Food allergy vs food intolerance in babies

Two different problems

When a baby gets a rash or an upset tummy after a new food, parents often wonder whether it’s an allergy or an intolerance. The words get used as if they meant the same thing, but they don’t, and the difference changes what you do next.

A food allergy involves the immune system. The immune system overreacts to a protein in a food (NHS, FDA). Reactions can affect the skin, breathing, gut or circulation, sometimes in more than one place at once (ACAAI). Most are mild, but some are severe and can be life threatening (NHS).

A food intolerance doesn’t involve the immune system. The body has trouble digesting a food or an ingredient in it. It can make your child feel unwell, but the NHS says it can’t cause a serious allergic reaction. The AAAAI describes it as uncomfortable, but not dangerous.

At a glance

• Food allergy: immune reaction to a food protein. Usually starts within minutes, and up to 2 hours (NHS). Hives, mild puffiness of the face or around the eyes, vomiting, coughing or wheezing; lips or mouth that suddenly swell are a severe sign. Can cause anaphylaxis. Diagnosed by a doctor or allergist.
• Food intolerance: trouble digesting a food. Usually starts a few hours after eating (NHS). Bloating, gas, tummy pain, diarrhea. Not an allergic reaction, so it can’t cause anaphylaxis, and it’s not usually serious (NHS). Symptoms tend to grow with the amount eaten: the AAP says this about lactose intolerance.

Delayed allergies: the exception to the rule

Not every allergy is fast. Some allergies don’t involve the antibody IgE and come on more slowly:

• Delayed (non-IgE) cow’s milk allergy can take up to 3 days to show (NHS), with symptoms such as vomiting, diarrhea, constipation, or eczema getting worse. See cow’s milk allergy.
• FPIES causes repeated vomiting, usually starting a few hours after a trigger food such as milk, soy, rice or oats (ACAAI). See FPIES.

These are still allergies, handled by a doctor, even though the timing looks more like an intolerance. That’s one reason not to guess at home. If your baby vomits once or twice a few hours after a food and is alert and like themselves, call your pediatrician or GP. Call 911 or go to the emergency room if your baby is vomiting again and again, or is pale, floppy, very sleepy or hard to wake.Call 999 or go to A&E if your baby is vomiting again and again, or is pale, floppy, very sleepy or hard to wake.Call your local emergency number (such as 112 in Europe or 000 in Australia) or go to the emergency department if your baby is vomiting again and again, or is pale, floppy, very sleepy or hard to wake. See FPIES.

Signs of a food allergy

Reactions usually start within minutes of eating, but can take up to 2 hours (NHS).

Mild to moderate: hives or a red, itchy rash; mild puffiness of the face or around the eyes; red, itchy or watery eyes; vomiting once, tummy pain or diarrhea; sneezing or a runny nose; eczema getting worse (NHS). If it’s only one of these, stop offering the food, stay with your baby and watch them closely for the next couple of hours, and call your pediatrician before you try it again. If signs show up in more than one part of the body at once, treat it as anaphylaxis (below).

Signs of anaphylaxis are trouble breathing, wheezing or a cough that won’t stop, swelling of the tongue or throat, lips or mouth that suddenly swell, a hoarse cry, turning pale or floppy, becoming very sleepy or unresponsive, or signs in more than one part of the body at once. If you see any of these and your baby has an epinephrine (adrenaline) auto-injector, use it right away, then call for an ambulance. Call 911.Call 999.Call your local emergency number (such as 112 in Europe or 000 in Australia). If you don’t have an auto-injector, call straight away. Keep your baby lying down, never standing or walking (on their side if they’re vomiting, head and shoulders a little raised if breathing is hard), and go to hospital even if they seem better. See signs of anaphylaxis.

Lactose intolerance is rare in babies

Lactose is the natural sugar in milk, including breast milk. Lactose intolerance means the body doesn’t make enough of the enzyme lactase to digest it (NHS).

The AAP says lactose intolerance is not very common in babies, and in babies born at term it usually only starts to show after age 3. All babies are born with lactase. The exceptions:

• Premature babies may have trouble digesting lactose for a short time, because their gut hasn’t fully developed. This usually gets better as they grow (NHS, AAP).
• Congenital lactase deficiency, where a baby can’t digest lactose from birth, is very rare (AAP). It causes severe diarrhea and needs medical care straight away.
• After a tummy bug, a child can be lactose intolerant for a while (AAP).

Lactose intolerance is not cow’s milk allergy. The AAP says milk allergy involves the immune system and tends to appear in the first year, while lactose intolerance is a digestive problem that usually shows up later and can’t cause anaphylaxis. Ask your doctor before switching formula: see cow’s milk allergy for which formulas help a milk allergy and which don’t.

How doctors tell them apart

Your pediatrician or GP will start by asking what your baby ate, how much, when the symptoms started, and what they looked like. Writing it down helps: the NHS suggests a food and symptom diary.

For a suspected food allergy, a doctor or allergist may arrange a skin prick test or a blood test for IgE antibodies to the food (NHS, ACAAI). ACAAI says neither test is conclusive on its own: a positive result doesn’t always mean a child will react, so an allergist reads the results alongside the history. If it’s still unclear, they may suggest an oral food challenge, where the child eats small, increasing amounts of the food under strict supervision, with emergency medicine on hand.

For a suspected intolerance, a doctor or dietitian may suggest taking the food out for a set time and then bringing it back to see if symptoms return (NHS). For lactose, a hydrogen breath test or a stool test may be used (AAP).

Tests to skip

• IgG food panels. These blood tests report IgG to dozens of foods and claim to find “sensitivities”. The AAAAI says they have never been shown to work, that IgG is likely a normal response to eating a food, and it recommends against using them to diagnose food allergy or intolerance.
• Home intolerance kits. The NHS says home tests that claim to diagnose food intolerances aren’t recommended: there’s limited evidence they’re accurate, and they may suggest avoiding several foods, which can be harmful.

Don’t cut foods out on your own

It’s tempting to drop a food that seems to upset your baby. But the NHS says not to cut foods out of your child’s diet without the advice of a GP or dietitian, because your child could miss out on vitamins and minerals that matter for growth. Don’t experiment by cutting out a major food such as milk (NHS).

If your doctor does suggest taking a food out, they’ll tell you for how long and how to bring it back. After an allergic reaction, never reintroduce the food at home to test it. Your allergist decides if and when, and some foods are reintroduced only under medical supervision.

Keep offering a wide range of other foods, including the common allergens your baby already tolerates: see keeping allergens in the diet.

When to see a doctor

Call your pediatrician or GP if:

• your baby has had any reaction to a food, even a mild one
• tummy symptoms keep coming back after eating
• your baby has blood in their poop, isn’t gaining weight, or has diarrhea that won’t settle
• you’re thinking of changing formula or cutting out a food

Where US and UK advice differs

• Lactose intolerance tests. The AAP describes a short break from milk products, a hydrogen breath test, or stool tests for young children. The NHS lists an elimination diet, blood tests, or a hydrogen breath test, arranged by a GP. Either way, start with your doctor rather than changing your baby’s diet yourself.
• Who to see. In the US, your pediatrician may refer you to an allergist or a pediatric gastroenterologist. In the UK, your GP or health visitor may refer you to a specialist or a registered dietitian.

For how to introduce new foods safely, see how to introduce allergens.