Based on the Hugenholtz-Van Hove theorem, it is shown that both the symmetry energy E sym (ρ) and its density slope L(ρ) at normal density ρ 0 are completely determined by the nucleon global optical potentials. The latter can be extracted directly from nucleon-nucleus scatterings, (p,n) charge-exchange reactions, and single-particle energy levels of bound states. Averaging all phenomenological isovector nucleon potentials constrained by world data available in the literature since 1969, the best estimates of E sym (ρ 0 ) = 31.3 MeV and L(ρ 0 ) = 52.7 MeV are simultaneously obtained. Moreover, the corresponding neutron-proton effective mass splitting in neutron-rich matter of isospin asymmetry δ is estimated to be (m * n − m * p )/m = 0.32δ.