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Mannose is a simple sugar (monosaccharide), so it belongs to simple carbohydrates. It is composed of the same elements (C6H12O6) as glucose, but has a different arrangement of atoms . The chemical form that appears in foods and most supplements is called D-mannose.
The name origin: the Greek manna = food miraculously supplied to the Israelites in the wilderness; -ose denotes sugar.
Is mannose an essential nutrient?
Mannose is not an essential nutrient, which means you do not need to get it from food to be healthy; mannose can be produced in your body from glucose .
Mannose Functions in the Human Body
- In the human body, mannose can be converted to glucose and provide 2-5 kilocalories per gram of energy [nutrition f. labels].
- Mannose contributes to the synthesis of glycoproteins, which mainly appear in the cell membranes .
Dietary Sources of Mannose
Mannose can be found in fruits (peaches, apples, oranges, blueberries, black currants, cranberries), legumes (green beans, kidney beans, lima beans, soybeans), vegetables (cabbage, turnip, tomatoes), ivory nuts, Aloe vera and baker’s yeast [2,3,10,11,12]. The amount of mannose that can be absorbed from these foods in not known [3,4].
Mannose syrup is a term used for konjac gum or glucomannan.
Oral mannose supplements have been successfully used in the treatment of a rare genetic disease called carbohydrate deficient glycoprotein syndrome (CDGS) type 1b [5,6].
Mannose is partially excreted in the urine . Some websites advertise D-mannose as a supplement that can prevent or even treat lower urinary tract infections (bladder, urethra) caused by E. coli bacteria, treat cancer and boost immunity, but no clinical studies on humans have been done so far to support such claims.
Side effects of mannose supplements may include bloating, diarrhea and (when used in high doses) kidney damage  and, maybe, birth defects .
D-mannose supplements should not be used during pregnancy and breastfeeding and in diabetes mellitus .
Cranberry juice, which is high in mannose, is believed by some people to prevent urinary tract infections (UTI), but in one study involving 230 young women, cranberry juice was even less effective than placebo . However, several other clinical trials have shown that cranberry juice may prevent UTI caused by E. coli bacteria [9,14], but the active substance in cranberry juice might be tannins , not mannose. Antibiotics are still a recommended treatment for established urinary infections.