Yes — not only can you take vitamin D3 and K2 together, most experts recommend you should when taking more than 2,000 IU vitamin D3/day. Vitamin D3 increases calcium absorption from the gut; vitamin K2 (specifically MK-7) activates the proteins that direct that calcium to bones (osteocalcin) and prevent its deposition in arteries (Matrix Gla Protein). Taking D3 without K2 may allow calcium to accumulate in soft tissues.
Key Facts at a Glance
| Safe to combine? | Yes — strongly recommended together |
| K2 dose with D3 | 90–200 mcg MK-7/day |
| Best K2 form | MK-7 (long half-life, once-daily dosing) |
| Warfarin interaction | K2 reduces anticoagulant effect — monitor INR |
| Fat-soluble? | Both — take with largest meal of the day |
| Combined in one capsule? | Yes — widely available |
Why Vitamin D3 and K2 Work Together
Vitamin D3 and K2 work on the same calcium metabolism system but at different steps. Vitamin D3 (cholecalciferol) → converted to calcitriol → increases intestinal calcium absorption by 2–4× and raises serum calcium. Vitamin K2 (menaquinone) → activates carboxylation-dependent proteins: (1) Osteocalcin — directs calcium into bone matrix; (2) Matrix Gla Protein (MGP) — the most potent inhibitor of vascular calcification in the body, preventing calcium from depositing in arteries and soft tissues. Without K2, the additional calcium absorbed through vitamin D supplementation has no specific direction — it may deposit in blood vessel walls. With K2, it is trafficked to bone.
Which Form of K2 is Best with Vitamin D3?
There are two main supplemental forms of vitamin K2: • MK-7 (menaquinone-7): Half-life of approximately 3 days — once-daily dosing is sufficient. Produced by fermentation (natto). Effective dose: 90–200 mcg/day. The most studied form for arterial health. • MK-4 (menaquinone-4): Half-life of only 1–2 hours — requires 3× daily dosing at much higher amounts (1,500 mcg per dose). Less practical as a supplement. For most people supplementing D3, MK-7 at 90–200 mcg/day is the preferred choice for its practical once-daily dosing and clinical evidence for arterial protection.
How Much K2 to Take with Vitamin D3?
90–200 mcg MK-7/day is the evidence-based range for vitamin D3 supplementation. Some practitioners scale K2 upward with higher D3 doses (e.g., 200 mcg K2 with 5,000 IU D3), though dose-response relationships in humans are not precisely established. The minimum effective dose appears to be 90 mcg/day based on carboxylation studies. There is no established upper limit for MK-7, and doses up to 360 mcg have been studied safely.
Does K2 Interact with Warfarin?
Yes — critically. Vitamin K2 directly competes with warfarin (coumadin). Warfarin works by inhibiting vitamin K-dependent clotting factor activation; K2 supplements can significantly reduce warfarin's anticoagulant effect, increasing clotting risk. Anyone taking warfarin or other vitamin K-dependent anticoagulants must consult their physician and have INR monitored closely before adding any vitamin K supplement. This includes both K1 and K2. This is a significant drug interaction.
Evidence for D3 + K2 Arterial Health
A key 3-year double-blind RCT (Knapen et al., 2015, Thrombosis and Haemostasis, n=244) found MK-7 (180 mcg/day) significantly decreased arterial stiffness in postmenopausal women vs placebo. The Rotterdam Study observational cohort (n=4,807) found the highest K2 intake was associated with 57% lower aortic calcification and 52% lower cardiovascular mortality. While observational data cannot prove causation, the mechanistic rationale is strong and RCT evidence continues to accumulate.
Frequently Asked Questions
Clinical References
- Knapen MH et al. (2015). Thromb Haemost. Vitamin K2 supplementation improves arterial stiffness in healthy postmenopausal women. → Source
- Geleijnse JM et al. (2004). J Nutr. Dietary intake of menaquinone and reduced coronary heart disease risk. → Source
- NIH Office of Dietary Supplements. Vitamin K Fact Sheet. → Source