Lactase activity begins to decrease in a genetically programmed fashion so that by adulthood it is low in many ethnic groups (9), highest prevalence being seen in Far East. Approximately 70% of world's population has primary lactase deficiency. (10,11) The percentage varies according to ethnicity and is related to use of dairy products in diet resulting in genetic selection of individuals with the ability to digest lactose (Table1). In populations with predominance of dairy foods in diet as few as 2% of population has primary lactase deficiency. In contrast prevalence of primary lactase deficiency is 50-80% in Hispanic people, 60 to 80% in black and Ashkenazi Jewish people and almost 100% in Asian and American Indian people. (12-14) Age of onset and prevalence differs among various population. Ethnic group with higher prevalence have an earlier decrease of lactase activity. Approximately 20% of Hispanic, Asian and black children younger than 5 yrs of age have evidence of lactase deficiency and lactose malabsorption (15) whereas white children typically do not develop symptoms of lactose intolerance until after 4 or 5 yrs of age. Molecular studies of lactase have correlated genetic polymorphism of messenger RNA expression with persistence of lactase activity, demonstrating early loss (at 1-2 yrs of age) in Thai children and late (10-20 yrs of age) loss of activity in Finnish children. (14,16) Clinical signs of lactose intolerance at an earlier age than is typical for a specific ethnic group may warrant an evaluation for an underlying cause, because primary lactase deficiency would otherwise be unusual at such a young age. Typically onset of primary lactase deficiency is subtle and progressive over many years although it may be acute. Most lactase deficient individuals experience onset of symptoms in late adolescence and adulthood.

Dietary history is an unreliable means to confirm or exclude the diagnosis of lactose intolerance because clinical symptoms are prone to subjectivity. (17) One glass of milk or a scoop of ice cream may be tolerated even in lactose intolerant adult whereas an additional glass of milk or other milk products may produce symptoms. Because of variation of dairy intake in each individual's diet and in amount of lactose contained in different products symptoms may vary and be modified by diet and milk containing foods.