ConditionDigestive system
Lactose Intolerance
Lactose intolerance is difficulty digesting lactose, the sugar in milk, because the small intestine makes too little of the enzyme lactase. Undigested lactose is fermented by gut bacteria, causing bloating, gas, cramps and diarrhea 30 minutes to 2 hours after eating dairy. It is very common: most of the world's adults lose some lactase after childhood. It is not an allergy and is not dangerous. Most people can manage it by limiting lactose, choosing low-lactose dairy or taking lactase enzyme.
Also called: Lactase deficiency, lactose malabsorption, dairy intolerance, milk intolerance
At a glance
- What it is
- Too little lactase enzyme, so milk sugar is not fully digested.
- Symptoms
- Bloating, gas, cramps, diarrhea, sometimes nausea, 30 minutes to 2 hours after dairy.
- Types
- Primary (gradual loss with age, most common), secondary (after gut injury), congenital (very rare).
- Tests
- Hydrogen breath test, or a trial of avoiding and reintroducing lactose.
- Management
- Smaller dairy portions, lactose-free milk, hard cheeses and yogurt, lactase tablets, enough calcium and vitamin D.
When to get help
Call 911 or go to the ER
- Hives, lip or throat swelling, wheezing or vomiting within minutes of dairy (milk allergy, not intolerance): use epinephrine if prescribed and call 911
See a doctor within a day or two
- Diarrhea with blood, fever, or signs of dehydration
- A baby with diarrhea, poor weight gain or blood in the stool after milk feeds
Book a routine appointment
- Bloating, gas or diarrhea after dairy that is affecting your life
- Symptoms that continue even when you avoid lactose
- Weight loss, anemia or diarrhea at night, which suggest another cause
Usually safe to manage at home
- Mild gas or loose stools after large amounts of milk or ice cream that settle within hours
How lactose intolerance happens
Lactose is a sugar made of glucose and galactose. Lactase, an enzyme on the lining of the small intestine, splits it so it can be absorbed. Without enough lactase, lactose passes to the colon, where bacteria ferment it into gases (hydrogen, carbon dioxide and methane) and short-chain fatty acids, and it draws water into the bowel. The result is bloating, gas, cramps and loose stools.
Types of lactose intolerance
| Type | Cause | Who |
|---|---|---|
| Primary (lactase non-persistence) | Genetically programmed decline in lactase after weaning | Most adults worldwide, especially of East Asian, West African, Native American and Ashkenazi Jewish descent. Less common in Northern Europeans |
| Secondary | Damage to the small intestine lining | After gastroenteritis, with celiac disease, Crohn's disease, chemotherapy or SIBO. Often temporary |
| Developmental | Immature gut | Some premature babies, usually improves |
| Congenital | Inherited absence of lactase from birth | Very rare. Severe diarrhea from the first feeds |
Symptoms
Symptoms usually begin 30 minutes to 2 hours after eating or drinking dairy and depend on how much lactose you consume and how much lactase you have:
- bloating and a swollen belly
- gas and rumbling
- cramping belly pain
- loose stools or diarrhea
- nausea and sometimes vomiting
Many people with lactase deficiency can drink a cup of milk (about 12 g of lactose) with little or no discomfort, especially with a meal.
Lactose intolerance vs milk allergy
| Lactose intolerance | Milk allergy | |
|---|---|---|
| Cause | Enzyme deficiency for milk sugar | Immune reaction to milk proteins |
| Typical age | Older children and adults | Babies and young children |
| Symptoms | Gut only: gas, bloating, diarrhea | Hives, swelling, vomiting, wheeze, anaphylaxis, or gut symptoms |
| Amount | Related to dose, small amounts often fine | Tiny amounts can trigger reactions |
| Danger | Not dangerous | Can be life-threatening |
Lactose intolerance is uncommon in babies under 2 to 3 years. Gut symptoms with milk in infants are more often from cow's milk protein allergy or infection.
Diagnosis
- dietary trial: remove lactose for 2 weeks, then reintroduce it, and see whether symptoms disappear and return
- hydrogen breath test: you drink a lactose solution and breath samples are taken over 2 to 3 hours. A rise in hydrogen shows lactose is being fermented by bacteria
- stool acidity test in infants and young children
- genetic test for lactase persistence variants, available in some places
- tests for underlying causes of secondary intolerance, such as celiac antibodies
Symptoms of lactose intolerance overlap with irritable bowel syndrome, and both can occur together.
Managing lactose intolerance
Adjust how you eat dairy
- have smaller portions, spread through the day
- drink milk with meals rather than alone
- choose lactose-free milk, which has lactase added and the same nutrients
- hard and aged cheeses (cheddar, Swiss, Parmesan) contain very little lactose
- yogurt with live cultures is often well tolerated because bacteria digest some lactose
- butter contains little lactose
Lactase enzyme
Over-the-counter lactase tablets or drops taken with the first bite or sip of dairy help many people. Follow the label dose.
Watch for hidden lactose
Lactose is added to some breads, processed meats, salad dressings, baked goods and instant foods, and is used as a filler in many tablets. Check ingredient labels for milk, whey, curds, milk solids and dry milk powder. Amounts are usually small.
Keep bones healthy
Avoiding dairy completely can lower calcium and vitamin D intake and raise the risk of osteoporosis. Good alternatives include lactose-free dairy, fortified plant milks, canned fish with bones, tofu made with calcium, leafy greens such as kale and bok choy, and calcium-fortified foods. Supplements may be advised.
Secondary lactose intolerance
Treating the cause, such as celiac disease or recovering from gastroenteritis, often restores lactase.
Frequently asked questions
What are the symptoms of lactose intolerance?
Typical symptoms are bloating, gas, belly cramps and diarrhea, sometimes with nausea, starting 30 minutes to 2 hours after eating or drinking dairy. Symptoms are stronger with larger amounts of lactose and usually settle within a few hours.
What is the difference between lactose intolerance and a milk allergy?
Lactose intolerance is a digestive problem with milk sugar caused by low lactase, causing gas, bloating and diarrhea. Milk allergy is an immune reaction to milk proteins that can cause hives, swelling, vomiting, wheezing or anaphylaxis, even with tiny amounts.
How is lactose intolerance diagnosed?
Often by a dietary trial: avoiding lactose for 2 weeks and seeing whether symptoms improve and return when it is reintroduced. A hydrogen breath test can confirm it. Doctors may check for other conditions such as celiac disease if symptoms are severe or unusual.
Can lactose intolerance develop later in life?
Yes. Lactase levels naturally fall after childhood in most people, so symptoms can begin in the teens or adulthood. Lactose intolerance can also appear after a gut infection, with celiac disease or Crohn's disease, or after some treatments that damage the gut lining.
What foods can I eat with lactose intolerance?
Many people tolerate lactose-free milk, hard aged cheeses, yogurt, butter and small portions of milk with meals. Plant milks fortified with calcium are good alternatives. Lactase tablets taken with dairy also help. Most people do not need to avoid lactose completely.
Sources
This guide was written from current patient and clinical guidance, including:
- National Institute of Diabetes and Digestive and Kidney Diseases: Lactose intolerance
- American Academy of Pediatrics: Lactose intolerance in infants, children and adolescents (clinical report)
- MedlinePlus: Lactose intolerance
- NIH Office of Dietary Supplements: Calcium fact sheet
- American College of Gastroenterology: Lactose intolerance