Vacuum fluctuations of \bar{q}q and values of low-energy constants

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.