  Using iso-spin symmetry the quark-connected and disconnected contributions to
the hadronic vacuum polarisation in a theory with N_f=2 flavours can be
described as independent correlation functions, respectively. We show how this
allows to use twisted boundary conditions for the connected contribution in
order to improve the q^2-resolution in lattice QCD. Furthermore we derive an
exact relation between the connected and the disconnected contributions at NLO
in chiral perturbation theory. We discuss extensions to theories with more than
2 dynamical flavours.
