Galactose, the milk sugar half, also helps decide your blood type
Split lactose and you get two simple sugars: glucose and galactose. The second one differs from glucose by the flip of a single group of atoms, yet our bodies cannot burn it directly. Galactose also turns up in nerve tissue, earning the nickname brain sugar, and it is part of what distinguishes blood groups A, B and O.
Chemically, galactose is a six-carbon sugar that mirrors glucose except at its fourth carbon. It is a white solid that dissolves easily in water and tastes somewhat less sweet: about 80 to 90% as sweet as glucose and roughly 65% as sweet as table sugar. Its name comes from the Greek for milk, just as lactose comes from the Latin, and both effectively mean milk sugar. Besides dairy, it occurs in avocados, sugar beets and plant gums, and the body makes its own for use in glycolipids and glycoproteins.
Before the body can use galactose for energy, it has to convert it into a form of glucose. The main route is the Leloir pathway, three enzymes working in sequence. Glucose is more stable and less prone to sticking randomly onto proteins and fats, which may be why rapid conversion has been conserved across so many species. People with galactosemia inherit a faulty enzyme in this pathway, so even small amounts are harmful to them.
In breastfeeding, the mammary glands need glucose and galactose in equal measure to make lactose. One study found that most of both sugars in the milk came straight from blood glucose, with a smaller share built from scratch out of smaller molecules. On blood cells, galactose units form part of the ABO antigens: A and O markers carry two, B markers three. A pair of linked galactose units known as alpha-gal is found in mammal meat and can trigger allergy, sometimes set off by lone star tick bites.
The sugar's story began in 1855, when E. O. Erdmann noticed that breaking down lactose produced something besides glucose. Louis Pasteur isolated it in 1856, Marcellin Berthelot gave it its current name in 1860, and in 1894 Emil Fischer and Robert Morrell worked out its structure.
Source: Galactose
