For D-Ribose, Dr. Myhill's website suggests 15gm per day:
http://drmyhill.co.uk/wiki/D-ribose
Not sure how this works, if one is to pursue the paleo/ketogenic diet that she recommends. One has to keep daily carb intake to under approx. 40 or 20 grams (I think it varies for each person) in order to get into ketosis. Some people report an energy boost when they shift into ketosis. I assume 20-30 grams of D-Ribose per day, plus whatever other carbs can't be avoided, would prevent ketosis. (Not knocking anyone here; I'm just personally trying to get into ketosis to see if it lives up to all the hype...)
Thanks for the discussion. I just found that the D-Ribose I've been using has Dextrose in it, increasing the carb load unfortunately. I just ordered some pure D-Ribose powder, with the goal of reducing carbs...