Effect of FS on Levels of Urinary 8-oxo-dGuo as a Biomarker of Systemic Oxidative Stress. 8-oxo-dGuo, one of the major products of DNA oxidation resulting from oxidative stress, was 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).