  The loop corrections to the couplings of Higgs bosons to quarks and charged
leptons are calculated within supersymmetric versions of the Standard Model,
extended by a gauge singlet. The effective couplings of the $SU(2)_L$ doublet
and singlet Higgs bosons to quarks and leptons, induced by sfermion loops, are
derived. Analytic expressions for the case of generic sfermion flavour mixing,
including the complete resummation of all chirally-enhanced contributions are
presented. These results are important in scenarios in which the mixing between
singlet and doublet components of Higgs bosons is small, and the (pseudo)
scalar component of the doublet is light. The calculated loop effects can have
important consequences in flavour physics, especially for $\Delta F=2$
processes.
