Lactose and IBS: Foods, Symptoms & Reintroduction

How lactose is absorbed

Lactose is the D — the disaccharide — in FODMAP. Unlike fructans and GOS, lactose can be fully absorbed, but only if your small intestine makes enough lactase to split it into glucose and galactose. Lactase production commonly declines after childhood, and how much you keep varies a lot between people and populations. When lactose outstrips your available lactase, the surplus travels on: it pulls water into the small intestine and is then fermented by colonic bacteria. This is why lactose intolerance is dose-related — most people with reduced lactase can still handle a small amount, such as the splash of milk in a coffee, without symptoms.

Typical symptoms

Lactose tends to act faster than the oligosaccharides because the water-drawing effect starts higher up the gut: bloating, cramping, wind, and often looser stools or diarrhoea, typically within a few hours of a dairy-heavy food. The severity scales with the amount of lactose and your personal lactase level.

Which foods carry lactose

The highest-lactose foods are milk, evaporated and condensed milk, soft and fresh cheeses (ricotta, cottage cheese), yoghurt, ice cream, and milk-based custards. Importantly, not all dairy is high in lactose: hard, aged cheeses like cheddar and parmesan are very low because most lactose is lost in whey and further broken down during ageing, and butter is essentially lactose-free.

High-lactose foods to limit

These foods reach a high lactose level at a realistic serving, so they're usually limited during the elimination phase. Each links to its full serving thresholds.

Safe picks low in lactose

These low-FODMAP foods don't carry a meaningful lactose load, so they make dependable swaps while you keep lactose low.

Aged hard cheeses (cheddar, parmesan) and butter are naturally low in lactose, so 'dairy' and 'high FODMAP' are not the same thing — the list below reflects lactose content specifically, not dairy in general.

How to reintroduce lactose

Lactose is one of the simpler groups to challenge because milk isolates it cleanly. Build the serving over three days while keeping the rest of your diet low-FODMAP.

Typical test food: Cow's milk

These are a general escalation pattern, not prescriptive doses. If you already know dairy reliably troubles you, lactose-free milk (real dairy with the lactase added) lets you keep dairy nutrition while you test — but plan the challenge itself with your dietitian so the result is meaningful.

For the full method — washouts, how to read your results, and the mistakes that quietly derail people — see our guide to the FODMAP reintroduction phase.

Lactose FAQ