Galactose is a monosaccharide carbohydrate, or simple sugar, that forms the terminal part of the blood group antigen of individuals with blood group B. Galactose is also present on the red blood cells of those with blood group AB in combination with N-acetylgalactosamine, the terminal carbohydrate of the antigen for individuals with blood group A.
Galactose is readily absorbed and rapidly eliminated following oral administration in humans. For example, following ingestion of 50 grams galactose in men, the plasma galactose concentration increases rapidly, reaches a peak at 1 hour, and subsequently returns to the basal level by 2.5 hours.
Dietary galactose is also rapidly metabolized and extensively incorporated into glycoconjugates. For example, following ingestion of 50 grams galactose in men, 20% appears as additional glucose in blood; in animals, significant amounts of galactose are stored as glucose in glycogen. In healthy fasting subjects, a 7 mg/kg body weight dose (equivalent to about 0.5 gram in a 150-lb human) of galactose was metabolized 3-6% within 1 hour and 21-47% within 5 hours; oxidation kinetics reached a maximum at 90-120 minutes. As much as 40% of ingested galactose is incorporated into glycoconjugates in humans.
Additional ADME data are available from studies with nursing newborns. For example, nursing infants are exposed to galactose naturally present in breast milk. In the lactating breast, galactose is actively transported from maternal blood into mammary gland epithelial cells, where it is used to synthesize lactose and oligosaccharides.Naturally occurring galactose from breast milk also appears in feces of nursing newborns.
A significant amount of dietary galactose is also incorporated into glycoconjugates and glycogen in breast milk. For example, galactose, given orally (27 grams) to a lactating female, was incorporated within 8 hours mainly into lactose and neutral oligosaccharides in breast milk and the urine of a breast-fed infant. However, it was not clear from the study data what the levels of free, unbound galactose were in either breast milk or infant urine. In a similar study, 15 breast-feeding women received 27 grams of galactose orally; during the following 24-36 hours, breast milk as well as urine from nursing infants were collected. The data showed that 7% of the oral dose of galactose appeared in breast milk during the first 2-8 hours, followed by a continual decrease.
There do not appear to be any published studies in animals or humans to indicate that dietary intake of galactose results in adverse biological effects in healthy subjects.