According to a recent Journal of ADA I have, "In the United States, the prevalence of lactose maldigestion has been estimated to be about 25% of the population. The condition occurs in 15% (6% to 19%) of whites, 53% of Mexican Americans, 62% to 100% of Native Americans, 80% of African Americans, and 90% of Asian Americans."
Inman-Felton, Amy. "Overview of Lactose Maldigestion (Lactase Nonpersistence). Journal of The American Dietetic Association . Vol. 99 (4). April 1999481-86.
Actually, this article quoted a 1998 article of J. ADA. titled " Gastrointestinal Disorders: "Roles of Nutrition and the Dietetics Practitioner".
I'm sure there are more recent studies/articles with similar and/or different averages (IN USA), but this is just one I had on hand in the drawer. It could be easily found if it's a big deal here.
If one is Lactose intolerant, or id one consumes large amounts of milk, then he/she could add some Lactase pills to aid in digestion.
MR. BMJ
ps, there are 3 different types of lactose intolerances which effect people world-wide.