Statistics. Statistical analyses were conducted using SPSS software (version 20 SPSS, Inc., Chicago, IL). For normally distributed variables, Student's t-test and one-way ANOVA were used for comparisons, and the data are presented as the mean ± SE. The Mann-Whitney U and Kruskal-Wallis tests were used for comparisons of variables that were not normally distributed. Correlations were assessed with Pearson's correlation coefficient for parametric distributions. Statistical significance was set at P < 0.05.