Sixty percent of adults can't digest milk

SAN FRANCISCO -- Got milk? If you do, take a moment to ponder the true oddness of being able to drink milk after you're a baby.

No other species but humans can. And most humans can't either.

The long lists of food allergies some people claim to have can make it seem as if they're just finicky eaters trying to rationalize likes and dislikes. Not so. Eggs, peanuts, tree nuts, fish, shellfish soy and gluten all can wreak havoc on the immune system of allergic individuals, even causing a deadly reaction called anaphylaxis.

But those allergic reactions are relatively rare, affecting an estimated 4% of adults.

Milk's different.

There are people who have true milk allergies that can cause deadly reactions. But most people who have bad reactions to milk aren't actually allergic to it, in that it's not their immune system that's responding to the milk

Instead, people who are lactose intolerant can't digest the main sugar —lactose— found in milk. In normal humans, the enzyme that does so —lactase— stops being produced when the person is between two and five years old. The undigested sugars end up in the colon, where they begin to ferment, producing gas that can cause cramping, bloating, nausea, flatulence and diarrhea.

If you're American or European it's hard to realize this, but being able to digest milk as an adult is one weird genetic adaptation.

It's not normal. Somewhat less than 40% of people in the world retain the ability to digest lactose after childhood. The numbers are often given as close to 0% of Native Americans, 5% of Asians, 25% of African and Caribbean peoples, 50% of Mediterranean peoples and 90% of northern Europeans. Sweden has one of the world's highest percentages of lactase tolerant people.