They make stuff called NO2 Black that expands your capilaries to help with blood flow to the muscles and another supplement called VO2 max that supposedly increases you oxygen intake. I don't know if they are safe or legal. I am not suggesting them just letting you know that they are out there at GNC and Vitamen shop. Most of the really good runners on these forums seem to recommend good diet and hydration.
On another note yet still nutrition related...Does anyone know a supplement to help carb load for a race? I eat whole wheat pasta for three days before my really long runs. I saw someone had posted something about carbohydrate powder or something. I was wondering if there is a more convenient method of carb loading then eating lots of pasta..Thanks.