A low rumble like thunder in the distance, a shriek like a race car or just a loud bubbling: our stomach makes a concert of sounds. Particularly striking – and embarrassing – in a quiet classroom or conference room. “My stomach is growling,” we say, or, “I’m rumbling with hunger.” What actually causes the grunting? And is hunger the culprit?
The drink talk on the internet is unanimous: the grunting occurs because the stomach continues to knead after the food has already been removed to the small intestine. What you hear is the compression of air in the empty stomach, sometimes with gastric juice sloshing as well. “Put some food in there quickly, then it won’t sound so hollow in there anymore,” says the unsurpassed children’s book Why your own farts don’t smell so bad sometimesby health scientist Esther Walraven.
But Geert Wanten, gastrointestinal specialist at Radboudumc in Nijmegen, is a bit more nuanced. “Often it’s not so much the stomach that you hear rumbling, but the intestines,” he says. The entire gastrointestinal tract is eight meters long. There is continuous transport of air, juices and thin food residues. “The better plumbing,” says Wanten. High in the system, against the diaphragm, it feels like it’s about the stomach. Only in the large intestine does the bubbling feel gut-like.
Is it cluttering up there especially when we’re hungry? “I think that’s a myth,” says Wanten, “but it’s hardly been researched as far as I know.” Digestion is a continuous process, he emphasizes, that always makes noise, even if you can’t hear it on the outside. Gastrointestinals listen to it with a stethoscope, just like a cardiologist listens to your heart.
“It is of course true that peristalsis, that interplay of muscle movements that propel the fluid, increases somewhat at certain times,” says Wanten. “Around mealtime, or when you’re just thinking about food. Digestive juices are then already produced. And it may be that the bubbling sounds louder when the stomach is empty, because then your belly is a hollow sound vessel. But I can’t say whether that really works.”
Like a drum
Mitten does get people at his office hours who often suffer from a bubbling stomach. “I can reassure most of them,” he says. “The bubbling is the most normal thing in the world. That’s part of it.”
Only in some cases is something really wrong. With a blockage, for example, you sometimes hear a high-pitched whistle when the intestines have to squeeze their contents along something. “Sink noise, we call it,” notes Wanten. And people with lactose intolerance or irritable bowel syndrome, who on average produce more intestinal gases, also report more rumbling in the gut. “Then the intestines work more like a drum.”
But where exactly does that sound arise, physically? Australian doctors and engineers investigated this together. They published “a mathematical model of gut sound generation” in The Journal of the Acoustical Society of America (2018). Because the air is forced through the intestine, the intestinal wall starts to vibrate. And the best part is: it happens at exactly the same frequency as our vocal cords vibrate – between 100 and 1,000 times per second. Not surprising, because it concerns comparable tissue, according to the Australians. That explains why our gut sometimes seems to sing or moan.
And it puts the notion of ventriloquism in a whole new perspective.