This is a sugar molecule. It’s found in milk and other dairy products, such as cheese and yogurt. For the human body to digest lactose, the sugar molecule must be broken down into smaller molecules. Lactose is broken down by an enzyme — a molecule that speeds up chemical reactions — called lactase. Nearly everyone is born with the ability to produce this enzyme. It’s what allows mammals to drink milk from their mothers. However, people can stop producing the enzyme as they grow up. About 65 percent of adults can’t digest lactose well — or at all — after childhood. This includes many people of East Asian, West African, Greek and Italian descent. Without lactase, a person who eats dairy products with lactose can experience stomach pain, gas and bloating.
In a sentence
Scientists have found that even people from the Stone Age could have been lactose intolerant.