Ribose is a special sugar made in your body. It doesn't come from food; it helps build the energy blocks of your body. Of all the things you can do to combat the effects of knee-dragging fatigue, taking a daily ribose supplement is the one that seems to really turbo-charge some people who have diseases associated with low energy. The only side effect is that some people feel too much energy, if that's possible.
The data aren't good enough to recommend ribose for all of us. But if you want to give it a try, start with 500 mg three times a day for a week or so until you get used to the taste (or find a smoothie, coffee or tea to put it in). Then go to 5 grams three times a day for three weeks to get a sense of the effect. Then you can scale back to 5 grams twice a day.
By the way, since we know you're wondering: Each 5-gram scoop contains only 20 calories, since ribose isn't metabolized as a sugar. In fact, since it is a bit sweet, you might think of it as a sugar substitute.
As an aside, ribose has been shown to relieve fatigue, soreness and stiffness after exercise, and some professional athletes have reported muscular benefits after taking ribose. But again, the data are too weak to say it does or doesn't work well, since the studies just haven't been done.