They initially thought that mannose might interfere with glucose uptake, since the two sugars use the same transporter to get into cells. But no, that wasn’t it.

Mannose enters cells using the same receptors as glucose. Once within the cell, it accumulates as mannose-6-phosphate. As this metabolite builds up, it slows down glucose metabolism.

Mannose, the least sweet of the sugars, formed longer and weaker hydrogen bonds with water than glucose or fructose. Fructose, the sweetest of the sugars, formed the shortest and strongest ...