Lactose Intolerance in Babies

What lactose intolerance actually is

Lactose is the sugar in milk. It is broken down by lactase, an enzyme in the small intestine, into glucose and galactose, which the body can then absorb. When there is not enough lactase, the lactose is not absorbed, and the symptoms follow from that. As the AAP puts it, "lactose intolerance is a digestive problem, while milk allergy involves the immune system".

Symptoms are gut symptoms. The NHS lists tummy pain or discomfort, bloating, wind, tummy rumbling, diarrhoea or constipation, and feeling or being sick. The AAP adds cramping, loose stools and watery diarrhoea with gas, and notes that severity tracks the amount consumed.

Why it is rarer in babies than the internet suggests

This is the part that changes decisions. The AAP states that "lactose intolerance is actually not very common in infants and typically only starts showing up after age three in children who were born full-term". The reason is developmental: all babies are born with lactase in their intestines, and the enzyme declines as they get older.

The NHS reaches the same conclusion from the formula side, noting on its types-of-formula page that lactose intolerance is rare in babies, and that a suspicion of it warrants speaking to a midwife, health visitor or GP.

Lactose is also the main carbohydrate in breast milk and in standard cows' milk formula. A baby genuinely unable to handle lactose would be struggling with their main food from very early on, which is not the usual picture in a baby who is gaining weight and thriving but windy.

The three forms that do exist

Congenital lactase deficiency

The AAP describes this as "a very rare disorder in which babies can't break down the lactose in breast milk or formula", caused by genes inherited from both parents. It produces severe diarrhoea from the earliest feeds, and without a lactose-free formula those babies can develop severe dehydration and weight loss. It is rare enough that it is not a reasonable first explanation for a grizzly six-week-old, but it is real and it presents immediately and dramatically.

Developmental lactase deficiency

Babies born prematurely are more likely to have this form, because lactase activity develops late in gestation. The AAP notes that it "usually lasts only for a short time after birth", and that the majority of premature infants will still be able to take lactose-containing formula and breast milk.

Secondary, or temporary, lactose intolerance

This is the version most families meet. The AAP confirms that lactose intolerance "can occur temporarily in children after a viral infection", which is why you may be advised to avoid milk containing lactose during a stomach bug, and that it can also accompany conditions that inflame the intestine, such as coeliac disease. The gut lining recovers and so does the enzyme. This is a temporary state, not a diagnosis for life.

Telling it apart from cows' milk allergy

The two conditions are confused constantly, and getting the distinction right changes what you should do next.

• Mechanism. Intolerance is enzymatic. Allergy is immune-mediated; NICE defines cows' milk allergy as "a reproducible immune-mediated allergic response to one or more proteins in cow's milk".
• Danger. The AAP states that lactose intolerance "will not produce a life-threatening reaction such as anaphylaxis". Cows' milk allergy can.
• Symptoms beyond the gut. Hives, swelling, wheeze, cough or hoarseness point towards allergy, not intolerance. The NHS notes that food allergy can cause difficulty breathing, itchy skin and swelling of the lips, face and eyes.
• Age. NICE notes almost all cows' milk allergy presents before one year. The AAP puts typical lactose intolerance after age three in term-born children.

Confusingly, both can produce diarrhoea and abdominal discomfort, which is exactly why guessing at home is unreliable and why NICE builds its cows' milk allergy pathway around elimination followed by a deliberate reintroduction.

How it is investigated

The AAP describes a pragmatic first step of removing milk products for two weeks and then slowly reintroducing them in small amounts each day to see whether symptoms return. Formal testing exists: a hydrogen breath test measures breath hydrogen before and after a known lactose drink, and for infants and young children who cannot manage the breath test, doctors can check stool acidity, since acidic stool may indicate lactose malabsorption.

None of this is a reason to run a two-week milk elimination on a young baby without advice. A baby's main food is milk, and removing it is a nutritional intervention.

Lactose-free formula: when it is indicated

Lactose-free formula is suitable from birth but, in NHS terms, only under medical supervision. It exists for babies who genuinely cannot absorb lactose. Ireland's HSE takes the same position on specialist formulas generally, directing parents to a health professional rather than the shelf.

One important warning: lactose-free formula is still made from cows' milk protein. It is not an allergy formula and it will not help a baby with cows' milk protein allergy, who needs an extensively hydrolysed prescription formula instead. Soy formula is sometimes recommended for babies unable to digest lactose, though the AAP notes that this situation "is very uncommon".

Calcium if dairy is reduced

If a child does need to reduce lactose, the AAP points out that lactose-free dairy products still provide calcium, as do dark green leafy vegetables, almonds, white beans, sardines and salmon, and calcium-fortified drinks. It also advises talking to a doctor before starting any alternative milk, and separately reminds parents that a mother who is lactose intolerant can breastfeed safely, with no increased risk of the baby becoming intolerant.

When to see someone

See a GP if gut symptoms keep recurring after feeds, if there is blood in the stools, if weight gain has stalled, or if your baby seems in pain. The NHS advises seeking help if symptoms keep coming back or do not go away. Call 999 for sudden swelling of the lips, mouth, throat or tongue, breathing difficulty, a tight throat, skin turning blue, grey or pale, or a baby who is limp, floppy or unresponsive: those are signs of anaphylaxis, which is an allergy emergency and not something lactose intolerance causes.