Lactose Intolerance in Babies: What You Need to Know

Lactose intolerance when our bodies struggle to digest lactose — the natural sugar found in milk including breastmilk, dairy products, and most infant formulas. This is usually due to a shortage of lactase, the enzyme that breaks lactose down in the gut.

While true lactose intolerance in babies is rare, it’s something parents may worry about when they have tummy troubles. Here’s what you need to know.

Types of Lactose Intolerance in Babies

1. Congenital Lactase Deficiency (Very Rare)

This is an extremely rare genetic condition where a baby is born with little or no lactase enzyme. It causes severe symptoms like watery diarrhoea, weight loss, dehydration, and failure to thrive within the first few days of feeding — even on breast milk. It’s diagnosed early and needs to be managed under specialist care.

2. Secondary Lactose Intolerance (More Common)

This type is temporary and happens when the gut lining is damaged — for example, after a bout of gastroenteritis, food allergy, or other illness. The damage affects the cells that produce lactase, reducing the baby’s ability to digest lactose for a short time.

• Symptoms can include: diarrhoea, bloating, gas, or fussiness after feeds.

• Usually resolves once the gut heals.

• A low-lactose or lactose-free formula may be recommended temporarily.

If your baby has recently been sick and is suddenly reacting to bresatmilk or formula, this could be the cause. Always check with your healthcare provider before switching formulas.

3. Primary Lactose Intolerance (Not Seen in Babies)

This is a normal, age-related reduction in lactase production that occurs in many people .

• It does not occur in babies.

• Symptoms typically appear in older children, teens or adults.

• It’s not a cause of lactose issues in infants under 12 months.

Babies are born with plenty of lactase to digest breastmilk and infant formula. 

What About Lactose Overload?

Lactose overload is not the same as lactose intolerance. It happens when a baby gets too much milk too quickly, and their digestive system can’t keep up. 

Signs can include:

• Frothy green stools

• Gassy tummy

• Lots of wind and fussiness

• Explosive poos

It’s about volume, not enzyme deficiency. Adjusting feeding patterns — not changing formula — is usually the solution.

 What to Do if You Suspect Lactose Intolerance

If your baby is unsettled after feeds, has frequent diarrhoea, or seems uncomfortable, it’s important to:

1. Speak to your healthcare provider
   They can help determine whether it’s lactose intolerance or something else (like a cow’s milk protein allergy, reflux, or normal baby digestion).

2. Don’t switch formulas without advice
   Low-lactose or lactose-free formulas are available but should only be used when recommended by a healthcare professional.

3. Keep a feeding and symptom diary
   Tracking feeds, poos, and symptoms can help your healthcare professional understand what’s going on.

In summary

Lactose intolerance in babies is often misunderstood. While temporary issues like secondary intolerance are fairly common, true lactose intolerance in infancy is rare. Many tummy symptoms are due to normal development, feeding volume, or temporary illness — not a permanent lactose problem.

If you’re ever unsure, trust your instincts and seek healthcare professional advice.