Effect of FS on Urinary F2a-Isoprostane Levels as a Biomarker of Systemic Oxidative Stress. F2a-isoprostane levels, a key biomarker of oxidative stress, were determined from urine samples collected during and after FS supplementation at week 0, 1, 2, 4, and 8. Levels were determined by high-performance liquid chromatography-electrospray tandem mass spectrometry and normalized to urinary creatinine. Panel A displays comparison between healthy controls (n = 5) and all cystic fibrosis patients (n = 10). Panel B displays comparison between healthy controls (n = 5), cystic fibrosis patients with low lignans (n = 6), and cystic fibrosis patients with high lignans (n = 4). * signifies p < 0.05 for comparison between any time point in each respective cohort versus baseline (pre-FS).