  Electronic interactions in multiorbital systems lead to non-trivial features
in the optical spectrum. In iron superconductors the Drude weight is strongly
suppressed with hole-doping. We discuss why the common association of the
renormalization of the Drude weight with that of the kinetic energy, used in
single band systems, does not hold in multi-orbital systems. This applies even
in a Fermi liquid description when each orbital is renormalized differently, as
it happens in iron superconductors. We estimate the contribution of interband
transitions at low energies. We show that this contribution is strongly
enhanced by interactions and dominates the coherent part of the spectral weight
in hole-doped samples at frequencies currently used to determine the Drude
weight.
