Violation of the Equivalence Principle in Modified Theories of Gravity

We show that the metric in $f(R)$ theories of gravity in Palatini formalism can be solved as the product of a rank-two tensor times a scalar function which is very sensitive to the local energy-momentum densities. This local dependence of the metric generates new gravitationally-induced microscopic interactions, which eventually would lead to self-accelerated test body trajectories. These facts make very unlikely the viability of Palatini $f(R)$ models designed to change the late-time cosmic evolution.