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. Psicose is one of the hexose monosaccharides and classified as a ketose.
Psicose is a ketohexose with a chemical formula of C6H12O6. It is an epimer of fructose. Fructose is also a ketohexose. However, unlike fructose that is abundant in nature and serves as one of the important energy sources, psicose is a low-energy carbohydrate that occurs in small quantities in natural products. It is one of the rare monosaccharides that exist naturally. The naturally-occurring psicose is in the dextrorotary (D-) configuration.
Mass production of D-psicose was first done in Japan when the enzyme D-tagatose 3-epimerase was found to convert fructose to D-psicose.1 Now, it is used as sweetener in beverages, ice cream, and baked goods. It has zero calorie with sweetness of about 70% as that of sucrose.
D-psicose has been observed to suppress the augmentation of postprandial blood glucose by inhibiting and facilitating glucokinase translocation. Furthermore, it reduces the accumulation of abdominal fat by suppressing lipogenic enzymes in the liver.2
1 Itoh, H., Sato, T., & Izumori, K. (1995). "Preparation of D-psicose from D-fructose by immobilized D-tagatose 3-epimerase". J Fermentation and Bioengineering. Elsevier B.V. 80 (1): 101–103.
2 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