Correlation Between Salivary Cortisol Levels and Diurnal Variation in Spirometric Parameters in Apparently Healthy Adults

Background: This study investigated diurnal variations in spirometric parameters and their correlation with salivary cortisol levels among apparently healthy adults.

Methods: Forty subjects, aged 18-22, were assessed. Spirometric parameters, including forced vital capacity (FVC), forced expiratory volume in one second (FEV1), forced expiratory flow during 25%-75% of expiration, and peak expiratory flow rate, were measured in the morning, afternoon, and evening. Salivary cortisol levels were also measured at these intervals. Statistical analyses included calculating means, standard deviations, confidence intervals, and Pearson correlation coefficients.

Results: The mean FEV1 values were 2.8325 L in the morning, 2.9263 L in the afternoon, and 2.9543 L in the evening, slightly increasing throughout the day but with overlapping confidence intervals, suggesting no statistical significance. The FEV1/FVC ratios displayed a similar pattern. FVC values decreased slightly from morning to evening, but these changes were not statistically significant. Pearson correlation analysis revealed a strong positive correlation between salivary cortisol and FVC (r = 0.829, p = 0.055), a moderate negative correlation with FEV1 (r = -0.649, p = 0.038), and a strong negative correlation with the FEV1/FVC ratio (r = -0.730, p = 0.040).

Conclusion: Spirometric parameters exhibited minor diurnal variations, with no statistically significant changes for most measures except for salivary cortisol levels, which significantly decreased throughout the day. Significant correlations were observed between salivary cortisol and certain spirometric parameters.