  We calculate the amplitude T_1 for forward doubly-virtual Compton scattering
in heavy-baryon chiral perturbation theory, to fourth order in the chiral
expansion and with the leading contribution of the gammaNDelta form factor.
This provides a model-independent expression for the amplitude in the
low-momentum region, which is the dominant one for its contribution to the Lamb
shift. It allows us to significantly reduce the theoretical uncertainty in the
proton polarisability contributions to the Lamb shift in muonic hydrogen. We
also stress the importance of consistency between the definitions of the Born
and structure parts of the amplitude. Our result leaves no room for any effect
large enough to explain the discrepancy between proton charge radii as
determined from muonic and normal hydrogen.
