<P> Lactase persistence is the continued activity of the lactase enzyme in adulthood . Since lactase's only function is the digestion of lactose in milk, in most mammal species, the activity of the enzyme is dramatically reduced after weaning . In some human populations, though, lactase persistence has recently evolved as an adaptation to the consumption of nonhuman milk and dairy products beyond infancy . The majority of people around the world remain lactase nonpersistent, and consequently are affected by varying degrees of lactose intolerance as adults . However, not all genetically lactase nonpersistent individuals are noticeably lactose intolerant, and not all lactose intolerant individuals have the lactase nonpersistence genotype . </P> <P> The distribution of the lactase persistence phenotype, or the ability to digest lactose into adulthood, is not homogeneous in the world . Lactase persistence - frequencies are highly variable . In Europe, the distribution of the lactase persistence phenotype is clinal, with frequencies ranging from 15--54% in the south - east to 89--96% in the north - west . For example, only 17% of Greeks and 14% of Sardinians are predicted to possess this phenotype, while around 80% of Finns and Hungarians and 100% of Irish people are predicted to be lactase persistent . </P>

When do most mammals lose their ability to produce lactase