Galactose is a simple sugar found naturally in various foods. It plays a role in human metabolism and is a component of larger sugar molecules.
What is Galactose?
Galactose is a monosaccharide, meaning it is a single sugar unit. It shares the same chemical formula as glucose (C6H12O6) but has a slightly different arrangement of atoms, making it an isomer. This structural difference impacts how the body processes each sugar. Galactose does not typically exist freely in large amounts in nature; instead, it is most commonly linked with glucose to form lactose, often referred to as milk sugar.
Its presence in lactose requires enzymatic breakdown before the body can utilize it. The bond between glucose and galactose in lactose is broken by the enzyme lactase during digestion.
Where is Galactose Found?
Galactose is primarily found in the diet as a component of lactose, the disaccharide present in milk and other dairy products. When consuming dairy, lactose breaks down into glucose and galactose in the small intestine. This makes dairy products like milk, yogurt, and cheese the most common dietary sources.
Smaller amounts of galactose are also found in certain fruits and vegetables, though not as part of lactose. Some legumes, such as dried beans and peas, contain free galactose. Fermented dairy products may also contain some free galactose due to the breakdown of lactose by bacterial enzymes.
How the Body Uses Galactose
Once ingested, galactose is absorbed from the small intestine and transported to the liver. In the liver, a series of enzymatic reactions, known as the Leloir pathway, converts galactose into glucose-1-phosphate. This compound then converts to glucose-6-phosphate, which enters glycolysis for energy production. The liver is the primary site for this conversion.
Galactose also serves as a building block for complex carbohydrates, including glycolipids and glycoproteins. These molecules are components of cell membranes, particularly in nerve tissue and the brain. Glycolipids and glycoproteins are involved in cell recognition, cell signaling, and maintaining cellular structural integrity.
Galactose and Health Conditions
Problems with galactose metabolism can lead to a group of genetic disorders known as galactosemia. Classical galactosemia, the most severe form, results from a deficiency in the enzyme galactose-1-phosphate uridylyltransferase (GALT). This enzyme converts galactose-1-phosphate into UDP-galactose. Without a functional GALT enzyme, galactose and its metabolites, like galactose-1-phosphate, accumulate in the body.
The accumulation of these toxic substances can lead to severe health issues, including liver damage, kidney failure, cataracts, and brain damage. Symptoms often appear shortly after birth when an infant begins consuming milk, including breast milk or lactose-containing formula. Early diagnosis through newborn screening is important, as immediate implementation of a strict galactose-free diet can prevent or minimize many severe complications. Lifelong dietary management, avoiding all sources of lactose and free galactose, is necessary for individuals with classical galactosemia.
References
1. Galactose metabolism. ScienceDirect. [https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/galactose-metabolism](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/galactose-metabolism)
2. Galactose. Wikipedia. [https://en.wikipedia.org/wiki/Galactose](https://en.wikipedia.org/wiki/Galactose)
3. Galactosemia. National Institute of Diabetes and Digestive and Kidney Diseases. [https://www.niddk.nih.gov/health-information/digestive-diseases/galactosemia](https://www.niddk.nih.gov/health-information/digestive-diseases/galactosemia)
4. Galactosemia. MedlinePlus. [https://medlineplus.gov/galactosemia.html](https://medlineplus.gov/galactosemia.html)