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What Monosaccharides Are Found in Lactose?

By Stephanie Chandler

Lactose is commonly referred to as the "milk sugar" because you can only find it in the milk produced by mammals. Classified as a carbohydrate, lactose serves as the primary source of energy in milk. Lactose consists of two simple sugar molecules bound together -- glucose and galactose. The prevalence of the medical condition known as lactose intolerance makes understanding lactose and the monosaccharides found in lactose important.

Types of Sugar Molecules

The National Institute of Medicine recommends that adults consume 45 to 65 percent of their daily calories in carbohydrates. Substances known as carbohydrates can be further classified into simple sugars also known as monosaccharides, disaccharides and polysaccharides. Scientists classify these sugar molecules based on their molecular structure. All sugar molecules consist of carbon, hydrogen and oxygen, but the structure of the three types of sugar differs.


The simple sugars, which include glucose, fructose, xylose, ribose and galactose possess a linear, unbranched backbone made up of carbon atoms and one carbon and oxygen double bond, known as a carbonyl functional group, and a number of hydroxyl -- hydrogen bound to oxygen -- groups on the remaining carbon atoms. The simplicity of this structure means that your body can absorb and use the energy from simple sugars without the need to break them down into smaller particles.


Because lactose contains a glucose molecule bound to a galactose molecule, it is known as a disaccharide -- meaning two sugar molecules. To utilize the energy provided by a disaccharide sugar, your body must break it down into the two individual sugar molecules and then absorb them through the wall of the small intestine. The cells lining the small intestine produce an enzyme known as lactase that breaks lactose apart into glucose and galactose.


Glucose serves as the main source of energy for most organisms, including humans. When you eat foods that contain glucose, you can immediately use that energy. When you consume other monosaccharides, such as fructose or galactose, your body easily converts them to glucose and then either uses or stores the energy.


Galactose is found in dairy products and sugar beets. In addition to serving as a source of energy through the conversion to glucose, the body uses galactose to incorporate into proteins known as glycoproteins and fats known as glycolipids. These glyconutrients support communication between cells, which in the brain can enhance long-term memory. For this reason, galactose is known as the brain sugar.

The Importance of Lactose

Because milk is an important whole food source of nutrition, the U.S. Department of Agriculture recommends adults consume 3 cups of milk per day. Each 1 cup serving of cow’s milk contains 11 g of lactose. For the 30 to 50 million people in the United States who fail to produce enough lactase to break down the lactose into the monosaccharides, the sugar remains intact as it moves into the large intestine. There bacteria attack the lactose to break it down into smaller particles. This causes excess gas, bloating, abdominal pain and diarrhea.

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