🧪 Lactose

Lactose is milk sugar, found only in milk. Its molecule is made of two simple sugars, glucose and galactose. Cow's milk contains 3.6 to 5.5% lactose, on average 4.8%. ² According to the USDA, 100 g of whole milk contains 4.81 g. ³

In different milks. For 100 g of mature human milk the USDA lists 6.89 g of sugars. ⁴ In milk, lactose is in true solution together with salts, not in droplets or particles like fat and proteins. ²

How we digest it. Lactose is split in the small intestine by the enzyme lactase. When the intestine cannot digest all the lactose, this is called lactose malabsorption. Not everyone with malabsorption gets bloating, gas or diarrhoea: only those with symptoms are called lactose intolerant. ¹

How common it is. Most infants digest lactose, but many people lose some of this ability after infancy. About 68% of the world's population has lactose malabsorption. It affects most people in Africa and Asia and fewer in northern Europe, where many carry a gene that keeps lactose digestion into adulthood. ¹

How much is tolerated. In 2010 EFSA concluded that there is no single threshold, because tolerance varies. Most people with malabsorption tolerate up to 12 g of lactose at once with no or minor symptoms, and more if spread over the day; some react to less than 6 g. ⁵

Bacteria and sour milk. Lactic acid bacteria split lactose into glucose and galactose and then turn them into lactic acid. That is how milk goes sour. ²

Heating. Above 100 °C lactose reacts with proteins, and milk turns brown and tastes of caramel: this is the Maillard reaction, which lowers nutritional value, notably lysine. Heated milk also forms lactulose from lactose; its amount is used to tell pasteurised, UHT and sterilised milk apart. ²

The label. In the EU, milk and milk products, including lactose, are on the list of allergens that must be emphasised in the ingredients. ⁶ But lactose intolerance is not a milk allergy: an allergy is an immune system disorder. ¹

Lactose and lactase are not the same thing: lactose is the milk sugar made of glucose and galactose, while lactase is the enzyme that splits it in the small intestine. Lactulose is a third thing: it forms from lactose when milk is heated, and its amount is used to tell pasteurised, UHT and sterilised milk apart. Lactose intolerance is not a milk allergy either: an allergy is an immune system disorder, while intolerance means digestive symptoms. Lactose is found only in milk.

On a label, look for “milk”, “milk sugar” or “lactose” in the ingredients. In the EU, milk and milk products, including lactose, are on the list of allergens that must be emphasised. A “lactose-free” claim tells you about lactose only, not about milk protein allergy. You find these products in the dairy aisle of supermarkets and corner shops: milk, yoghurts and cheeses. A 250 ml glass of milk contains about 12 g of lactose.

Why it matters to you. Intolerance does not mean giving up dairy entirely: milk is a main source of calcium and vitamin D. ¹ A 250 ml glass of milk contains about 12 g of lactose, exactly the amount most people with malabsorption tolerate at once. ³⁵