The symptoms of lactose Intolerance (LI) are diarrhea, flatulence, borborygmi (the noisy rumbling of air moving through the intestines), cramps, bloating, and sometimes vomiting. You get them because undigested lactose does two things, both bad: it pulls excess water into your intestines and it gets fermented by the bacteria in your colon, creating huge amounts of gas. The combination of water and gas and stool leads to the symptoms mentioned.
Let’s review what happens when someone with LI drinks a glass of milk as part of a meal.
Food first goes to the stomach, where it is broken down (mostly by stomach acid) into smaller bits that can pass through to the intestines. This takes about an hour on average. A liquid, such as milk, may move through faster, since an individual milk drop is already small, but nothing much happens immediately.
In the second part of the small intestine, a place called the jejunum, the milk washes over the cells that manufacture lactase, the enzyme that digests lactose. Even those people with LI usually have some lactase activity, just not enough to make a difference. Almost all the undigested lactose stays in the intestinal tract, finally reaching the colon after six to ten hours.
There are billions upon trillions of bacteria that live in everyone’s colon, living on the waste food that reaches them. Certain kinds of these bacteria happen to love lactose. They break it down by fermenting it, a process that releases gas into the intestines. It’s this gas that causes the cramps and flatulence associated with LI. And this gas won’t stop until all the undigested lactose is gone, a process that can take up to a couple of days.
This is the standard explanation for LI, and it’s all true. But when I first read it, I had to ask: if it takes ten hours, why do my symptoms start no more than a half-hour after I drink the milk?
It turns out that the standard explanation leaves out a few things. Three major ones, actually.
First, if it takes ten hours for food just to reach the colon and a day or two for all of it to be gone, then your intestines are always working on several meals at any one time. Any symptoms you feel may have nothing to do with what you just ate. (A fact that often makes it extremely difficult to initially guess exactly which foods are bothering you.)
Second, lactose has another role to play that isn’t always mentioned. All foods contain water. Fruits and vegetables are mostly water, and so is anything soft, including most dairy products. In normal digestion this water is absorbed by the intestines, leaving just a little bit left so that your stool doesn’t get hard and dry. Undigested lactose, however, reverses this process so that water is actually drawn into the intestines. This excess water mixes with the undigested food and gas to create the sometimes explosive diarrhea so common with LI.
Third, your intestines work by pushing food along in a process called peristalsis. Food entering the stomach acts as a trigger to start the pushing, all the way through the intestines. The urgent need to go to the bathroom that often comes soon after a meal is from the peristalsis pushing on the gassy, watery stool sitting in your colon, not from any lactose that happened to be in the food you just ate.
In short, you can expect to see any of a variety of symptoms anywhere from a half-hour to several days after eating dairy products, the exact timing depending on how much you eat of which foods, whether your intestines still produce enough lactase to do a little digesting and, perhaps most of all, just what’s going on in your colon.
What can you do to relieve these symptoms? Prevent them by taking lactase pills or avoiding dairy. The other options aren’t as good.