Why Does Your Stomach Growl?
― Your Empty Gut Does a "Big Clean" About Every 100 Minutes
Have you ever been embarrassed when your stomach let out a loud growl during a quiet meeting or class? That sound is not your stomach crying "I'm hungry." It is the sound of an empty gut carrying out a strong cleaning movement at regular intervals. And this cleaning repeats many times while you are asleep, too.
It is 11:30 a.m., just before lunch. About four hours have passed since breakfast, and you are reading papers in a quiet room.
Suddenly a long "grrrr… gurgle" rises from deep in your belly. You press a hand to your stomach, wondering whether anyone heard.
Ten minutes later, though, the noise has died away as if it never happened. After lunch, it hardly sounds all afternoon. Why are there times when it growls and times when it doesn't?
There are just two reasons
Once the food is gone, strong waves of contraction sweep from the stomach through the small intestine about every hour and a half to two hours. It is a kind of big clean, pushing leftover debris onward.
With no food inside, the strong contractions push air and stomach fluid through narrow gaps. Nothing is there to soak up the sound, so the gurgling rings out loudly.
So the sound is less a "hunger signal" and more a signal that "the gut has entered cleaning time." Let's look at each part in turn.
The gut has a "fed mode" and a "fasting mode"
After a meal, the gut keeps moving gently. These movements mix food with digestive juices and pass it along bit by bit. This "fed mode" is said to last several hours, depending on how much you ate.
Once the food is gone, the gut switches to "fasting mode." From here, its movements are surprisingly regular and repeat three phases in order. First comes a quiet period with almost no movement. Next comes a period of occasional, irregular contractions. Finally, for the last 5 to 10 minutes, strong contractions travel like a wave from the stomach to the small intestine.
This last strong wave is the "big clean." It is thought to sweep undigested leftovers, shed cells and even bacteria that threaten to multiply too much onward toward the large intestine. One cycle lasts about 100 minutes, and it repeats as long as you stay hungry. In Figure 1 below, move the slider to change the time since your meal.
Because it is empty, the sound rings out
So why do the cleaning waves make noise? Besides food, your stomach and intestines hold small amounts of swallowed air and digestive juices. When a strong contraction squeezes them, air and liquid mix and rush through narrow gaps. It is close to the slurp you hear when you suck the last of a drink from the bottom of a cup through a straw.
The gut also moves after a meal, but then it is packed with food. Food acts like a cushion that soaks up sound, and it does not narrow the air's path. A fasting gut, on the other hand, is almost empty, so the sounds of air and liquid ring out as they are. The abdominal wall is thin, so the sound carries outside.
Incidentally, belly noises are not only made when you are hungry. Small sounds are made after meals, too. But the sounds loud enough to hear from outside are most likely to come during the fasting-mode cleaning.
Shortly before a cleaning wave, a hormone called motilin is released from the entrance of the small intestine. A Belgian research group reported that in people, hunger also grows stronger in step with the stomach's cleaning waves. It may be no coincidence that you tend to feel hungry just when your stomach growls.
Cleaning waves stop once food enters the stomach, and the gut switches to fed mode. Eating a little something before an important occasion is said to make growling less likely. Still, the growling itself is a sign that your gut is working well.
Summary
When you are hungry, the gut produces strong waves of contraction about every 100 minutes, pushing whatever remains onward. Because the gut is empty at that time, the sound of air and liquid being squeezed rings out loudly. That is what a stomach "growl" really is. Eating stops it, and it repeats regularly for as long as you stay hungry.
A growling stomach is not a cry of hunger.
It is the sound of a clean-up that your gut started right on schedule.
For why the stomach does not dissolve itself in its own strong acid, see "The stomach makes strong acid, so why doesn't it dissolve itself?" For how the body keeps burning energy even when you are not thinking about anything, see "Does thinking hard really make you hungry?"
- Choose a day when you will go a long time without eating, such as a weekend with a late breakfast. Note the time you last ate.
- When your stomach growls loudly, note the time. If it keeps growling for a few minutes, count it as one.
- Work out the gaps between the times it growled. If the gaps are around an hour and a half to two hours, that is your cleaning cycle.
If you poke a small hole in the bottom of a paper cup and hold it against a family member's belly with your ear to it, you can hear small sounds that are not audible from outside (ask their permission first). The cycle varies from person to person, and it will not be exactly the same every time. If you feel unwell, or have a medical condition that limits your eating, do not force yourself to stay hungry.
For those who want more ― terms, formulas and links to textbooksEach section is labelled with its level, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school biology
- High school+Advanced high-school material, or textbook sidebar content
- UniversityUniversity specialist courses (physiology, gastroenterology) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught even at university as settled fact
Middle schoolTerms: this phenomenon has a name
- Borborygmus: the medical name for a rumbling stomach, or for the sound itself.
- Peristalsis: the movement in which the muscles of the digestive tract contract in sequence and push the contents onward. Middle school covers it as part of how digestion works.
- Fasting-state propagating contraction: the strong wave of contraction that travels from the stomach to the small intestine during fasting. It corresponds to the "cleaning wave" in this article.
Middle schoolHigh schoolChecking with a formula: how many cleanings happen in one night?
After dinner, your gut also enters fasting mode while you sleep, and the cleaning repeats. The number of times a repeating event occurs can be estimated by dividing the time by the cycle length. The cycle is said to vary from about 90 to 120 minutes depending on the person and the day, so we use 100 minutes as a representative value here.
| In symbols | N ＝ t ÷ T (total time of strong movement ＝ N × τ) |
| In words | Number of cleanings = fasting time ÷ length of one cycle. Total cleaning time = number of cleanings × length of one cleaning |
| Where it comes from | It is simply the definition of the "period" of an event that repeats at fixed intervals. If one cycle has a constant length, dividing the time by the period gives the number of cycles |
| N | Number of cleaning waves (unit: times) |
| t | Time spent in fasting mode (unit: minutes) |
| T | Length of one fasting-mode cycle = period (unit: minutes) |
| τ | Duration of one cleaning wave (unit: minutes) |
| Time in fasting mode (8 hours of night) | 480 minutes |
| Length of one cycle (representative value) | said to be about 100 minutes |
| Length of one cleaning wave | said to be 5 to 10 minutes (here, 10 minutes) |
| Cleanings in one night | 480 ÷ 100 ＝ 4.8 |
| Share of one cycle spent moving strongly | 10 ÷ 100 ＝ 0.1 |
| Total strong-movement time in one night (minutes) | 4.8 × 10 ＝ 48 |
By this calculation, about four to five cleanings happen while you sleep. The strong movement takes up about a tenth of each cycle, adding up to nearly 50 minutes over a night. Your stomach growls in the daytime when you are hungry and happen to hit that tenth of the time. Put the other way, even when you are hungry, the gut is unlikely to growl for nine-tenths of the time.
High schoolHigh school+Who moves the gut?
High schoolThe wall of the gut is made of smooth muscle, which you cannot move at will. Its movement is regulated by the autonomic nervous system (sympathetic and parasympathetic) and by hormones released from the digestive tract. When food arrives, the stomach releases gastrin and other hormones, which drive the digestive movements after a meal.
High school+The stomach muscle contains cells similar to the heart's pacemaker, which are said to set an electrical rhythm of about 3 beats per minute. This rhythm sets the upper limit on how many times the stomach can contract per minute. During a cleaning wave, the stomach contracts strongly in step with nearly every beat of this rhythm, so its movement is at its maximum.
UniversityThe migrating motor complex and motilin
In physiology, this movement repeated during fasting is called the migrating motor complex (abbreviated MMC). The quiet period is called phase I, the irregular contraction period phase II, and the period when strong contractions travel along the gut phase III. Motilin, secreted from the duodenum, is involved in starting phase III, and ghrelin from the stomach is also said to be involved in both fasting-state movement and appetite. The stomach's electrical rhythm is said to be generated by the interstitial cells of Cajal. This movement first became known in 1969, when it was reported as a periodic electrical activity in a dog's small intestine.
📖 Derivations of the formula and further reading: Motilin (Wikipedia) ／ Ghrelin (Wikipedia)
ResearchWhat is still not clear
- How far does the cleaning's real role go? The idea that it prevents bacteria from overgrowing in the small intestine is a strong one. But how much effect it has in people is still being studied.
- The link with hunger. There are reports that phase III in the stomach coincides with hunger. But it is not clear whether the movement produces the hunger, or whether the same signal triggers both.
- The relationship with how often we eat. Frequent snacking reduces the time spent cleaning. How that affects the body has not yet been settled.
In other words, the content of this article is also "an explanation of what is known so far." In particular, the cycle and loudness of the sound in people vary greatly from one person to another, and the 100 minutes used here is only a guide value.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, Year 2: "Digestion and absorption" | Peristalsis, flow from stomach to small intestine |
| High school | Biology: "Autonomic nerves and hormones" | Smooth muscle, autonomic nerves, digestive hormones |
| High school+ | Biology (advanced) | The stomach's pacemaker and its rhythm of about 3 beats per minute |
| University | Physiology, gastroenterology | Migrating motor complex, motilin, ghrelin |
| Research | Research on gut motility | Role of the cleaning, link with hunger |
| ― | Links to daily life | Eating a little before an important occasion, understanding hunger noises |
- Szurszewski JH. A migrating electric complex of the canine small intestine. American Journal of Physiology, 1969.
- Deloose E, Janssen P, Depoortere I, Tack J. The migrating motor complex: control mechanisms and its role in health and disease. Nature Reviews Gastroenterology & Hepatology, 2012.
- Tack J, et al. Motilin-induced gastric contractions signal hunger in man. Gut, 2016.
- Wikipedia "Motilin" (モチリン, Japanese)
- Hongo Toshinori and Hiroshige Tsutomu (supervising eds.), Standard Physiology (標準生理学), Igaku-Shoin (医学書院), chapter on gut motility.
*This article is a general-audience science explainer. The numbers given are guides and rough estimates meant to help you understand the mechanism. If stomach noises come with severe pain, diarrhoea, vomiting, weight loss or similar symptoms, do not rely on your own judgement; consult a medical professional.