The role of compressible uctuations in the energy cascade of fast solar wind turbulence is studiedusing an exact law derived recently for compressible isothermal magnetohydrodynamics and in-situobservations of the THEMIS spacecraft. For the first time, a direct turbulent energy cascade isevidenced over three decades of scales which is signicantly broader than the previous estimatesmade from an exact incompressible law or from a compressible heuristic model. Unlike previousworks, our evaluation gives an energy ux which keeps a constant sign over the inertial range. Aterm-by-term analysis reveals that the dominant contribution to the energy ux comes from purecompressible uctuations. Furthermore, the compressible turbulent cascade rate is shown to providethe adequate energy dissipation required to account for the local heating of the non-adiabatic solarwind.