  This paper studies a maximal $L^q$-regularity property for nonlinear elliptic
equations of second order with a zero-th order term and gradient nonlinearities
having superlinear and sub-quadratic growth, complemented with Dirichlet
boundary conditions. The approach is based on the combination of linear
elliptic regularity theory and interpolation inequalities, so that the analysis
of the maximal regularity estimates boils down to determine lower order
integral bounds. The latter are achieved via a $L^p$ duality method, which
exploits the regularity properties of solutions to stationary Fokker-Planck
equations. For the latter problems, we discuss both global and local estimates.
Our main novelties for the regularity properties of this class of nonlinear
elliptic boundary-value problems are the treatment of equations with a zero-th
order term together with the analysis of the end-point summability threshold
$q=d(\gamma-1)/\gamma$, $d$ being the dimension of the ambient space and
$\gamma>1$ the growth of the first-order term in the gradient variable.
