You’ve probably heard of lactose intolerance before, since in adults, it’s a reasonably common digestive problem. Known as late onset lactose intolerance, it occurs when, over a number of years, a person’s body begins to slowly decrease in their production of lactase, the enzyme that breaks down lactose. The body then becomes unable to digest lactose, a type of sugar mainly found in milk and dairy products.

But how might lactose intolerance affect infants, who for the first 6 months of life rely exclusively on milk for their nutrition and as their main source of nutrition even after solids have been introduced for several months beyond that?  And what might cause them to be unable to tolerate the most abundant component found in human milk?

On very rare occasions, babies can be born with primary lactose intolerance. I want to stress that this is not at all common. Because milk is the natural first food of all humans, it stands to reason that babies are typically born ready, willing and able to drink (and digest) milk. As such, primary lactose intolerance is an incredibly rare, inherited metabolic disorder with only 16 reported cases in 17 years. With this type of lactose intolerance, babies are born with a lack of the enzyme, lactase. It’s a serious condition that would become apparent soon after birth. A baby with primary lactose intolerance would not thrive and would show obvious symptoms of malabsorption and dehydration and would require a special lactose-free formula to survive.

Babies who are born prematurely can also suffer from a form of lactose intolerance called developmental lactose deficiency because their bodies have not started producing their own lactase, which typically increases during the third trimester. However, this resolves with time as the infant’s system matures and ingesting lactose can help to accelerate this maturation process.

Far more commonly, infants can experience periods of secondary lactose intolerance. This type of lactose intolerance occurs when irritation to the gut lining occurs. Since lactase is produced in the villi, which are the very tips of the microscopic folds in the gut, even very subtle damage to the gut can affect the delicate villi and thus go on to reduce the amount of lactase produced, resulting in secondary lactose intolerance. Damage to the villi commonly occurs through an infectious, allergic or inflammatory process, for example, as a result of:


-          A food sensitivity; In breastfed babies, this can come from food proteins, such as in cows’ milk, wheat, soy or egg, or possibly other food chemicals that enter breastmilk from the mother’s diet, as well as from food the baby has eaten.

-          Gastroenteritis

-          Antibiotic use

-          The oral rotavirus vaccine

-          A parasitic infection

-          Coeliac disease

-          Crohn’s disease

-          Bowel surgery


So, what does secondary lactose intolerance look like? Well, it can vary, but typically you might expect liquid, frothy stools which are sometimes green, due to the rapid transit through the digestive system with stringy mucus. Your baby may also be more irritable and gassy than usual.

What can we do about secondary lactose intolerance in the breastfed infant? This is the important bit: The solution is to find out what caused the secondary lactose intolerance and tackle that rather than to attempt to eliminate lactose. Bear in mind that lactose is present in ALL breastmilk, no matter what diet the mother consumes. It is also present in much the same quantities in both foremilk, the watery milk present at the beginning of the feed, and in the hindmilk, the richer milk with a higher fat content that is present at the end of the feed. In a formula fed infant, a quick switch to lactose-free formula will cause a reduction in symptoms as the gut heals. However, human milk remains the best food and will assist with gut healing, so, for a breastfed infant, replacing breastmilk with a lactose-free formula is NOT the solution unless there are serious concerns about the infant’s growth. Because breastmilk can help to heal the gut, in the vast majority of cases, it is important for breastfeeding to continue.

Gut healing times can vary, depending on cause and age of the infant. For example, the gut of a baby with severe gastroenteritis will take 4 weeks on average to heal, but that could double for a baby under 3 months. For babies over 18 months, recovery may be as rapid as 1 week.

Whatever your baby’s age, you should always seek professional advice from a breastfeeding friendly pediatrician and/or lactation specialist before introducing lactose-free formula. Except for the very rare primary type, lactose intolerance in babies always has a cause. Establishing the cause and fixing that is the key to resolving the baby’s symptoms.