Did Persistence of Intestinal Lactase Into Adult Life Originate on the Arabian Peninsula?

Abstract

Persistence of the intestinal enzyme lactase at high concentration into adult life (PL) occurs in some sections of the world's human population. For the mammalian kingdom that is an abnormal situation; although high at birth, lactase is usually present at a very low concentration in adult life. The three major groups affected are (i) inhabitants of the Arabian peninsula, (ii) African Hamites, and (iii) most northern Europeans and their descendants in northern America and Australasia. There are also limited data showing that some north-west Asian populations have a high incidence of PL. It is suggested that the initial population to be so affected lived on the Arabian peninsula, and that the gene dispersed from that origin; there might also have been additional foci. The reason for the selective advantage of PL is not yet clear. As affected adults would have tolerated camels' milk better than those with a low lactase concentration (adult hypolactasia), it seems possible that this was the basis for an advantage in Arabia. Because milk causes intestinal symptoms (including severe diarrhoea) in subjects with hypolactasia, it is possible that among desert nomads the selective advantage of PL resulted from repletion of body-water and electrolytes after drinking camels' milk; and that might have been coupled with a nutritional advantage. Incidence of PL in population surveys constitutes a valuable method for mapping gene dispersal, and consequently population migrations, in Africa and Asia; more studies are required in north-west Asia and countries surrounding the Arabian peninsula. I Intestinal lactase is an enzyme which is necessary for digestion of milk sugar-lactose--prior to absorption. Persistence of its activity into adult life (PL) is rarely found in the animal kingdom (Cook I969; 1973; Morris et al. 1977), and is confined to a small proportion of homo sapiens. That conclusion is contrary to opinion held two decades ago, when low lactase activity in man was thought to be very unusual. Working in Uganda, Cook & Kajubi (I966) demonstrated that whereas lactase was absent in most African adults (adult hypolactasia), those with an Hamitic ancestry had PL. Furthermore they produced evidence that PL was not substrate related (i.e. dependent upon milk-drinking); neither was it related to any other environmental factor examined. It seemed therefore to have a genetic basis, and they suggested that the condition might be a valuable ethnological marker (Cook I969). The report from Uganda was closely followed by one from Bayless & Rosensweig (I966) working in the U.S.A., who confirmed an earlier observation of Cuatrecasas et al. (1965) which showed a significantly higher incidence of PL in white Americans than in American negroes. They also considered that