Blood vampires, also known as Sanguinarians, are people who have a need to feed on the blood of others in order to maintain a state of good health. Sanguinarians do not produce a sufficient amount of pranic or "life" energy, and therefore need to replace this energy by drinking blood, which is a high source of pranic energy.
As stated above by Sanura, this is in fact true to the best of my knowledge.
I am a Sang which is short for Sanguinarian and I haven't had blood since I was about 17 and my health is not well at all despite my appetite and diet.
I have found a web site that is very good and it even covers the science behind sang vampires, yes science, some to hard to understand unless your excelent at boilogy and chemistry though the author makes his best attempt to explain in less scientific manor.
I hope to provide some web space for the author to use instead of the traffic jamed, ad zone geocities and hope to hear a reply from hime soon!