| Summary: | Vitamin D supplementation has been widely promoted to restore 25-hydroxyvitamin D concentrations; however, experimental evidence suggests a nutrient interaction with vitamin K. We assessed the effects of 1200 IU vitamin D<sub>3</sub> per day versus placebo for six months on vitamin K status in a randomized, double-blind, placebo-controlled trial with participants aged 60⁻80 years with depressive symptoms and ≥1 functional limitation for a secondary analysis. Stored baseline and six-month follow-up blood samples were available for 131 participants (<i>n</i> = 65 placebo vs. <i>n</i> = 66 vitamin D supplementation). We measured dephosphorylated uncarboxylated matrix gla protein (MGP) (dp-ucMGP) concentrations—a marker of vitamin K deficiency. Mean age was 68 years, and 89 participants (68%) were women. Vitamin K antagonists were used by 16 participants and multivitamin supplements by 50 participants. No differences in change between intervention and placebo were found (−38.5 ± 389 vs. 4.5 ± 127 (pmol/L), <i>p</i> = 0.562). When excluding vitamin K antagonist users and multivitamin users, dp-ucMGP at follow-up was significantly higher in the vitamin D group (<i>n</i> = 40) compared to placebo (<i>n</i> = 30), with a difference of 92.8 (5.7, 180) pmol/L, adjusting for baseline dp-ucMGP and sex. In conclusion, vitamin D supplementation for six months did not affect vitamin K status; however, among participants without vitamin K antagonist or multivitamin use, vitamin D supplementation influenced dp-ucMGP concentrations.
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