Dear Jules,
Sometimes my stomach growls so loudly that other people can hear it. It’s pretty awkward in meetings.
I asked my friend Ed Johnson why that happens. He teaches classes about the human body in the School of Biological Sciences at Washington State University.
He told me the rumbling noise happens because of what’s going on inside your gut.
“The sounds you hear are the fluid and gas that’s in your digestive tract moving around,” Johnson said.
The digestive system cycles through a lot of fluid—about 9 liters every single day. That’s the same as four-and-a-half 2-liter soda bottles.
Some of that liquid comes from the water you drink. Some is gastric juice made by specialized cells in your stomach’s lining.
There’s a lot of gas in there, too. It’s made by friendly bacteria that live in your gut. They help you break down food and make vitamins. As they do that work, they release gas—up to 1.5 liters each day.
This is bacteria called Bifidobacterium longum. It lives in the digestive tract and helps digest sugars from plants and human milk. It’s the most abundant bacteria in the guts of babies. Li Zhao et al. CC BY 4.0
All that fluid and gas doesn’t just sit there. There’s a thick layer of smooth muscle in your digestive tract that moves them along and stirs them around.
“The muscle squeezes down on the contents to push it along,” Johnson said. “It’s an action like you’re milking a cow, squeezing and pushing the food through. And it also makes a mixing motion to move digested nutrients around so they can reach the lining and be absorbed.”
Your digestive tract moves so much that you have a whole section of your nervous system dedicated to controlling that movement. Johnson told me there are about half a million nerve cells doing that job—about the same number you have in your spinal cord.
As the fluid and gas move around, they make a grumbling noise. But why does it happen when you’re peckish?
When your stomach is empty, your body releases the hormone ghrelin. It’s a chemical messenger. It signals your brain to give you the hungry feeling. It also tells your digestive tract to get ready for food.
Your digestive system responds by cranking up the squeezing, pushing, mixing motions—because it expects food to show up soon.
When all that liquidy-, gas-y movement happens to an empty stomach, the rumbling sounds are louder. There’s nothing in there to muffle the noise. So, you—and maybe the people around you—can hear a big, resounding growl from your guts.
Scientists call that sound “borborygmus.” The term comes from an ancient Greek word made to mimic the sound.
I guess people have been wondering about that rumbling sound for a long time. They had no idea it involves a whole ecosystem inside your tummy—and a complex messaging network that links your brain to your gut.
It’s honestly a lot to digest.
Sincerely,
Dr. Universe