Lactose is the natural sugar in milk. To absorb it, your small intestine needs an enzyme called lactase, and in most people worldwide lactase activity falls during childhood. Lactose that isn't absorbed travels on to the colon, where bacteria ferment it, and that can cause pain, bloating and diarrhea — the same symptoms as IBS.[1] The two also overlap: people with IBS are no more likely than anyone else to malabsorb lactose, but they're more likely to get symptoms from it.[2]
What's the difference between IBS and lactose intolerance?
Doctors separate two things. Lactose malabsorption means your gut doesn't fully digest or absorb lactose. Lactose intolerance means you get symptoms, such as pain, bloating or diarrhea, after eating lactose because of it. Many people with malabsorption can drink a glass of milk without symptoms. Whether you get them depends on how much lactose you have, how much lactase you make, your gut bacteria and how sensitive your gut is.[1]
That last factor is where IBS comes in. In a meta-analysis of 14 studies, people with IBS were no more likely than healthy controls to malabsorb lactose, but they had about 3.5 times the odds of reporting lactose intolerance and 2.5 times the odds of having symptoms when tested.[2] Cutting lactose alone often helps only modestly, though, because lactose is just one of several poorly absorbed carbohydrates that cause symptoms in the same way.[1]
IBS vs celiac vs lactose intolerance
| IBS | Lactose intolerance | Celiac disease | |
|---|---|---|---|
| What happens | Disorder of gut–brain interaction[3] | Unabsorbed lactose is fermented in the colon[1] | Immune reaction to gluten that injures the small intestine[4] |
| Trigger | Many, including FODMAPs[3] | Lactose (milk sugar)[1] | Gluten in wheat, barley and rye[4] |
| Main tests | Symptom criteria plus normal basic blood and stool tests[5][3] | Breath test, blood glucose test or genetic test[1] | Antibody blood test (tTG), then a small-intestine biopsy[4] |
| Treatment | Diet, soluble fiber, psychological therapies and medicines[3] | Less lactose, lactose-free products or lactase tablets[1] | Strict lifelong gluten-free diet[4] |
Celiac disease is the one not to miss, because it needs a strict gluten-free diet. That's why British guidelines advise a celiac blood test for everyone first seen with IBS symptoms.[3] See IBS or celiac disease.
Is lactose a FODMAP?
Yes. FODMAPs are fermentable oligo-, di-, monosaccharides and polyols, and lactose is the disaccharide. It's one of six groups on the low-FODMAP diet, alongside fructans, galacto-oligosaccharides, excess fructose, sorbitol and mannitol.[6] It only causes trouble if you don't absorb it well: if you tolerate lactose, dairy foods can be included freely on a low-FODMAP diet, according to Monash University's FODMAP team.[7]
When it's tested blind, lactose turns out to be a less common trigger than many people expect. In a Belgian study, people with IBS who improved on a low-FODMAP diet were given powders of single FODMAPs, or plain glucose as a control, without knowing which. Lactose brought symptoms back in 28%, about the same as glucose at 26%, while fructans did in 56% and mannitol in 54%.[8] That's what the reintroduction phase is for: finding out which foods actually trigger your symptoms instead of avoiding them all.[9]
How is lactose intolerance tested?
- Hydrogen breath test: you drink a set dose of lactose, and your breath is checked for hydrogen, a sign that bacteria are fermenting it. Symptoms during the test can be recorded too.[1]
- Lactose tolerance test: checks whether your blood glucose rises after you drink lactose.[1]
- Genetic test: shows whether you carry the gene variant for lasting lactase production, but not whether lactose gives you symptoms.[1]
For typical IBS, British guidelines say there's no role for hydrogen breath testing to rule out lactose intolerance, partly because the tests can be falsely positive when food moves through the gut quickly.[3] How people feel doesn't always match the test either: among people referred for breath tests, about half of those who digest lactose normally still report discomfort after a lactose drink when they know what it is.[1] A practical alternative is a planned trial: cut back on lactose for a few weeks, then reintroduce it in steps, ideally with a dietitian.[9]
Do I have to give up dairy with IBS?
Usually not. Monash University's FODMAP team says a dairy-free diet is only needed if you're sensitive to cow's milk protein. Many dairy foods are very low in lactose, and higher-lactose foods such as milk and yogurt are fine in small servings.[7]
- Most people tolerate some lactose. A systematic review found that most adults with lactose intolerance or malabsorption can handle 12–15 g of lactose, about a cup of milk.[10] Larger amounts are often tolerated with food or spread over the day.[1] If you have IBS, your own limit may be lower.[2]
- Hard cheeses are very low in lactose. Cheddar and Gruyère contain about 0.1 g per 100 g, and butter very little. Yogurt has roughly half the lactose of milk.[1]
- Lactose-free milk has lactase added and contains almost no lactose.[1]
- Lactase tablets improve lactose digestion and symptoms, though the effect is modest.[1]
- Keep the calcium. Dairy is a major source of calcium, so if you cut back, replace it. A dietitian can help.[7]
To answer the question for yourself, note how much dairy you had and how you felt over the next few hours, and compare dairy-heavy days with lactose-free ones. IBS Clue estimates the lactose level of each meal you log, so a latte or a creamy sauce counts, and over a few weeks it ranks lactose against the other FODMAP groups as a possible trigger.
When to see a doctor
See a doctor before you cut out whole food groups, and promptly if you notice warning signs: blood in your stool, unintentional weight loss, anemia, fever, symptoms that wake you at night, a family history of bowel cancer, inflammatory bowel disease or celiac disease, or symptoms that started after about age 50.[11][3] New problems with dairy in adulthood are worth mentioning too. Gut infections, inflammatory bowel disease and celiac disease can all cause a temporary lactase shortage, and many people with celiac disease digest lactose again after 6–12 months on a gluten-free diet.[1]
British guidelines advise checking for disordered eating before recommending restrictive diets, so tell your doctor if you have, or have had, an eating disorder.[3] Get advice first, too, if you're pregnant or changing a child's diet. More on warning signs in IBS red flags.