Lactose intolerance or IBS: which is it?

What lactose intolerance is

The NIH explains that lactose intolerance comes from lactose malabsorption: your small intestine makes low levels of lactase, the enzyme that breaks down lactose. Undigested lactose moves into the colon, where bacteria break it down and make fluid and gas.

The NHS says symptoms can start a few minutes or a few hours after having lactose, and lists:

• tummy pain or discomfort
• bloating and farting
• tummy rumbling
• diarrhea or constipation
• feeling or being sick

What IBS is

The NIH describes IBS as repeated abdominal pain together with changes in bowel movements, with no visible damage in the gut. The NHS says IBS flare-ups can be triggered by alcohol, caffeine, spicy or fatty foods, stress and anxiety, or happen for no obvious reason.

Side by side

	Lactose intolerance	IBS
Main link	Food or drink containing lactose	Many triggers, or none obvious
Timing	Minutes to hours after lactose (NHS)	Can come and go over days and weeks
Common symptoms	Pain, bloating, wind, rumbling, diarrhea or constipation, feeling sick	Pain or cramps eased by pooing, bloating, diarrhea, constipation, or both
How it’s checked	GP may suggest cutting lactose to see if it helps; NIH lists a hydrogen breath test	Pattern of symptoms over time, plus tests to rule out other conditions

This table is a guide for your GP chat, not a self-diagnosis tool.

Can they overlap?

Yes. Lactose is the “D” (disaccharide) in FODMAP. Monash lists milk, yoghurt and soft cheeses as high in lactose, and these are tested during the low FODMAP diet’s challenge phase. So someone with IBS can also find dairy is one of their triggers.

The NIH also notes that injury to the small intestine, from conditions like coeliac disease or Crohn’s disease, can lower lactase levels (called secondary lactose intolerance). That’s one more reason to get it checked rather than guess.

A simple log to bring

Mon 08:00  Breakfast (photo): cereal with milk  tags: dairy
Mon 10:30  Poop: type 6
Tue 08:00  Breakfast (photo): cereal, lactose-free milk
Tue  (no poop logged)
Wed 08:15  Breakfast (photo): yoghurt, berries  tags: dairy
Wed 11:00  Poop: type 6

A dairy-tagged pattern like this is worth showing your GP. PoopAI lets you tag meals with dairy and see what your Bristol types did afterwards. How you felt is a note you’d keep alongside.

When to see a GP

The NHS lactose page says see a GP if symptoms keep coming back after eating, or if you’ve had changes in your poo, blood in your poo, or bloating and discomfort for 3 weeks, or weight loss without trying. The NHS IBS page suggests a GP visit after 4 weeks of IBS-like symptoms. Call 999 if your lips, mouth, throat or tongue suddenly swell, as that can be an allergic reaction.

Wrap-up

If symptoms reliably follow dairy, lactose is a strong suspect. If they wander in on stress, coffee or nothing at all, IBS might be part of the picture. Either way, keep notes and let a GP join the dots.