Monosaccharides are the simplest form of carbohydrates. They may be classified based on the number of constituent carbon atoms. For instance, hexose is a monosaccharide with six carbon atoms. They may also be classified based on the functional group present: aldose and ketose. Tagatose is one of the hexose monosaccharides and classified as a ketose.
Naturally-occurring tagatose is found but only in small amounts in fruits, cacao, and dairy products. Commercially, tagatose is used as a sweetener. It is regarded as tooth-friendly and therefore it is used as an ingredient in certain dental products. Tagatose is artificially produced by initially hydrolyzing lactose to glucose and galactose. The galactose, in turn, is isomerized to D-tagatose through calcium hydroxide.
D-tagatose was found to increase HDL cholesterol levels. Similar to the mechanism employed by D-psicose, D-tagatose can suppress the postprandial blood glucose elevation.1
1 Iida, T. & Okuma, K. (2013). Properties of Three rare Sugars D-psicose, D-allose, D-tagatose and Their Applications. J-STAGE, 13(9): 435-440. https://doi.org/10.5650/oleoscience.13.435