Cow's milk protein allergy and lactose intolerance: how the mechanisms, symptoms and when to see a doctor differ

One is the immune system mistaking something as a threat; the other is a shortage of the enzyme that digests lactose

Cow's milk allergy is when the immune system treats the proteins in milk as an "enemy". The body makes antibodies against these proteins. When it meets them again, it releases substances such as histamine, causing reactions in the skin, the digestive tract and even the airways. The problem lies in the immune system, and it involves the proteins in milk.

Lactose intolerance is a digestive problem. Milk contains lactose, which needs to be broken down by the enzyme lactase in the small intestine before it can be absorbed. When there is not enough lactase, lactose reaches the large intestine and is fermented by bacteria, producing gas and water. This leads to bloating, rumbling in the gut and diarrhoea. The problem lies in the enzyme, and it has nothing to do with protein.

This basic difference determines two things: with cow's milk allergy, even a very small amount can cause a reaction; with lactose intolerance, how severe the symptoms are usually depends on how much lactose was eaten, and small amounts are often tolerated.

How soon symptoms appear: minutes to two hours, or half an hour to several hours

Reactions in cow's milk allergy can be fast. Immediate reactions appear within minutes to two hours after drinking milk, commonly raised welts on the skin, swelling of the lips, or vomiting. Some are delayed, appearing slowly over hours to days, such as repeated eczema or blood-streaked stools.

Symptoms of lactose intolerance usually appear half an hour to several hours after eating food containing lactose, mainly as stomach discomfort. NHS information notes that symptoms can start within minutes to a few hours, and there is considerable variation between people.

Timing is only a clue, not a basis for diagnosis. Within the same time window, either problem can occur, so it is also important to look at which body system the symptoms affect.

Comparing symptoms: skin, blood in stool, vomiting, or bloating, passing gas, diarrhoea

Cow's milk allergy more often involves the skin and the whole body. Infants may have worsening eczema or a hives-like rash. In the digestive tract, there may be repeated vomiting, diarrhoea or blood streaks in the stool. In severe cases there is swelling of the lips and eyelids, wheezing, or poor responsiveness. These severe reactions need emergency treatment.

Lactose intolerance is centred on the stomach: bloating, rumbling in the gut, passing a lot of gas, abdominal pain, loose stools or diarrhoea. NHS information also notes that some people have symptoms outside the digestive tract, such as headache, tiredness, or joint or muscle aches, but not everyone has these.

A simple way to observe: if symptoms are mainly rash, swelling, blood in the stool or projectile vomiting, think of allergy; if they are mainly gas, passing wind and sour-smelling loose stools, it is more like lactose intolerance. But the two can exist together, and a doctor needs to consider the history to judge.

Management differs: avoid the protein, or control the amount of lactose

The core of managing cow's milk allergy is avoiding foods containing cow's milk protein, including milk, yoghurt, cheese and butter, as well as processed foods containing milk powder, whey protein or casein. For infant formula, a doctor should assess the situation and choose a replacement; do not switch to soy milk or goat's milk on your own.

Managing lactose intolerance means managing lactose intake. Most people do not need to avoid it completely; they can take small amounts spread out, take it with meals, or choose low-lactose dairy products. A doctor may suggest avoiding lactose-containing foods for a short time to observe symptoms, or doing a hydrogen breath test to assess digestion.

For both, long-term wide-ranging restriction without medical advice is not recommended. This is especially important for children who are growing: reducing dairy intake needs a nutritional replacement plan, and a doctor or nutrition professional should be involved.

When to see a doctor: do not wait if you see these signs

Seek medical care as soon as possible, and call 120 (the emergency number in China) if necessary, if any of the following occur: sudden swelling of the lips, tongue or throat; rapid breathing, wheezing, or a feeling of being choked; tightness in the throat or difficulty swallowing; blue, grey or pale skin, tongue or lips; sudden severe confusion, drowsiness or dizziness; a child who is floppy, responds poorly, or cannot lift their head. These may be signs of anaphylaxis.

If there are no such emergencies but symptoms keep coming back, you should also see a doctor: repeated rash or worsening eczema after feeding; blood streaks in the stool; repeated vomiting; bloating, abdominal pain or diarrhoea that does not improve for three weeks; a clear change in bowel habits lasting three weeks.

Where to go: if cow's milk allergy is suspected, you can see the allergy department or paediatrics; if bloating and diarrhoea are the main problem and lactose intolerance is suspected, you can see gastroenterology or paediatric gastroenterology. Bringing a record of foods and symptoms will be more helpful than a verbal description.

This article is for general information only and is not medical advice. Please follow your doctor's instructions.