Vitamin D3 and vitamin K2 (MK-7) act on different steps of calcium handling.
D3 supports calcium absorption from the gut, and K2 serves as a cofactor for the carboxylation of osteocalcin, which binds to hydroxyapatite and supports bone mineralization, and of matrix Gla protein, described in this literature as a calcification inhibitor in the vessel wall (Ma 2022, PMID: 36033779; Diederichsen 2022, PMID: 35465686).
That mechanism is the stated rationale for pairing the two.
The "calcium paradox" idea, that D3 taken without K2 shifts calcium toward arteries, remains a hypothesis rather than a demonstrated outcome: the largest trial to test it directly gave 720 μg MK-7 plus 25 μg vitamin D daily for 24 months and found no difference in aortic valve or coronary artery calcification progression versus placebo, even though the K2 biomarker moved strongly in the expected direction (PMID: 35465686).
Pooled bone mineral density data across 8 trials of the combination favor it modestly (PMID: 32219282), while a 1-year trial adding vitamin D and calcium on top of K2 found no additional bone benefit (PMID: 32060566).
MK-7 doses in the trials below range from 90 to 720 μg/day. These statements describe published research findings. They are not medical claims about any disease.