What is the difference between a milk allergy and lactose intolerance?

These two get used interchangeably in conversation and they are not remotely the same thing. One is an immune system reacting to a protein; the other is a missing digestive enzyme. Getting them the wrong way round in a kitchen is one of the more consequential confusions in home cooking.

Safety

Step by step

1. Understand cow's milk protein allergy.The immune system reacts to the proteins in milk — casein and whey. Reactions can include hives, swelling, vomiting, wheezing and, in severe cases, anaphylaxis. It is most common in infants and many children grow out of it, but it can persist into adulthood. Avoidance must be complete.
2. Understand lactose intolerance.The small intestine produces too little lactase, the enzyme that breaks down milk sugar, so undigested lactose ferments in the gut. The result is bloating, wind, cramps and diarrhoea — unpleasant, but not an immune reaction and not dangerous. Tolerance varies enormously between individuals.
3. Note who is affected.Milk allergy affects a small percentage of infants and far fewer adults. Lactose intolerance is very common worldwide and increases with age, and is much more prevalent in some populations than others. Someone saying they are 'dairy intolerant' most often means the latter.
4. Apply completely different avoidance rules.Milk allergy: no milk protein at all, in any quantity, plus attention to cross-contamination — shared equipment, shared utensils, 'may contain milk' warnings. Lactose intolerance: reduce the quantity, spread it across the day, and use lactase drops or tablets. There is no cross-contamination concern.
5. Know that lactose-free is not milk-free.Lactose-free milk is ordinary cow's milk with the lactose broken down by added lactase. It contains all the milk proteins and is dangerous for someone with a milk allergy. This is the single most important practical point and it is very commonly got wrong.
6. Know which dairy foods are naturally low in lactose.Hard aged cheeses — cheddar, parmesan, gouda — contain very little lactose, and live yoghurt and kefir contain bacteria that help digest it. Many people with lactose intolerance eat these without difficulty. None of that helps someone with a milk allergy, for whom they are all off-limits.
7. Read labels differently for each.For a milk allergy, look for milk emphasised in the ingredients list and for whey, casein, caseinate, ghee, butter, curds and milk solids — and take 'may contain milk' seriously. For lactose intolerance, the ingredients list is a guide to quantity rather than a safety check.
8. Get a diagnosis rather than self-managing.Both conditions are diagnosable, and cutting dairy out on suspicion risks both an unnecessary restriction and, in children, a real nutritional problem. Calcium, iodine and vitamin B12 all need replacing if dairy goes. See a GP, and a dietitian for a child.

Tips

• If someone tells you they are dairy-free, ask which it is. The follow-up question — 'is it an allergy or an intolerance?' — determines whether you need separate equipment or just a substitute.
• Plant milks vary a great deal nutritionally. Choose one fortified with calcium and, ideally, iodine and B12, especially for children.
• Soya and cow's milk allergy sometimes occur together in infants, so a soya substitute is not automatically the answer.
• Butter contains milk protein. It is not a safe fat for a milk allergy, however small the amount.

Common mistakes

• Offering lactose-free milk to someone with a milk allergy — It is standard cow's milk with the sugar broken down. All the protein is still there, so it is exactly as dangerous as ordinary milk for an allergic person. This is the most common and most serious confusion between the two.
• Telling someone with lactose intolerance they cannot have any dairy at all — Most people with it tolerate hard cheese, butter and small amounts of milk without trouble, and unnecessary total avoidance costs them calcium and variety for no benefit. The quantity matters, not the presence.

Questions people ask