An air supply system is configured to provide cooling air with reduced heat pickup to a turbine rotor of a gas turbine engine. The system comprises a first cooling passage extending between a hollow airfoil and an internal pipe extending through the airfoil. The airfoil extends through a hot gas path. A second cooling passage extends through the internal pipe. The coolant flowing through the second cooling passage is thermally isolated from the airfoil hot surface by the flow of coolant flowing through the first cooling passage. The first and second cooling passages have a common output flow to a rotor cavity of the turbine rotor where coolant flows from the first and second cooling passages combine according to a predetermined ratio.