  Recent experimental and theoretical works have shown that giant fluctuations
are present during diffusion in liquid systems. We use linearized fluctuating
hydrodynamics to calculate the net mass transfer due to these non equilibrium
fluctuations. Surprisingly the mass flow turns out to coincide with the usual
Fick's one. The renormalization of the hydrodynamic equations allows us to
quantify the gravitational modifications of the diffusion coefficient induced
by the gravitational stabilization of long wavelength fluctuations.
