  Fits of the p^4 covariant SU(2) baryon chiral perturbation theory to lattice
QCD nucleon mass data from several collaborations for 2 and 2+1 flavors are
presented. We consider contributions from explicit Delta(1232) degrees of
freedom, finite volume and finite spacing corrections. We emphasize here our
Nf=2+1 study. We obtain low-energy constants of natural size that are
compatible with the rather linear pion-mass dependence of the nucleon mass
observed in lattice QCD. We report a value for the pion-nucleon sigma term of
41(5)(4) MeV for the 2 flavor case and 52(3)(8) MeV for 2+1 flavors.
