Why Does Your Stomach Rumble?

It always chooses its moment. The exam hall goes silent, the meeting pauses for a thoughtful moment, the cinema reaches its quietest scene, and from somewhere around your middle comes a long, gurgling growl, like a small whale clearing its throat. Everyone pretends not to hear it. You pretend it was somebody else. And you probably assume it is your stomach announcing, loudly, that it would like some food now, please.

The noise has a splendid name. Doctors call it borborygmus, and more than one rumble are borborygmi. The word comes through French and Latin from Ancient Greek, and it was probably meant to sound like the noise itself, which, if you say it slowly, it rather does. Doctors listen for these sounds on purpose with a stethoscope, because a working gut is supposed to be a little noisy.

The first surprise is that much of the rumbling does not come from the stomach at all. It is mostly made in the small intestine, close by, as muscle contractions called peristalsis squeeze liquid and gas along the tube. Gas can come from swallowing air while you talk, eat and drink, and from fizzy drinks. When fluid and bubbles are pushed through a squeezing tube, they gurgle, in much the same way that water does in old plumbing.

So why is it so much louder when you are hungry? Here is the clever part. When the stomach and small intestine have been empty for a while, the gut begins a routine called the migrating motor complex. Every so often, a strong wave of contractions sweeps from the stomach down through the small intestine, carrying away leftover scraps of food and other material, and helping move bacteria along to the large intestine where they belong. It has been nicknamed the gut's housekeeper.

That sweeping wave pushes through a tube that is now mostly empty, full of air and a little fluid, and the empty space acts like a sounding box. A full gut muffles the noise. An empty one amplifies it. Meanwhile an empty stomach also releases a hormone called ghrelin, often called the hunger hormone, which helps make you feel peckish. The growl and the hunger arrive together, so it is easy to believe one is causing the other.

How do we know all this? The migrating motor complex was first discovered and described in 1969, in studies of fasting dogs at the Mayo Clinic in the United States, which also showed that the pattern stops when a meal is eaten. Later measurements in people found that between meals the cycle comes round roughly every one and a half to four hours, with a peak of intense activity lasting only a few minutes. It is thought to be set off by a gut hormone called motilin.

Eating switches the housekeeper off. One study found that a continental breakfast of about 450 calories made the cycle disappear for around three and a half hours, and fatty food keeps it away longer than carbohydrates or protein do. So the next time your middle speaks up in a quiet room, you can explain, with great dignity, that it is not rumbling at all. It is tidying.