  We discuss the influence of the vacuum fluctuations of \bar{q}q pairs on
low-energy constants and condensates. The analysis of the Goldstone boson
masses and decay constants shows that the three-flavour condensate and some
low-energy constants are very sensitive to the value of L_6, which measures the
Zweig-rule violation in the scalar channel. A chiral sum rule based on
experimental data in this channel is used to constrain L_6, confirming a
significant decrease between the two- and the three-flavor condensates.
