Reply To: I am a collegiate runner that runs a high weekly mileage (averaging about 65 mile per week when healthy).I began feeling very weak on runs in October and I was also feeling very tired and I would get headaches and I was slightly depressed. After several blood tests and doctor appointments we finally found that my Vitamin D (25, Hdroxy) was low on December first (20.8ng/ml). After 6 weeks of prescription strength vitamin D (unfortunetely D2 Drisdol was prescribed). I did not feel significantly better and was still taking off time from running and I got my blood tested on on March 9th and was still only 21.5ng/ml. I am now going for a few runs a week, but still don't feel that great. How much Vitamin D3 should I be taking/ is healthy to take?