  The possible constraints on the poorly determined symmetry part of the
effective nuclear Hamiltonians or effective energy functionals, i.e., the
so-called symmetry energy S(rho), are very much under debate. In the present
work, we show that the value of the symmetry energy associated with Skyrme
functionals, at densities rho around 0.1 fm^{-3}, is strongly correlated with
the value of the centroid of the Giant Dipole Resonance (GDR) in spherical
nuclei. Consequently, the experimental value of the GDR in, e.g., 208Pb can be
used as a constraint on the symmetry energy, leading to 23.3 MeV < S(rho=0.1
fm^{-3}) < 24.9 MeV.
