What causes lactose intolerance?

Declining lactase enzyme production after childhood — the default state for most of the world's adults, especially in Asia and Africa. It causes bloating, gas, and diarrhoea after dairy. It's a digestive issue, not an allergy, and small amounts or fermented dairy are often tolerated.

Fermentation in curd converts much of the lactose to lactic acid, and the bacteria supply their own lactase, which is why curd and buttermilk are usually tolerated.

Most people with lactose intolerance tolerate about 12 g of lactose (roughly one cup of milk) if taken with food and spread through the day.