Galactose is a simple sugar, which belongs to simple carbohydrates (monosaccharides). Galactose is composed of the same elements as glucose, but has a different arrangement of atoms. Galactose is less sweet than glucose and fructose. When combined with glucose (monosaccharide), through a condensation reaction, the result is the disaccharide lactose.
Galactose can bind to glucose to make lactose (in breast milk), to lipids to make glycolipids (for example, molecules that constitute blood groups A, B and AB), or to proteins to make glycoproteins (for example, in cell membranes).
In the human body, most of the ingested galactose is converted to glucose, which can provide 4.1 kilocalories per gram of energy, which is about the same as sucrose. Glucose is the primary metabolic fuel for humans. It is more stable than galactose and is less susceptible to the formation of nonspecific glycoconjugates, molecules with at least one sugar attached to a protein or lipid. Many speculate that it is for this reason that a pathway for rapid conversion from galactose to glucose has been highly conserved among many species.
Galactose metabolism, which converts galactose into glucose, is carried out by the three principal enzymes in a mechanism known as the Leloir pathway. Although the main pathway of galactose metabolism is the Leloir pathway; humans and other species, however, have been noted to contain several alternate pathways, such as the De Ley Doudoroff pathway.
The Leloir pathway consists of the latter stage of a two-part process that converts β-D-galactose to UDP-glucose. The initial stage is the conversion of β-D-galactose to α-D-galactose by the enzyme, mutarotase (GALM). The Leloir pathway then carries out the conversion of α-D-galactose to UDP-glucose via three principal enzymes: Galactokinase (GALK) phosphorylates α-D-galactose to galactose-1-phosphate, or Gal-1-P; Galactose-1-phosphate uridyltransferase (GALT) transfers a UMP group from UDP-glucose to Gal-1-P to form UDP-galactose; and finally, UDP galactose-4’-epimerase (GALE) interconverts UDP-galactose and UDP-glucose, thereby completing the pathway.
The main dietary source of galactose is lactose mostly found in dairy products. The lactose is digested to its respective constituents, galactose and glucose when ingested. Other sources of galactose are natural foods like fruits, vegetables, nuts, grains, fresh meats, eggs and milk.
Galactose is also synthesized by the body, where it forms part of glycolipids and glycoproteins in several tissues; and is a by-product from the third-generation ethanol production process (from macroalgae).
Galactose is not an essential nutrient, which means you do not need to get it from food to be healthy; galactose can be synthesized in the human body from glucose in a process known as lactation. In human lactation, glucose is changed into galactose via hexoneogenesis to enable the mammary glands to secrete lactose. However, most lactose in breast milk is synthesized from galactose taken up from the blood, and only 35±6% is made from galactose from de novo synthesis. Glycerol also contributes some to the mammary galactose production.
Galactose like other sugars can promote dental caries.
Most of the absorbed galactose enters the liver, where it is mainly converted to glucose contributing to increases in blood sugar levels though not at the same rate as ingesting pure glucose.
The World Health Organization recommends that both adults and children reduce the intake of free sugars to less than 10% of total energy intake. A reduction to below 5% of total energy intake brings additional health benefits, especially in what regards dental caries (cavities in the teeth).
Sugar found naturally in milk, fruit and vegetables doesn't count as free sugars. We don't need to cut down on these sugars, but remember that they are included in the "total sugar" count.