Welcome to BrainStuff, a production of I Heart Radio.

Hey Brain Stuff, Lauren Vogelbaum here.

When people can't digest milk, we call that lactose intolerance. This might seem to signify that it's an unusual

condition that most people are just fine consuming dairy products like milk, cheese, and ice cream. But it turns out that we poor souls

who get gassy, crampy, and otherwise digestively miserable after eating dairy products

are actually in the majority worldwide. It's the people who can handle milk who are the weird ones.

The lactose is the main sugar in all mammal milk, and everyone is born with a gene that codes for producing lactase

which is the enzyme in our bodies that processes lactose so when we're babies we all

have the ability to digest milk the human body produces lactase in the small intestine where it breaks down

lactose into glucose and lactose which our bodies can easily absorb into our

bloodstream but for reasons unknown the gene can easily absorb into our bloodstream.

But for reasons unknown, the gene for lactase tends to shut off at about the time that most

of us would be weaned off of breast milk and onto solid foods.

Most people's bodies don't create any lactase or very little of it by the time they're five years old or so.

In the absence of lactase, the excess sugar sitting around in your small intestines causes your body to try to flush it out by secreting fluids and moving things along faster than normal.

By the time the lactose reaches your colon, the mostly helpful

microorganisms there eat it, and poop things like carbon dioxide, hydrogen, and

methane. In combination, this can lead to those unpleasant symptoms of diarrhea, gas, and cramps.

Some lucky people, however, carry a genetic mutation that allows the lactase gene to keep working.

Sometimes it continues for just a few more years, sometimes for a lifetime.

Up to 90% of Americans have a functional lactase gene,

so in the US it's a little unusual to be lactose intolerant.

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