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Lactose

Lactose is a sugar of animal origin, found in mammals’ milk. This disaccharide consists of a glucose molecule and a galactose molecule. Lactose is digestible when lactase is present, an enzyme that remains in the human body even after weaning. If this enzymatic activity declines or is interrupted, it causes lactose intolerance.
AL032-03 lait
© Shutterstock / kubais

A source of energy for infants

Lactose is a disaccharide sugar of animal origin, synthesised from two monosaccharides, namely glucose and galactose. It can be found in mammal’s milk. The highest concentration is in human milk (7%) and the lowest in reindeer milk (2.5%).

AL032-03 Structure chimique lactose
© Shutterstock / chromatos - Molecular structure of lactose


Lactose plays an essential role in the initial phase of baby mammals’ lives, except for pinnipeds (sea lions, walruses, etc.) whose milk does not contain any lactose. An intestinal enzyme called lactase enables us to digest lactose by hydrolysing it, which means it breaks down this disaccharide into two monosaccharides, glucose and galactose. These pass through the intestinal wall and into the bloodstream. Lactase activity declines after weaning and lactose can no longer be tolerated. Humans are the only mammals where one quarter of the world population is able to digest lactose in adulthood. The remaining three quarters show a decline in lactase activity and suffer from lactose intolerance. Undigested lactose descends into the large intestine and, once in contact with bacteria, it ferments. This process produces gas and uses up water, leading to bloating, colic and diarrhoea.

Lactose-free but not sugar-free

The amount of lactose in dairy products varies depending on the transformation and maturation process used. Although milk contains the most lactose, hard cheeses have none. Nowadays, we can buy ‘lactose-free’ cow’s milk, yoghurt and cream. During the production process, an enzymatic process breaks down the lactose into its two components, glucose and galactose, so that the small intestine can easily absorb them. Owing to the fact that glucose has greater sweetening power than lactose, such lactose-free products have a slightly sweeter taste.

We can also ingest lactase if and when necessary, in the form of capsules, chewable tablets or powder, preferably before or during a meal.

La lactase peut aussi être ingérée en cas de besoin ponctuel sous forme de capsules, de tablettes à mâcher ou de poudre, et elle devrait être prise avant ou pendant le repas.

Calorific value

Lactose has the same calorific value as galactose and glucose, i.e. 4 kcal per gram.

Mutant humans

Scientists have shown that lactase activity does not depend on the presence of lactose in a person’s diet. Therefore, there is a theory that the persistence of lactase activity is due to a genetic adaptation to milk consumption.

Dairy farming and the appearance of a genetic mutation in human beings are thought to have occurred simultaneously: The totally distinct populations of Europe and some regions of Africa each separately developed the ability to digest milk in adulthood. As individuals with persistent lactase activity proved hardier, natural selection ensured the propagation of this mutant gene within the populations. Thus, dairy farming and milk tolerance are thought to have evolved side by side.