Over 70 percent of the world’s population is unable to produce lactase[i] which is an enzyme that breaks down lactose[ii] for digestion. Lactase is produced in the small intestines where it simplifies lactose allowing it to be absorbed into the bloodstream. With that said, the deficiency of lactase and the malabsorption of lactose may lead to a phenomena is known as Lactose intolerance.
Lactose intolerance is common among Asian and African populations where the frequency of milk consumption is little to none. Geneticists determined this to be the result of an early encounter with Sleeping Sickness [iii] which exterminated most of the cattle (and other livestock) in parts of Africa and Asia.
People of Northern European descent, for the most part, maintain their ability to produce lactase and are able to consume milk throughout their life. While people of Asian and African descent tend to lose their lactase-producing ability during maturation. Most African-Americans are descendants of slaves that were taken from West and Central Africa in areas that lie within the Tsetse belt[iv]. Therefore, approximately 75% of African-Americans are lactase deficient. It is important to note that areas outside of the Tsetse belt were able to herd cattle (and other livestock); and thus, people within those areas may have the ability to produce lactase due to the availability of milk.
Now, that’s africentriguing!
 Schwarcz, Joe. “Dr.Joe: Milk- To Drink or Not To Drink?” CJAD, last modified September 14, 2015, http://www.cjad.com/dr-joe/2015/9/14/dr-joe-milk–to-drink-or-not-to-drink/print
 “Lactose Intolerance.” The National Institute of Diabetes and Digestive and Kidney Diseases: U.S. Department of Health and Human Services, last modified June 2014, http://www.niddk.nih.gov/health-information/health-topics/digestive-diseases/lactose-intolerance/Pages/facts.aspx#wha
 Schwarcz, Joe. 2015
 Lang, Susan S. “Lactose intolerance seems linked to ancestral struggles with harsh climate and cattle diseases, Cornell study finds.” Cornell Chronicle: Cornell University, last modified June 1, 2005, http://www.news.cornell.edu/stories/2005/06/lactose-intolerance-linked-ancestral-struggles-climate-diseases
[ii] Sugar originating from the glucose and galactose in milk
[iii] Trypanosomiasis, a disease transmitted by the Tsetse Fly
[iv] Area where the Tsetse fly is most prevalent