Lactose is a mammalian sugar and the reason infants love the taste of breast milk so much. It is extremely rare for infants to be born lactase-deficient, so nearly all babies should be able to tolerate lactose. A common cause of lactose intolerance is transient intolerance, which occurs for a short period after diarrhoea, illness, or a course of antibiotics.

Symptoms and signs in infants

Lactose intolerance typically presents with:

Watery, acidic diarrhoea
Abdominal distension
Flatulence
In severe cases, continued intake can lead to failure to thrive and dehydration

Importantly, hives, immediate-onset rashes, and tissue swelling are signs of Cow’s Milk Protein Allergy (CMPA), not lactose intolerance. Symptoms usually appear after ingesting breast milk or standard infant formula, both of which contain lactose. Secondary lactose intolerance — for example after a gut infection — presents with similar but often milder symptoms and typically resolves once lactose is removed from the diet.

How is it diagnosed in infancy?

In infants, clinical history with dietary elimination-and-rechallenge is used: symptoms should resolve with lactose elimination and recur on reintroduction. In older children, the hydrogen breath test is the most widely used non-invasive method (a rise in breath hydrogen of ≥20 ppm above baseline indicates lactose malabsorption), interpreted alongside symptoms. Stool acidity and reducing substances may support the diagnosis where breath testing isn’t feasible.

Which milk is safe for my infant?

Breastfeeding should be continued wherever possible, as human milk is generally well tolerated except in the exceptionally rare cases of congenital lactase deficiency. For lactose-intolerant infants, recommended formulas are rice- or soy-based, or lactose-free cow’s-milk-protein-based formulas. Extensively hydrolysed or amino-acid formulas are not indicated for isolated lactose intolerance — they are reserved for infants with severe cow’s milk protein allergy. Avoid soy milks if there is a coexisting soy allergy.

Distinguishing intolerance from a true milk allergy guides which test is appropriate, and our team is experienced in guiding families through this diagnostic process.