Lactose Intolerance: Why Does Dairy Upset My Stomach?

Lactose intolerance is a common reason dairy upsets the stomach. Your small intestine may not make enough lactase, the enzyme that breaks down lactose, the sugar in milk. Undigested lactose can cause bloating, gas, cramps and loose stools, usually within a few hours of eating dairy. This article explains why it happens, how it differs from a milk allergy, how clinicians evaluate it, what can help, and which symptoms need prompt medical attention.

Lactose intolerance: why dairy upsets your stomach

Lactose is the main sugar in milk. To absorb it, your small intestine uses an enzyme called lactase, which splits lactose into two simpler sugars that the body can take in. MedlinePlus (NIH) describes lactose intolerance as trouble digesting lactose because the body does not make enough lactase.

When lactose is not digested, it travels on to the large intestine. Gut bacteria ferment it, which can produce gas, and the unabsorbed sugar can draw water into the bowel. Several factors contribute to how strongly a person reacts, and the mechanism is not identical in everyone.

Common symptoms

Symptoms often start some time after eating or drinking dairy. They can include:

• Bloating and a swollen, full feeling in the belly
• Gas and flatulence
• Belly cramps or pain
• Loose stools or diarrhea
• Rumbling or loud stomach noises
• Nausea, and sometimes vomiting

Some people with low lactase have mild symptoms or none. Others react to a small amount of milk. If gas is your main complaint, the MedlinePlus page on gas covers general causes. Loud belly sounds have their own explanation in our article on stomach growling, and excess burping is covered separately as well.

Why the body makes less lactase

There are several routes to low lactase, and they differ in how long they last.

Lactase decline with age

Lactase activity in many people falls after childhood. Research reviews, including Adult lactose digestion status and effects on disease, describe wide differences between populations in how long lactase stays active into adulthood. Genetics play a role, and the molecular details are reviewed in The molecular basis of lactose intolerance. Being able to digest lactose as an adult is common in some regions and less common in others.

Injury or illness in the small intestine

Conditions that affect the lining of the small intestine can lower lactase temporarily or longer term. Examples include celiac disease, Crohn disease, and a bad stomach infection. In these cases, lactose symptoms may improve when the underlying problem is treated. MedlinePlus describes celiac disease and malabsorption syndromes if you want background.

Rare congenital forms

Some babies are born with very little or no lactase. This is much less common than the age-related form, and it usually causes problems very early in life, so it is evaluated by a pediatrician.

Lactose intolerance versus milk allergy

These are different problems. Lactose intolerance is a digestion problem and does not involve the immune system. A milk allergy is an immune reaction to milk proteins and can cause hives, swelling of the lips or face, wheezing or vomiting. A severe allergic reaction needs emergency care, which is why those signs appear in the emergency box below. If you have never been told which one you have, tell your clinician about your symptoms so they can sort it out.

Other conditions can look similar, including irritable bowel syndrome, celiac disease and other food sensitivities. A single symptom after dairy does not establish lactose intolerance on its own.

How clinicians evaluate it

Clinicians combine your history, an examination and, when needed, testing. A common first step is asking how you feel after dairy and whether symptoms improve when you cut it back. If the picture is unclear, a clinician may consider options such as a hydrogen breath test, which measures hydrogen in your breath after you drink a lactose solution, or other tests to look for related conditions. Which approach is appropriate depends on your age, symptoms and other health history. Do not start a long, strict elimination diet without discussing it with your clinician, especially for a child.

What may help

Adjusting how you eat dairy

Many people with lactose intolerance can handle some dairy. Smaller portions, dairy taken with a meal rather than alone, and some aged cheeses or yogurt may be better tolerated for some people. Individual responses vary, so it can help to keep a simple food and symptom diary and bring it to your appointment.

Lactose-free products

Lactose-free milk and other dairy products have the lactose broken down or removed, and they provide the same kinds of nutrients as regular dairy. The review Lactose-Free Dairy Products: Market Developments, Production, Nutrition and Health Benefits describes how they are made.

Lactase enzyme supplements

Over-the-counter lactase supplements may reduce symptoms for some people when taken with dairy. Evidence on how well they work varies. Ask your pharmacist or clinician which product and amount fit your situation and read the label, particularly if you are pregnant, take other medicines, or are giving one to a child.

Hidden lactose

Lactose can appear in foods that are not obviously dairy, such as some breads, processed meats, sauces, salad dressings and some medicines or supplements. Reading ingredient lists for words like milk, whey, curds and milk solids can help. Pharmacists can tell you whether a medicine contains lactose as an inactive ingredient.

Getting enough calcium and vitamin D

If you cut back on dairy, you may take in less calcium and vitamin D. Over time, low intake can matter for bone health, and in growing children, very low vitamin D and calcium are associated with rickets, a bone-softening condition described by MedlinePlus. The review Lactose Intolerance, Dairy Avoidance, and Treatment Options discusses these nutrition concerns. Non-dairy sources include calcium-fortified foods and drinks, canned fish with soft bones, and some leafy greens. Ask your clinician or a dietitian whether you need a supplement, and avoid giving a child a restrictive diet without pediatric guidance.

When symptoms may point to something else

Lactose intolerance by itself usually does not cause bleeding, unexplained weight loss, fever or symptoms that wake you at night. These features can suggest a different condition and deserve medical evaluation. Persistent diarrhea has many possible causes; the MedlinePlus page on diarrhea lists some of them, and our article on diarrhea lasting more than two weeks goes deeper. Diarrhea can also cause dehydration, particularly in infants, young children and older adults.

Another cause sometimes considered when foods upset the stomach is histamine intolerance, which is still debated. A study in PubMed Central looked at histamine levels in people with suspected histamine intolerance, though the condition is not fully understood and is much less commonly diagnosed than lactose intolerance.

Planning your next step

If dairy reliably upsets your stomach, write down what you ate, how much, and when symptoms began. Note any other symptoms, family history of bowel disease, and any medicines you take. Bringing this to a clinician helps them decide whether testing or a trial of dietary changes is appropriate for you.

When to Seek Medical Care