What is D-mannose? NOW D-mannose is a popular new supplement that supports urinary tract health. Mannose is a type of sugar that occurs in some fruits like the cranberry. People wouldn't ordinarily use a sugar as a type of dietary supplement, but d-mannose has the interesting property of inhibiting the ability of bacteria to adhere to urinary tract walls. Used under the supervision of a doctor, D-mannose may be useful for people who experience recuurent UTI's.1 D-mannose is not metabolized like other simple sugars and so has a negligible effect on blood sugar and insulin levels.