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Lactose intolerance vs milk allergy

Dr Roger Henderson
Reviewed by Roger HendersonReviewed on 29.04.2024 | 7 minutes read
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It’s easy to get confused between an intolerance to lactose, a component of cow’s milk, and an allergy to milk. Both are very common, but each has a different body response, and therefore needs a different approach to treatment.

We’ll take you through the key differences, so you know the signs to spot if you or someone you know shows symptoms and will give you advice on what to do.

Doctor’s advice

What's the difference between an allergy and intolerance?

A food allergy is a reaction by the immune system, even if a small amount is consumed. The body's immune system mistakenly thinks proteins in food are a threat and as a result, reacts to get rid of it, causing symptoms which can be severe or life-threatening.

If you have a food intolerance, it affects the digestive system. You may be able to eat a small amount of the offending food and not have any symptoms. It can also be possible to prevent a reaction by taking medication. For example, if you have a lactose intolerance, you could take lactase enzyme pills (Lactaid) to help support digestion. There is also lactose-free milk.

A food intolerance is different from an allergy and is not life-threatening like an allergy can be.

What is a milk allergy?

A milk or dairy allergy provokes the immune system to react to one of the milk proteins like they’re a dangerous invader. Allergy symptoms are released.

Cows' milk allergy can cause a wide variety of symptoms including:

  • Skin reactions like an itchy red rash, swollen lips, face and around the eyes.
  • Tightness in the throat.
  • Wheezing.
  • Digestive problems such as a stomach ache, vomiting, diarrhoea or constipation.
  • Symptoms similar to hay fever like a runny or blocked nose.
  • Eczema that doesn't clear up even with treatment.

A milk allergy is a very common allergy, especially in children under the age of three. Around 2% are allergic to milk with most infants developing an allergy before their first birthday. Children will usually grow out of it by the time they turn six.

What is lactose intolerance?

Lactose is a type of sugar found in cow’s milk and dairy products. Most of us can process and absorb this as it passes through the gut with the help of an enzyme called lactase.

Lactose intolerance is typically the result of your body not being able to produce a sufficient amount of lactase.

Lactase is an enzyme (a protein that causes chemical reactions) usually produced in your small intestine. It’s important as it helps you to digest lactose. If your body doesn’t produce enough lactase, then you have a lactase deficiency and are lactose intolerant.

Undigested lactose causes symptoms when it’s broken down in the bowel by bacteria. This breakdown causes unwanted byproducts, like gas and bloating.

Symptoms of lactose intolerance can be similar to a dairy allergy but don’t impact the skin. They include:

The severity of symptoms can vary depending on the individual or the amount of lactose they’ve had.

Some people may be able to have some lactose such as a glass of milk without triggering any symptoms. Others may be unable to have even the smallest amount.

Types of lactase deficiency

There are four different types of lactase deficiency that can cause someone to generate a lactose intolerance. They are:

Primary lactase deficiency

70% of all adults are affected by a primary lactase deficiency. It is also known as lactase non-persistence and is the most common type.

It’s caused by an inherited genetic fault passed down by a child’s parents. It develops when your lactase production decreases as a result of your diet being less dependent on milk and dairy products. It usually happens after the age of 2, when a child is no longer being breastfed or bottle-fed. It’s not always easy to spot symptoms and sometimes you won’t realise you have a primary lactase deficiency until adulthood.

Secondary lactase deficiency

Secondary lactase deficiency is a transitory condition that occurs from an injury to the small intestine. It’s the most common cause of lactose intolerance in the UK.

It can develop at any age but is most common in babies and children. It’s possible as an adult to develop a secondary lactase deficiency even without something else causing it as your body’s lactase production decreases with age.

Possible causes of secondary lactase deficiency or symptoms are:

  • Chemotherapy - a cancer treatment
  • Coeliac disease - an autoimmune condition where you react to eating gluten which can damage the bowels and cause stomach pain.
  • Crohn's disease – a long-term condition that causes inflammation of the lining of the digestive system
  • Gastroenteritis – a gut infection that causes abdominal pain, diarrhoea and nausea or vomiting.
  • An extended course of antibiotics
  • Ulcerative colitis – a condition where there is inflammation of the large bowel and rectum.

Secondary lactase deficiency can sometimes be improved by treating the cause.

The reduced production of lactase in secondary lactase deficiency can be both temporary and a lifelong issue. If it's caused by a long-term condition, this can result in it being permanent.

Developmental lactase deficiency

Babies that are born prematurely have a short-term lactose intolerance due to the lactase-expressing cells in the small intestine not being fully developed. Developmental lactase deficiency will typically last for a short time after the baby is born and will get better as they get older.

Congenital lactase deficiency

Congenital lactase deficiency is an extremely rare hereditary condition that can be found in newborn babies.

It’s an inherited genetic fault where the affected baby's small intestine doesn’t produce enough lactase enzyme and in some cases produces none at all.

Congenital lactase deficiency is an autosomal recessive inheritance pattern meaning it is passed onto a child by both parents who have the abnormal gene.

How can I tell if my child is lactose intolerant or has a milk allergy?

Lactose intolerance develops in slightly older children and adults – compared to cow’s milk allergy, it’s much less common in young children. With an intolerance, undigested lactose passes through, and the activity of gut bacteria to break it down can cause bloating, excessive wind, tummy pain and diarrhoea. This typically occurs a couple of hours after eating dairy products, and sufferers will usually have eaten a fair amount, compared to cow’s milk allergy, where only a small amount can provoke a reaction.

Cow’s milk allergy occurs much younger, usually in babies less than 1-year-old. It’s more common in formula-fed babies, if they aren’t on a special formula, but can occur in breastfed babies as cow’s milk protein passes from the mother's diet.

Symptoms start when the formula or dairy food is first introduced, and babies may get any combination or severity of different problems. Digestive symptoms are common to both allergy and intolerance, but there is less bloating in allergy, and more likely colic or tummy pain, vomiting, diarrhoea or constipation. Skin problems are common, such as a red itchy rash, swelling around the face, lips or eyes, and hay fever symptoms (a blocked or runny nose).

Symptoms may be immediate, within minutes of taking cow’s milk, or delayed, so several hours or even days afterwards. Many children grow out of this allergy – usually by the time they are 5 years old – it's uncommon for this allergy to persist into adulthood.

While we’ve referred to cow’s milk here, formulas based on goat’s milk, or even drinking whole goat’s milk, is not a viable alternative if your child has an allergy. It contains similar proteins, so they are just as likely to react to this.

When should I speak to a doctor?

If you spot any of the above symptoms in your baby or child, you should book an appointment with your doctor. It’s particularly important when it comes to young children. If you or your health visitor notices that your baby is not growing as quickly as expected (they drop below their centile line on the weight or height chart), if your baby or child has any persistent gut symptoms or persistent or recurring rashes.

It is helpful to write down a food and symptom diary for your child as this can help inform the doctor when you see them and help with determining the diagnosis. For babies, it’s good to get them weighed and measured with your health visitor prior to the appointment, and take your red book with you.

If your doctor agrees that an intolerance or allergy is likely, they may refer you to a specialist allergy service. This is likely to involve a children’s dietician, who will talk you through how to avoid dairy products, and reassess regularly.

At some point, they may suggest reintroducing dairy too, but they will take you through this step-by-step when it comes to it. With a diagnosis, you may be able to get specialist lactose-free or hypoallergenic formula prescribed, and it’s best to only use these under medical supervision.

Can it be life threatening?

Lactose intolerance is never life threatening. In very rare cases, a cow’s milk allergy can be extremely serious. This is because in a severe food allergic reaction there is the potential for something called anaphylaxis to develop, which is life threatening and can develop quickly. They may be coughing, wheezing or have noisy or fast breathing. You may see the face, lips and tongue swelling, but the risk is if the throat swells, blocking the airways.

Any signs of sudden onset swelling of the tongue, lips leading to breathing difficulties should be treated as an emergency, you should call 999 for an ambulance.

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Dr Roger Henderson
Reviewed by Roger Henderson
Reviewed on 29.04.2024
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