Tagatose is not really a polyol--it is a sugar. However, it is not efficiently absorbed and digested by humans, so it behaves more like a polyol when consumed.
Tagatose is isomeric with fructose, differing only in the stereochemistry at carbon atom 4.
Molecular formula: C6H12O6
Molecular weight: 180.16
Tagatose has water solubility of about 0.2 g per mL.
The Joint FAO/WHO Expert Committee on Food Additives (JECFA) has determined the "Laxative Threshold Value" (LTV) for a number of polyols and low-digestible sugars. Tagatose is one of the less laxative polyols, with an LTV of 40 grams per meal. See my essay "Polyols--Digestive Issues" for a discussion of the potential gastrointestinal effects of polyol consumption.