In Search of Normality for Vitamin K1: Establishing Age-Dependent Reference Intervals in the Danish Population.

BACKGROUND: A growing body of evidence suggests that vitamin K has beneficial effects on human health, especially cardiovascular and bone health. Vitamin K1 (phylloquinone), the predominant form of vitamin K in blood, is regarded as an indicator of vitamin K status, but to our knowledge no reference intervals (RIs) have been established for vitamin K1. METHODS: In this population-based study, vitamin K1 was measured in serum from 3808 Caucasian individuals without diabetes from 26 to 78 years of age. The need for gender- and age-partitioned vitamin K1 reference intervals was evaluated using Lahti's method, and exclusion criteria were defined to obtain as healthy a study group as possible. The excluded subgroups were tested for differences in mean serum vitamin K1 levels. Serum vitamin K1 levels were quantified using an in-house newly developed, validated, and highly sensitive online SPE-LC-MS/MS method with a limit of quantitation of (LOQ) 0.05 nmol/L. RESULTS: The reference interval for serum vitamin K1 was 0.22 to 3.95 nmol/L for individuals aged 26 to 44 years and 0.35 to 3.70 nmol/L for individuals aged 45 to 78. Similar age-specific reference intervals were established for vitamin K1-triglyceride ratio being 0.20 to 3.16 and 0.31 to 3.44, respectively. No significant difference was found between genders. Serum vitamin K1 was detectable in all serum samples. Individuals with known comorbidity were found to have significantly lower serum vitamin K1 compared to those without comorbidity. Current smokers had lower serum vitamin K1 compared to nonsmokers. CONCLUSION: Age-dependent reference intervals were established for serum vitamin K1 and vitamin K1-triglyceride ratio in a well-defined, healthy Caucasian population. Lower serum vitamin K1 levels were found in individuals with known comorbidity, suggesting an association between serum vitamin K1 and disease status. Further studies are needed to determine an optimal serum vitamin K1 level.