When your small intestine doesn’t produce enough of an enzyme called lactase, it can’t digest lactose, a sugar found in dairy products, says Ryan Ungaro, M.D., assistant professor of gastroenterology at Mount Sinai Hospital in New York.
So your small intestine directs lactose directly to your colon—instead of your bloodstream—where your gut bacteria ferment it. This can cause loose, foul-smelling stool, bloating, and smelly gas, Dr. Ungaro says.
Lactose intolerance is fairly common: In fact, an estimated 65 percent of people have trouble digesting dairy, according to the National Institutes of Health. But the reaction—including painful stomach cramping, nausea, diarrhea, and vomiting, as well as smelly farts and poop— can vary from person to person, says Men’s Health nutrition advisor Alan Aragon, M.S.
For most people, though, it only takes 12 grams of lactose—about 8 ounces of milk or a cup of ice cream—to set off unsettling symptoms, he says.
So if your gas or poop smells particularly pungent after you down some milk, you might want to check in with your doctor, says Dr. Ungaro. (If you’re passing wind more than usual, rule out these five other reasons you could be gassy first.)
He or she can diagnose the problem, or even rule out more concerning intestinal issues, like Crohn’s disease, as a culprit behind the stench. Then, your doctor can help you determine how much lactose you can take in per day without causing a problem. (You can also pop a Lactaid pill, which contains the lactase enzyme and allows you to digest dairy within 45 minutes, if you just must have a sundae, Aragon says.)