The necessity of calculating reliable nuclear matrix elements for neutrinoless double beta decay has further stimulated the research on the mechanisms responsible for the renormalization of the beta-decay operator. We approach this problem starting from a nuclear Hamiltonian and electroweak currents derived consistently by way of the chiral perturbation theory, and adopt the many-body perturbation theory to derive effective Hamiltonian and operators for nuclear shell model calculations. I will present recent results obtained for medium-mass nuclei and discuss the contribution of both electroweak currents and many-body correlations to the quenching of the axial coupling constant gA.