D-Mannose (also called Mannose or D-mannopyranose) is a hexose or a six-carbon sugar. It is also classified as an aldohexose. It is fermentable monosaccharide and an isomer of glucose. Mannose commonly exists as two different-sized rings, the pyranose (six-membered) form and the furanose (five-membered) form. Formally, D-Mannose is the 2-epimer of glucose and exists primarily as sweet-tasting alpha- (67%) or as a bitter-tasting beta- (33%) anomer of the pyranose form (PMID: 24931670).  Mannose is not an essential nutrient, meaning that it can be produced in the human body from glucose or converted into glucose. Mannose is ~5x as active as glucose in non-enzyamtic glycation, which may explain why evolution did not favor it as a biological energy source (PMID: 24931670).  Mannose occurs in microbes, plants and animals. Free mannose is found in small amounts in many fruits such as oranges, apples and peaches and in mammalian plasma at 50‚Äì100 uM (PMID: 24931670).   More often, mannose occurs in homo-or hetero-polymers such as yeast mannans (alpha-mannose) where it can account for nearly 16% of dry weight or in galactomannans. Coffee beans, fenugreek and guar gums are rich sources of galactomannans. However, these plant polysaccharides are not degraded in the mammalian GI tract and, therefore, provide very little bio-available mannose for glycan synthesis. The digestion of many polysaccharides and glycoproteins also yields mannose. Once mannose is released, it is phosphorylated by hexokinase to generate mannose-6-phosphate. Mannose-6-phosphate is then converted to fructose-6-phosphate, by the enzyme phosphomannose isomerase, whereupon it enters the glycolytic pathway or is converted to glucose-6-phosphate by the gluconeogenic pathway. Mannose is a dominant monosaccharide in N-linked glycosylation, which is a post-translational modification of proteins. N-linked glycosylation is initiated by the transfer of Glc3Man9GlcNAc2 to nascent glycoproteins in the endoplasmic reticulum in a co-translational manner as the protein enters the transport system. Typically, mature human glycoproteins only contain three mannose residues buried under sequential modification by GlcNAc, galactose, and sialic acid. High-mannose-type oligosaccharides have been shown to play important roles in protein quality control. Several intracellular proteins such as lectins, chaperones, and glycan-processing enzymes, are involved in this process. These include calnexin/calreticulin, UDP-glucose:glycoprotein glucosyltransferase (UGGT), cargo receptors (such as VIP36 and ERGIC-53), mannosidase-like proteins (e.g. EDEM and Htm1p) and ubiquitin ligase (Fbs). They are thought to recognize high-mannose-type glycans with subtly different structures. Mannose-binding lectin (MBL) is an important constituent of the innate immune system. This protein binds through multiple lectin domains to the repeating sugar arrays that decorate many microbial surfaces and is then able to activate the complement system through a specific protease called MBL-associated protease-2. Mannose (D-mannose) is used as a nutritional supplement, packaged as "D-mannose", to prevent recurrent urinary tract infections (PMID: 21105658). D-mannose prevents FimH-mediated bacterial adhesion in the urinary tract through a competitive inhibition mechanism. This mechanism is based on the structural similarity between D-mannose and urothelial mannosylated receptors exposed by the epithelium of the urinary tract (PMID: 21105658). When D-mannose is administered in sufficient amounts, it is rapidly absorbed and then excreted by the urinary tract where it saturates bacterial FimH, thereby preventing bacterial binding to urothelial cells.