Lactose intolerance, the inability to break down lactose — the natural sugars found in milk and milk products — is extremely common. Approximately 30 million American adults have some degree of lactose intolerance by age 20, according to the U.S. National Library of Medicine.
Humans cannot digest lactose without the help of an enzyme called lactase. If your digestive system produces and maintains sufficient levels of lactase, which splits lactose into the much-more-absorbable sugars glucose and galactose, you should be able to tolerate milk products.
But most people across the globe, as well as a large portion of the U.S. population, are deficient in lactase. Genetics play a huge role in determining whose bodies do and do not produce enough of this enzyme. Lactase deficiency is far more prevalent among certain ethnic groups, especially people of Native American, African-American, Asian, Mediterranean and Jewish descent.
Genetics aside, anyone’s body can lose the ability to maintain adequate lactase levels as they age. It’s also common to stop producing the enzyme temporarily following a bout of infectious gastroenteritis. And if you take a lot of NSAID painkillers like ibuprofen or aspirin, or if you injure your small bowel in any way, your lactase levels will likely be lower.