Some info I found about it:

<<D-Ribose is a sugar (derived from glucose) that has been shown to boost levels of the energy compound ATP. Some studies have shown that CFS patients have lower than normal levels of ATP and D-Ribose could be used to increase energy production in the muscles and heart.
The increased levels of ATP may help relieve fatigue by provide the muscles with extra energy. D-Ribose also helps the body to recycle ATP, and use its own energy it has more efficiently.>>

It goes on to describe a study of 36 patients, most of whom showed improvement. (At http://www.chronicfatiguetreatments...ments/d-ribose/ .)

There is a little bit of discussion of effects on blood sugar in the replies.