An incidence survey of milk intolerance and lactase deficiency was conducted in 40 healthy nonpatient volunteers. There were 20 Negroes and 20 whites. Nineteen of the 20 Negro subjects and two of the 20 white subjects gave a history of milk intolerance. The majority had noted the onset during or after adolescence. Lactose intolerance occurred in 20 of the 21 milk-intolerant subjects; 18 of the 20 were Negro. There was also a prominent racial difference in the incidence of low levels of intestinal lactase activity. Deficient levels were observed in 70% of the 20 Negroes in contrast to only one of 20 whites. These data strongly favor a genetic etiology for the isolated lactase deficiencies seen so commonly in adults.
Bayless TM, Rosensweig NS. A Racial Difference in Incidence of Lactase Deficiency: A Survey of Milk Intolerance and Lactase Deficiency in Healthy Adult Males. JAMA. 1966;197(12):968–972. doi:10.1001/jama.1966.03110120074017
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