Sorbitol is not a sugar, since it lacks a keto or aldehyde group.
It is normally a minor component of dietary carbohydrates. Sorbitol is also formed by metabolism from glucose in the polyol pathway. Sorbitol converted into fructose. NADPH is used in the first step and NAD+ is used in the second.
Aldose reductase can reduce galactose into galactitol. Elevated level of galactose in the blood causes galactitol to accumulate in the lens and causes cataract. Conversion of glucose to fructose via the polyol pathway occurs in the seminal Vesicles in male. Fructose is found in the sperm fluid. Sperm cells require fructose to sustain their motility
Mannose is the C2 epimer of glucose. Mannose is a substrate for hexokinase which converts it into Mannose 6-phosphate. An enzyme similar to phosphoglucose isomerase, phosphomannose isomerase isomerizes mannose 6-phosphate into fructose-6-phosphate. Fructose 6-phosphate is the substrate for PFK-1.