What does lactose mean?
Lactose is the sugar naturally found in milk and dairy products. It is a disaccharide, meaning that each molecule consists of two linked sugar units: glucose and galactose. During digestion, the enzyme lactase separates those units so they can be absorbed normally. People whose systems produce too little lactase can develop lactose intolerance, especially when drinking large amounts of milk. The word also appears on ingredient labels, where “lactose-free” indicates that the sugar has been removed or broken down. Although it is often discussed in a medical context, lactose itself is simply the principal carbohydrate of milk, not a protein. Understanding it helps distinguish a digestion problem caused by reduced enzyme activity from other reactions to dairy products. The term is thus both a precise biochemical substance and a familiar food component.
nounA naturally occurring disaccharide composed of glucose and galactose, found in milk and dairy products.
- The naturally occurring disaccharide composed of glucose and galactose that forms the principal sugar in mammalian milk.
"Many people with lactose intolerance need to limit their intake of lactose."
"People with lactose intolerance may experience digestive discomfort after consuming large amounts of lactose."
"Lactose is one of the carbohydrates naturally present in milk and dairy products."
The plural is rare; lactose is normally used as a mass noun in food, nutrition, and biological contexts.
"Different lactoses can be distinguished by analytical methods."
Lactase persistence let some pastoral ancestors keep drinking milk as adults, turning a digestive enzyme into a genetic lifeline.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does lactose come from?
The scientific term was formed from the combining form lacto-, meaning “milk” and ultimately derived from Latin lac, lactis (“milk”), together with -ose, a chemical suffix used in naming sugars. The same Latin milk root underlies English cognates including lactic, lactate, lactase, and lactation.
Related word forms
How lactose is actually used
Lactose is standard in biochemistry, food science, nutrition, and medicine. In general contexts, “milk sugar” is often clearer. Lactose intolerance is a digestive malabsorption problem and is not the same as a milk allergy.
Easily confused with lactose
Lactase is the enzyme that breaks lactose down into glucose and galactose, whereas lactose is the sugar itself.
Lactate refers to a salt or ion derived from lactic acid, not to the milk sugar lactose.