Lactose

Lactose is the carbohydrate in milk. To digest it, the small intestine produces lactase, an enzyme that splits lactose into glucose and galactose for absorption. In most mammals — and most humans globally — lactase production declines sharply after weaning. The genetic mutation that keeps lactase active into adulthood ("lactase persistence") arose independently in several populations and spread alongside dairy farming, which is why tolerance varies so much by ancestry.

When lactose isn't fully digested, it reaches the colon, where bacteria ferment it — producing gas, drawing water into the bowel, and causing bloating, cramps, and diarrhea hours after dairy. This is lactose intolerance. It's distinct from milk protein allergy: same food, different mechanism.

Why some adults still digest lactose

Roughly 35% of adults globally retain enough lactase to digest large amounts of lactose; ~65% have some degree of intolerance. Rates are above 90% in Northern Europeans, around 50% in Middle Eastern and South Asian populations, and below 10% in many East Asian and West African populations. Many people who consider themselves "intolerant" can still handle small amounts.

What still has lactose

Milk and cream have the most. Yogurt has some, but live cultures help digest it. Hard aged cheeses (cheddar, parmesan) have very little — most lactose-intolerant people tolerate them. Lactose-free milk is regular milk with lactase added; the protein is unchanged. Butter is mostly fat with only trace lactose. Hidden lactose appears in many baked goods, sauces, processed meats, and some medications — read labels if you're strictly avoiding.