Lactose intolerance or cows' milk allergy? How to tell the difference

These two get confused constantly — by parents, and often by professionals. They can also co-exist, and each can cause the other, which is why sorting them out needs a proper feeding history rather than a guess.

The core difference

The complication is that they interact. Lactose intolerance damages the gut, and that damage can allow allergy to develop. Allergy damages the gut, and that damage causes lactose intolerance. They frequently co-exist, and working out which came first takes a detailed feeding history and a real understanding of lactation.

The three types of lactose intolerance

Primary — not relevant to babies

This is the adult-onset kind, and it's normal. Lactase production naturally declines once children no longer need to process their mother's milk, so from the natural weaning age of around four to seven years, many people become less able to tolerate large amounts of lactose.

In some populations — many Europeans and some African groups — lactase production persisted into adulthood because it allowed extra calories from domesticated animals' milk, and was selected for genetically. Globally, it is actually a minority of people who go on consuming milk into adulthood.

Because it's adult-onset, primary lactose intolerance is by definition not the cause of symptoms in a baby.

Secondary — the one you'll actually meet

Also called transient or temporary. Something damages the gut, the lactase-producing cells are lost, and lactose stops being broken down properly. This is the common one.

Perhaps two thirds of babies, breast or bottle fed, experience some degree of lactase insufficiency at some point in their early months — without it causing most of them any serious or ongoing harm.

Congenital — very, very rare

A tiny number of people have a genetic defect meaning they can never process the galactose released from lactose. This causes galactosaemia, an autosomal recessive disorder inherited from two carrier parents.

Incidence is around 1 in 60,000 births for people of European ancestry — though notably about a hundred times more common, roughly 1 in 480, within the Irish Traveller community.

It is almost always picked up because the baby is gravely ill in the first week of life, often before leaving hospital. Most practitioners will never see a case.

Why lactose matters

It's worth knowing what you'd be removing. Lactose isn't an incidental ingredient — it's the major carbohydrate in human milk, at about 4% in colostrum rising to 7% in mature milk, and human milk contains the highest concentrations of any species.

It provides about 40% of a baby's energy needs. It's metabolised into glucose for energy and galactose, which is a constituent of the galactolipids needed for central nervous system development. It helps the absorption of calcium and iron. And it promotes colonisation with Lactobacillus bifidus, which creates an acidic environment in the gut that inhibits pathogenic bacteria, fungi and parasites.

That's why removing lactose from a breastfed baby's diet is not a casual decision, and why it should be temporary when it's necessary at all.

What causes secondary lactose intolerance

Lactase is produced in the brush border of the small intestine, on tiny fingerlike projections called villi. When the villi register lactose, they release lactase to split it. Anything that damages or flattens the ends of those villi stops that happening.

Common causes:

• Infections — viruses such as rotavirus, unfriendly bacteria (Cronobacter, Salmonella, E. coli strains), fungi such as thrush, or parasites
• Allergens in the milk being fed, whether formula or breastmilk
• Toxins such as lead or aluminium in water used for bottle feeding
• Irritants — inappropriate stool pH from medications or herbal teas
• Antibiotics, altering the gut microbiome
• Reflux medications and chemotherapy, which alter faecal pH
• Pressure — from very tight swaddling, over-tight disposable nappies, immobility, or trapped gas from fermentation

Then a vicious cycle sets in: too little lactase means bacteria ferment the free sugar, producing gas and acid; high acidity and pressure flatten the villi further. Breaking that cycle is the job.

The good news is that recovery is rapid once the irritation stops — cell turnover in the gut is high, and breastmilk contains stem cells and other healing agents. A formula-fed baby, lacking those factors, may need lactose removed temporarily to allow healing.

Lactose intolerance symptoms

• Frequent loose stools, often frothy or greenish
• Inflamed peri-anal skin — "burnt bum syndrome" — needing a rinse after every stool and a barrier cream to prevent acid scald
• Intense crying and signs of gut discomfort, often labelled colic
• Rapid improvement of all symptoms once lactose is removed

The mechanism explains the stools: the gut contracts to speed transit and expel the damaging substances, so water isn't reabsorbed and the result is very loose, sometimes explosive, output.

Diagnosis of transient lactose intolerance is confirmed by improvement within 2–3 days of starting a lactose-free diet, with complete resolution within two weeks. It isn't immediate, because the villi take time to recover — in extreme cases, or where an irritant is still present, it can take five or six weeks.

Lactose overload in breastfed babies

This is the distinction most often missed, and it matters enormously.

The amount of lactose in breastmilk is not affected by the mother's diet. She cannot reduce it by cutting out dairy — and shouldn't want to, given everything lactose does. But if the irritant or allergen flattening the villi is in her milk, changing her diet resolves the problem quickly, because the villi recover fast.

For most breastfed babies, though, the presenting problem is not intolerance of lactose but simple overload of it — usually caused by feeding patterns that reduce the fat content of what the baby takes. Fat slows gut transit; less fat means lactose arrives faster than the lactase can cope with.

Underlying causes include:

• Inefficient suckling — from positioning and attachment through to tongue-tie or other physical anomalies
• Frequency and duration of feeds
• Excessive pumping to build a stash or to donate, with low-fat expressed milk then bottle-fed back to the baby

Dehydration risk can be managed with oral rehydration solutions while the underlying cause is identified. Frequent comfort suckling raises maternal prolactin and increases milk volume within around 48 hours — during which the mother may feel the baby is getting very little, though the fat content may be higher than she realises.

If a mother does follow medical advice to switch to formula, she should be advised to continue expressing for at least three weeks. There's no guarantee the formula won't cause its own problems, and relactation is difficult once supply has gone completely.

Cows' milk protein allergy

CMPA is the most recognised infant food allergy, affecting an estimated 3–7% of infants worldwide.

There's a strong intergenerational link, both inherited and via experience in the womb — allergic siblings usually indicate allergic parents. Some infants are born already sensitised during pregnancy. Others inherit a tendency to react but only develop symptoms after exposure.

Neonatal exposure to milk proteins in formula or fortifiers, at the point when the gut is most permeable soon after birth, is understood to increase the risk of developing the allergy. That's a large part of why exclusive breastfeeding is increasingly seen as a necessity for normal gut and microbiome development, not merely a preference.

Timing matters for recognition: in formula-fed babies not already sensitised before birth, symptoms may take 14–21 days after first exposure to become obvious.

And exclusively breastfed babies can develop CMPA symptoms from milk proteins in their mother's diet — despite levels of cows' milk protein in breastmilk being 100,000 times lower than in cows' milk itself. That figure tells you how sensitive a reacting infant can be.

IgE vs non-IgE symptoms

Allergy can be IgE-mediated or non-IgE-mediated. The distinction matters because they present differently and are diagnosed differently.

IgE-mediated — faster, more obvious

• An itchy red rash, sometimes raised
• Swelling of the face, mouth or elsewhere
• Nausea or vomiting
• Abdominal pain or diarrhoea
• Wheezing and coughing
• Hay fever-like symptoms — sneezing, itchy eyes
• Misery: these babies simply look unhappy

Non-IgE-mediated — slower, easily missed

Symptoms can take several days to appear after a single exposure, are much less obvious, and are often attributed to something else entirely.

• Stools becoming more frequent or looser, though not necessarily diarrhoea
• Constipation in the formula-fed infant
• Acid reflux and GORD
• Blood and mucus in the stools
• Redness around the anus, rectum and genitals
• Unusually pale skin
• Failure to grow at the expected rate
• Excessive, inconsolable crying despite being fed and clean — labelled "colic"

Getting to a diagnosis

Allergy is in many ways a diagnosis of exclusion. Before concluding CMPA, rule out the ordinary explanations: how the feed is made up, the amount, the type, the method of delivery, the water supply. In a breastfed baby, is it simply lactose overload? Is there a non-feeding cause?

If CMPA is still suspected, an allergy-focused clinical history should cover:

• Any history of atopic disease — asthma, eczema, allergic rhinitis — or food allergy
• Family history of atopic disease or food allergy in parents or siblings
• Foods avoided in the family, and why
• The infant's weight gain, sleep and stooling since birth
• Full feeding history from birth, including any exposure to formula or fortifiers in hospital
• Any previous treatment — antihistamines, specialist formulas — and the response

Also ask, for any suspected reaction: age when symptoms started, speed of onset after food contact, duration, severity, frequency, and whether symptoms reproduce on repeated exposure.

Smoke exposure raises allergy risk, so household details are relevant too.

The classic presentation

Maureen Minchin's description in Milk Matters remains the most recognisable picture — crying babies whose sleep is disturbed and whose crying is erratic, persistent, prolonged and heartbreaking:

• Either hyperactive before birth and persistently miserable after, or starting to scream in the second or third week of life
• Giving many indications of gut discomfort and distress
• Progressively developing other minor symptoms — unexplained fevers, night sweats, cradle cap, patches of rough dry skin
• Not responding more than briefly to babycare skills and comforting
• Looking miserable or worried most of the time

Find qualified feeding support through Lactation Consultants of Great Britain. If everything else has been excluded and CMPA still looks likely, referral to a paediatrician via the GP or health visitor may be needed.

Common questions