The calcium absorption hypothesis was recently subjected to critical review by

The calcium absorption hypothesis was recently

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The calcium absorption hypothesis was recently subjected to critical review by Simoons (2001), who concluded that there is scant osteoarchaeological, historical, or biomedi- cal evidence to support it. Further, while this may help ex- plain the very high frequencies of lactase persistence found among northern Europeans, it has little relevance for un- derstanding lactase persistence in populations at lower lat- itudes. There, other nutritional advantages to adult milk consumption, such as hydration or the use of lactose as a carbohydrate, a nutrient rare in the diets of exclusive pas- toralists, may have accrued to those able to digest fresh milk Another hypothesis that has received relatively little attention is that since the genetic markers associated with lactase persistence are known in geographically diverse ar- eas, lactase persistence could have spread prior to the rise of dairying cultures. B. Anderson and C. Vullo (1994) pro- posed that lactase impersistence was selected for as a de- fense against falciparum malaria, the most deadly form of the parasitic disease. Milk is a very rich source of the B- vitamin riboflavin, and malarial parasites require riboflavin to multiply in red blood cells. In the context of riboflavin deficiency at a level tolerated by a human host, malarial reproduction is significantly inhibited (Dutta et al. 1985). Thus, early reduction in lactase production would lead to earlier termination of breastfeeding; this in turn would gen erate riboflavin deficiency (from decreased milk intake) suf- ficient to reduce malarial infection without being overly deleterious to the child. Because populations vary in the age at which lactase activity may decline, later age of onset of lactase impersistence would have been tolerated in popula- tions not exposed to deadly forms of malaria (e.g., northern Europeans, who have both high rates of lactase persistence and later ages of onset of impersistence among those with that genotype). However, a study in Sardinia showed that three villages with varying exposure to malaria did have dif- ferent frequencies of other known adaptations to malaria (e.g., G-6PD deficiency and f-thalessemia) but did not vary in their rates of lactase persistence (Meloni et al. 1998). This hypothesis also fails to explain why some populations would remain lactase persistent throughout life, given that this is the derived (i.e., evolutionarily recent) condition for mammals. While selection probably played a key role in the spread of lactase persistence among some dairying populations, other evolutionary forces have also contributed to global diversity in adult lactase activity. Genetic drift may have been important in reducing diversity in genes associated with lactase activity in non-African populations, given that there is greater variation within contemporary African pop- ulations (Hollox et al. 2000). Clearly, gene flow has also This content downloaded from 184.98.45.72 on Mon, 22 Oct 2018 21:21:17 UTC All use subject to
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Wiley * Biological Diversity and Milk Consumption in the United States 509
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