lactose

American  

[lak-tohs] / ˈlæk toʊs /

noun

1. Biochemistry. a disaccharide, C 12 H 22 O 11 , present in milk, that upon hydrolysis yields glucose and galactose.

2. a white, crystalline, sweet, water-soluble commercial form of this compound, obtained from whey and used in infant feedings, in confections and other foods, in bacteriological media, and in pharmacology as a diluent and excipient.

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lactose British  

/ ˈlæktəʊs, -təʊz /

noun

1. Also called: milk sugar.  a white crystalline disaccharide occurring in milk and used in the manufacture of pharmaceuticals and baby foods. Formula: C 12 H 22 O 11

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lactose Scientific  

/ lăk′tōs′ /

1. A white crystalline disaccharide consisting of a glucose and a galactose molecule, found in milk and used in the manufacture of various other foods. Chemical formula: C ₁₂ H ₂₂ O ₁₁ .

2. The inability to digest lactose properly is called lactose intolerance. It is caused by a deficiency of the enzyme lactase and marked by abdominal cramping and other symptoms after ingesting lactose.

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Etymology

Origin of lactose

First recorded in 1855–60; lact- + -ose ²

Explanation

Lactose is the kind of sugar that's found in milk and other foods made from milk. Some people have trouble digesting lactose and avoid eating dairy products. No cheese stick for you! You might learn about lactose in chemistry class, including the fact that it's made of two sugars, glucose and galactose. Lactose is broken down into these simpler sugars by lactase, an enzyme that most of us have plenty of. However, if you're one of the people without enough lactase, eating food with lactose can be unpleasant. The lacto- comes from the Latin lac, or "milk," and the chemical suffix -ose tells you it's a sugar.

Example Sentences

He said tourist food eaten by macaques was "extremely rich in calories, sugar, salt and dairy" - with primates intolerant of lactose found in "hugely popular" ice-cream.

From BBC • Apr. 22, 2026

Shortly after Meadow Lane opened, customers posted online about undercooked chicken nuggets; another shopper, who was lactose intolerant, said that she’d bought chili topped with cheese that wasn’t on the list of ingredients.

From The Wall Street Journal • Feb. 10, 2026

“Being a backtalker is like being lactose intolerant,” she writes in “A Note From Kim.”

From Los Angeles Times • Jan. 30, 2026

It appears in milk and other dairy products when lactose breaks down under heat or enzymatic activity, including during the production of yogurt, cheese, and kefir.

From Science Daily • Jan. 13, 2026

When the sugar source was switched to lactose, the bacteria turned on an entire module of genes—several lactose-metabolizing genes—to digest lactose.

From "The Gene" by Siddhartha Mukherjee

Related Words

• cellulose
• glucose
• starch
• sugar