI complaint in adults. Although initially considered a “disorder”, most now consider it normal. That is, normal adults loose the ability to digest lactose. It is actually a genetic variant that allows adults to continue to produce lactase, the enzyme which breaks down lactose. If the lactose is not broken down, the typical symptoms of bloating, cramps, diarrhea, and gas occur.
So, is there a reason that a genetic variant allowing for lactase persistence would be useful? In early humans, there was no use of the enzyme beyond infancy. Early hunter gatherers had no milk in their diet. With the advent of agriculture, and domestication of animals, milk and milk products were introduced into the diet. Obviously, this would benefit only those able to break down the lactose in the milk. Research into the persistence of lactase has produced several truly amazing findings. First, there are several mutations that account for this ability that have evolved in different populations around the world. There are at least 4 gene mutations that have been found in geographically disparate populations that allow for lactase persistence. Second, this is a very recent adaption. Testing of DNA from Neolithic individuals who lived roughly 4000 to 5000 years ago has shown a very low prevalence of the mutation. This strongly suggests that there has been strong evolutionary pressure with those with the gene surviving to reproduce at much higher rates. For example, the Neolithic DNA from Sweden showed 95% of those studied were lactose intolerant, whereas the modern Swedish population has only 25% of individuals with this. Areas with a warmer climate such as around the Mediterranean and Africa have much higher rates of lactose intolerance, presumably because there was greater availability of different food sources, hence, less evolutionary pressure for the gene to spread. Isolated populations such as the Australian Aborigines, the Japanese, and Native Americans also have extremely high rates of lactose intolerance.
This is just one of many fascinating examples of how our bodies do interact with food and how rapid evolution can occur.
- GB Vogelsang, M.D.