Patients with Cystic Fibrosis (CF) are at high risk of fat-soluble vitamin deficiencies, even with supplementation. The contribution of a suboptimal vitamin K status to respiratory and endocrine pathophysiology in CF has been inadequately characterized. Cross-sectional study in adult CF patients (≥ 18 years old) from the Montreal Cystic Fibrosis Cohort. Vitamin K1 (VK1) was measured with high performance liquid chromatography (HPLC), using fasted serum samples collected during an oral glucose tolerance test (OGTT: 2h with plasma glucose & insulin every 30 min) (n = 168). Patients were categorized according to VK1 status (suboptimal defined as <0.30 nmol/L). Suboptimal VK1 levels were observed in 66 % of patients. Patients with a suboptimal VK1 status have a higher risk of colonization with Pseudomonas aeruginosa (p=0.001), lower body mass index (BMI) (p=0.003) and were more likely to have exocrine pancreatic insufficiency (p=0.002). Using an established threshold for VK1, we did show significantly reduced OGTT-derived measures of insulin secretion in patients with a VK1 status below 0.30 nmol/L (1st and 2nd Phase AUCINS/GLU (p=0.002 and p=0.006), AUCINS (p=0.012) and AUCINS/GLU (p=0.004)). Subclinical vitamin K deficiency is more common than other fat-soluble vitamin deficiencies in patients with CF. We demonstrate an association between a suboptimal VK1 status and measures of insulin secretion. We highlight the potential associations of mild vitamin K deficiency with pseudomonal colonization and lower BMI, although these need to be validated in prospective studies.