Epidemiology

Lactose malabsorption is present in about 70% of the world's population, although not all demonstrate the symptoms of lactose intolerance.[4]​ The prevalence of lactose intolerance is unknown as it is more difficult to measure.[5][6][7]​​ The primary form of lactase deficiency is the most common, while congenital lactase deficiency is extremely rare (<50 cases reported worldwide).[1][3][4]​​​​​​​​

In the US, primary lactase deficiency is far more common in African-American, American Indian, Hispanic, Latino, and Asian-American people than in white Americans.[2][8][9][10]​ North Americans, white Northern Europeans, and Australians have the lowest rates, ranging from 2% to 15%.[2][8]​ By contrast, the prevalence of lactase deficiency is 50% to 80% in South American people; around 60% to 80% in African and Jewish people; and almost 100% in American Indian and some East Asian populations.[2][9][10][11]

The age-related decline in lactase expression is usually complete during childhood, but the decline has also been reported to occur later, in adolescence, especially in white people.[10][12] The eventual level and the time course of loss of lactase activity vary considerably according to ethnicity.[1][10] Chinese and Japanese people lose 80% to 90% of activity within 3-4 years after weaning; Jewish people and Asians lose 60% to 70% over several years after weaning; and in white Northern Europeans and North Americans it may take up to 18-20 years for lactase activity to reach its lowest levels.[1][3] A lower prevalence of lactase non-persistence is observed in patients from mixed ethnicity, where a high prevalence is detected in the native ethnic group.[13]

Onset is typically subtle and progressive in primary lactase deficiency, and most people first experience intolerance symptoms in late adolescence and adulthood.[2][3]​ Compared with white Northern Europeans, North Americans, and Australians, presentation is earlier in Native Americans, African-Americans, Asian-Americans, and Hispanics/Latinos.[2][3][9][10]​ Secondary disease is more common in children, especially in developing countries where infections are the common cause.[14]

The sexes are affected equally.

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