Lactose intolerance is an inability to digest milk sugar, characterized by abdominal cramps, nausea, and diarrhea after ingesting milk products. It develops in adolescence or adulthood among those whose bodies naturally stop producing the lactose-digesting enzyme lactase after childhood. Although uncommon in the United States, lactose intolerance actually occurs in the majority of people around the world. The ability to continue producing lactase, and thus to digest milk products, is a relatively recent evolutionary development.

After presentation of an illustrative clinical example, the biology of lactase production and lactose intolerance is reviewed. The "culture-historical" hypothesis of Simoons in the 1970's held that persistent lactase production developed in a dairy herding population in Northern Europe during a time of food scarcity, when the ability to digest milk would have provided a survival advantage. Simoons demonstrated that low prevalence of lactose intolerance was associated with pastoral practices among early populations around the world.

Hirschhorn recently provided strong genetic evidence for the culture-historical hypothesis using contemporary techniques of genomics and single nucleotide polymorphisms. His research group found that European derived persistent lactase production is due to a mutation near the lactase gene that occurred only about 10 million years ago and rapidly increased in prevalence.

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