  The general theory of electromagnetic--fluctuation--induced interactions in
dielectric bodies as formulated by Dzyaloshinskii, Lifshitz, and Pitaevskii is
rewritten as a perturbation theory in terms of the spatial contrast in
(imaginary) frequency dependent dielectric function. The formulation can be
used to calculate the Casimir-Lifshitz forces for dielectric objects of
arbitrary geometry, as a perturbative expansion in the dielectric contrast, and
could thus complement the existing theories that use perturbation in
geometrical features. We find that expansion in dielectric contrast recasts the
resulting Lifshitz energy into a sum of the different many-body contributions.
The limit of validity and convergence properties of the perturbation theory is
discussed using the example of parallel semi-infinite objects for which the
exact result is known.
