http://www.arabidopsis.org:1555/ARA/NEW-IMAGE?type=PATHWAY&object=RIBOKIN-PWY
looks like you need transketolase (vitamin B1 is a needed cofactor) to use d-ribose.. in the pathway from d-ribose-5-phosphate to D-glyceraldehyde-3-phosphate ..
that's why initially I was better with d-ribose, then I got worse.. seemed to dam up.. and it probably was after it used up a lot of my B1!!!