Lactose, or milk sugar, is a disaccharide composed of galactose and glucose and has the molecular formula C12H22O11.

[8] Lactose is hydrolysed to glucose and galactose, isomerised in alkaline solution to lactulose, and catalytically hydrogenated to the corresponding polyhydric alcohol, lactitol.

[citation needed] Infant mammals nurse on their mothers to drink milk, which is rich in lactose.

This enzyme cleaves the lactose molecule into its two subunits, the simple sugars glucose and galactose, which can be absorbed.

[13] By descent, more than 70% of western Europeans can digest lactose as adults, compared with less than 30% of people from areas of Africa, eastern and south-eastern Asia and Oceania.

Depending on ingested dose, combination with meals (either solid or liquid), and lactase activity in the intestines, the caloric value of lactose ranges from 2 to 4 kcal/g.

[15] Its mild flavor and easy handling properties have led to its use as a carrier and stabiliser of aromas and pharmaceutical products.

Infant formula is a notable exception, where the addition of lactose is necessary to match the composition of human milk.

Yeast belonging to the genus Kluyveromyces have a unique industrial application, as they are capable of fermenting lactose for ethanol production.

Surplus lactose from the whey by-product of dairy operations is a potential source of alternative energy.

Lactose is added to tablet and capsule drug products as an ingredient because of its physical and functional properties (examples are atorvastatin, levocetirizine or thiamazole among many others).

[21] In 1700, the Venetian pharmacist Lodovico Testi (1640–1707) published a booklet of testimonials to the power of milk sugar (saccharum lactis) to relieve, among other ailments, the symptoms of arthritis.