Why Does Milk Make Your Stomach Rumble?
― Most Adults Have Less of the Enzyme That Splits Milk Sugar
You drink milk, and soon your belly feels bloated and starts to gurgle. That doesn't mean your gut is weak. The enzyme that broke down milk when you were a baby fades as you grow up. For a mammal, this is actually a perfectly normal change.
You're in a hurry one morning, and you gulp down a glass of cold milk.
About 30 minutes into your train ride, your belly starts to swell and rumble. Yogurt and cheese never bother you, yet milk somehow does.
Even in one family, some people drink the same milk without trouble and others don't. Where does this difference come from?
Two reasons: sugar that wasn't split reaches the large intestine
The sweetness in milk comes from lactose. It is two sugars joined together, and in that form your body can't absorb it. An enzyme in the small intestine called lactase cuts it in two, but in many people this enzyme declines after weaning.
Lactose that isn't cut flows on to the large intestine. There it draws water into the gut, and it also feeds the bacteria living there, which make gas. That water and gas are what cause the bloating and rumbling.
So milk itself isn't the problem. The place where lactose gets broken down has shifted from the small intestine, where it belongs, to the large intestine. Let's look at it step by step.
Why does the enzyme decline in adults?
For a baby, milk is the only food. So the inner wall of the small intestine is lined with plenty of lactase to split lactose.
But in most mammals, once the age of nursing is over, the body makes much less of this enzyme. It's like putting away a tool you no longer use. Humans were originally part of this group too.
The exceptions are people from regions that have drunk cow or goat milk for thousands of years. In places such as northern Europe and East Africa, many people carry a genetic trait that keeps the enzyme going into adulthood. Worldwide, roughly one third of adults are estimated to keep enough of the enzyme.
In East Asia, including Japan, people with less of the enzyme are said to be the majority. So a rumbling belly after milk is not a minority complaint. In Figure 1, the left side shows what happens in a body where the enzyme works, and the right side shows a body with little of it.
What happens in the large intestine? ― Water and gas
Lactose that isn't cut in the small intestine flows straight into the large intestine. Two things happen there.
The first is water. Across the gut wall, water tends to move toward the side where more is dissolved. Leftover lactose makes the contents of the gut more concentrated, so water is drawn in from the body. A glass of milk's worth of lactose keeps about 100 mL of water in the gut (we'll check this in the fold-out section at the end). This is what makes your stomach loose.
The second is gas. Huge numbers of bacteria live in the large intestine. To them, lactose is a feast, and as they break it down they give off gases such as hydrogen and carbon dioxide. When gas stretches the gut from the inside, you feel bloated, and when it moves along with the water, it rumbles.
Hospital tests use this mechanism in reverse. After you drink lactose, if the hydrogen in your breath rises, that is proof that bacteria in the large intestine have broken the lactose down.
The lactic acid bacteria in yogurt have already eaten some of the lactose and reduced it. The enzymes of the live bacteria are also thought to help out inside your gut. In long-aged hard cheese, much of the lactose drains away with the liquid (whey) during production, and the rest gets broken down, so it is said to contain almost none.
Cats also lose the enzyme after weaning. Many adult cats have trouble breaking down lactose, and milk can upset their stomachs. Milk made for pets has had its lactose reduced beforehand.
So do I have to give up milk?
Not at all. According to a report that pooled the research, many people with reduced enzyme can take up to about 12 g of lactose at a time with no symptoms or only mild ones. In milk, that is a little over one glass.
There are three tips. Drink less at a time, and split it into several servings. Drink it with a meal, so it moves on to the gut more slowly. And choose lactose-free milk, yogurt or cheese.
Some reports say that if you keep drinking a little every day, the mix of bacteria in your large intestine changes and symptoms ease for some people. But the enzyme in the small intestine itself is thought not to increase.
One more point: if you get hives or trouble breathing, it may be an allergy to milk protein rather than to lactose. The mechanism is completely different. In that case, don't push yourself, and talk to a doctor.
Summary
Milk makes your stomach rumble because lactase, which cuts lactose, declines in adulthood, and the leftover lactose draws water into the large intestine while bacteria make gas. This is a perfectly ordinary change seen in most adults around the world.
Milk isn't the culprit.
The lactose is simply passing the place where it gets broken down.
How bacteria break down food is covered in What is the difference between fermentation and rot?, and animals that leave digestion to microbes are covered in Cows eat only grass, so how do they grow so big?
- On a morning off, drink just 100 mL of milk (half a glass) with a meal.
- For the next few hours, note any bloating or sounds. If you feel fine, try a slightly larger amount on another day.
- Compare with a day when you drink the same amount on an empty stomach in one go, or a day with lactose-free milk.
Symptoms change with the amount, how you drink it and how you feel that day. Don't judge from a single try. If it's uncomfortable, don't keep forcing it.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school biology and chemistry
- High school+Advanced high school material, or textbook sidebar content
- UniversityUniversity-level topics not taught in high school (biochemistry, human genetics)
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Middle schoolTerms: this phenomenon has a name
- Lactose (milk sugar): a sugar made of one glucose and one galactose joined together. It gives milk its sweetness.
- Lactase: an enzyme on the inner surface of the small intestine that cuts lactose into two sugars.
- Lactose intolerance: a condition in which, with too little lactase, taking in lactose causes bloating, rumbling, loose stools and the like.
- Osmotic pressure: the force that makes water move toward the more concentrated side when two solutions are separated by a membrane.
Middle schoolHigh schoolChecking with a formula: how much water does one glass of milk pull into the gut?
Lactose that reaches the large intestine makes the fluid in the gut more concentrated than body fluid. Water moves into the gut until the concentrations match. Let's estimate how much water that is.
| In symbols | V ＝ ( m ÷ M ) ÷ C |
| In words | Water drawn in ＝ number of lactose particles (weight ÷ weight per mole) ÷ concentration of body fluid |
| Where it comes from | Balance of osmotic pressure. By van 't Hoff's equation (osmotic pressure ＝ particle concentration × gas constant × temperature), osmotic pressure depends on the concentration of dissolved particles. We assume water moves until the concentration in the gut equals that of body fluid. |
| V | Amount of water drawn into the gut (unit: L) |
| m | Weight of lactose reaching the large intestine (unit: g) |
| M | Weight of one mole of lactose (unit: g) |
| C | Concentration of body fluid (moles of particles dissolved in 1 L) |
| Share of lactose in 100 g of milk (from the carbohydrate content in the Standard Tables of Food Composition in Japan) | About 4.8 g, a fraction of 0.048 |
| One glass of milk (taking 200 mL as about 200 g) | 200 g |
| Weight of one mole of lactose | 342 g |
| Concentration of body fluid (dissolved particles) | About 0.29 mol/L |
| Amount of lactose per serving said to be unlikely to cause symptoms | About 12 g |
| Lactose in one glass (g) | 200 × 0.048 ＝ 9.6 |
| Number of lactose particles (mol) | 9.6 ÷ 342 ≒ 0.028 |
| Water drawn in (L) | 0.028 ÷ 0.29 ≒ 0.097 |
| Convert to mL | 0.097 × 1000 ＝ 97 |
| How many glasses is the 12 g guideline? | 12 ÷ 9.6 ＝ 1.25 |
③ In everyday terms: one glass (200 mL) of milk keeps about 100 mL of water, roughly half a glass, in the gut. If bacteria then break each lactose particle into several small acids, the number of particles rises, so even more water is thought to be drawn in. The 12 g said to be unlikely to cause symptoms equals about 1.25 glasses of milk. Keeping each serving to one glass is one useful rule of thumb.
High schoolHigh school+Enzymes are picky, and fermentation in the large intestine
High schoolAn enzyme works only on a specific partner. This is called substrate specificity. Lactase cuts lactose but does not cut table sugar (sucrose). A "disaccharide", two sugars joined together, is absorbed from the small intestine only after it has been hydrolysed into single sugars, or "monosaccharides".
High school+When bacteria break down lactose in the large intestine, it is fermentation in a place without oxygen. It produces short fatty acids (short-chain fatty acids) such as acetic, propionic and butyric acid, along with hydrogen and carbon dioxide. In some people methane is produced as well. The hydrogen breath test, which measures hydrogen in the breath after drinking lactose, uses this fermentation.
UniversityWhy some adults keep the enzyme: a difference in a gene "switch"
The trait of continuing to make lactase into adulthood is called lactase persistence. The difference lies not in the lactase gene (LCT) itself but in an upstream region that regulates it. This region sits inside the neighbouring gene (MCM6), and single-nucleotide polymorphisms there are thought to be involved. Separate mutations are said to have spread independently in Europe and East Africa, and this is known as a classic example of "gene–culture coevolution", in which the culture of dairy farming changed gene frequencies.
📖 Derivations and further reading: Lactose intolerance (Wikipedia) ／ Lactase (Wikipedia)
ResearchWhat we still don't fully know
- Symptom strength isn't set by enzyme amount alone. Even with the same amount of lactose, symptoms differ greatly from person to person. The mix of gut bacteria and how sensitive the gut is are thought to play a part, but the full picture is still unclear.
- Is "getting used to it" real? There are reports that symptoms ease if you keep drinking milk daily. But it is hard to separate this from the effect of expectation, and study results don't agree.
- Why did the mutation spread so fast? There are several hypotheses: the nutrition in milk, a source of fluid, calcium absorption, and more. Which one mattered in which region is still debated.
In other words, this article too is "an explanation within what is known today". Even if you feel you are sensitive to milk, there is plenty of room for change through amount and the way you drink it.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science (digestion and absorption, digestive enzymes) | Reason 1: how sugar is broken down and absorbed in the small intestine |
| High school | Biology (enzyme substrate specificity, disaccharides and monosaccharides) | Lactase cuts only lactose |
| High school+ | Chemistry (osmotic pressure, van 't Hoff's equation) | Checking with a formula: how much water is drawn in |
| University | Biochemistry and human genetics (lactase persistence, gene–culture coevolution) | Why some adults keep the enzyme |
| Research | Gastroenterology and gut microbiology | Individual differences in symptoms, the effect of getting used to it |
| ― | Link to daily life | Splitting the amount, drinking with meals, choosing lactose-free milk |
- Lactose intolerance ― Wikipedia (Japanese)
- Shaukat A, et al. (2010) Systematic Review: Effective Management Strategies for Lactose Intolerance. Annals of Internal Medicine 152(12).
- Ministry of Education, Culture, Sports, Science and Technology (文部科学省), Standard Tables of Food Composition in Japan (Eighth Revised Edition) (『日本食品標準成分表（八訂）』), composition values for regular cow's milk
- Swallow DM (2003) Genetics of lactase persistence and lactose intolerance. Annual Review of Genetics 37.
- Itan Y, et al. (2009) The origins of lactase persistence in Europe. PLoS Computational Biology 5(8).
※This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanism. If symptoms are severe, last a long time, or you are losing weight, don't decide for yourself; talk to a medical provider.