We extend the three-layer model for energy loss calculation to include many-body effects in an ab initio framework. The electron energy loss spectrum of the C(100)2×1 surface is calculated and compared with the existing experimental results. We show how many-body effects, namely self-energy, local-fields, and electron-hole interaction, deeply influence the dielectric response and how their inclusion is essential to have good agreement with the experiment. A strong anisotropic behavior of local-field effects on the dielectric response has been observed, while the inclusion of the electron-hole attractive interaction produces a surface-state exciton with binding energy of about 1 eV, confirming a recent theoretical-experimental study of the optical spectra of this surface.