In brief:
- Vitamin D increases calcium absorption from the gut, but K2 is needed to get that calcium into the bones rather than the arteries.
- K2 (especially the MK-7 form) activates osteocalcin and MGP protein, which direct calcium into the bones and prevent it from depositing in blood vessel walls.
- A three-year randomized controlled trial showed that 180 μg of MK-7 a day significantly reduced bone loss in postmenopausal women.
- Estonia’s climate and the time spent indoors mean that for most adults it makes sense to take both vitamins as supplements.
Vitamin D and K2 are two fat-soluble vitamins whose cooperation in the body is more than simply additive. Vitamin D and K2 complement each other at a mechanistic level — one unlocks calcium absorption, the other sends the calcium to the right place. In this article, we look at what science knows about this combination.
How do vitamin D and K2 work together?
Vitamin D — more precisely, its active form calcitriol — significantly increases calcium absorption from the small intestine, by up to 30–40% (Holick, 2007). Without adequate vitamin D levels, only 10–15% of dietary calcium is absorbed. This matters for bone mineralization and for normal muscle function. For a more detailed look at how vitamin D works, see our vitamin D article.
Vitamin K2 (menaquinone) plays a different role: it activates the proteins that regulate calcium transport. The two main ones are osteocalcin (synthesized in the bones) and matrix Gla protein, or MGP (found in blood vessel walls). Both need K2-dependent gamma-carboxylation to be activated. Inactive MGP is known in the scientific literature as one of the strongest promoters of arterial calcification (Maresz, 2015).
So: vitamin D opens the calcium “hose”, and K2 directs the flow the right way.
Why can a high dose of vitamin D without K2 cause problems?
If vitamin D raises calcium absorption but K2 is lacking, MGP stays inactive. Inactive MGP cannot prevent calcium from depositing in blood vessel walls — one of the pillars of the arteriosclerosis process (van Ballegooijen et al., 2017).
Observational studies have shown that low vitamin K2 levels are linked to an increased risk of vascular calcification, especially when vitamin D levels are high at the same time. Although more randomized controlled trials are needed to prove causality conclusively, taking the combination as a precaution is scientifically justified.
A parallel can be drawn with vitamin B: it, too, is a group of vitamins in which the individual compounds work better together. The same synergy logic applies to D3 and K2.
Which form of K2 should you choose, and what dose is recommended?
Vitamin K2 has two main forms:
- MK-4 (menaquinone-4): found naturally in foods of animal origin; short half-life in the blood
- MK-7 (menaquinone-7): produced through fermentation (e.g. natto soybeans); longer half-life, stays in the blood for more than a day
In clinical studies, MK-7 has proved more effective. Knapen et al. (2013) showed in a three-year randomized controlled trial that 180 μg of MK-7 a day significantly reduced bone loss in postmenopausal women compared with placebo.
As for vitamin D, the NIH Office of Dietary Supplements recommends 600–800 IU a day for adults, but in the winter months the need may be higher. You can find more detailed information on different dose ranges in our vitamin D overview. To assess your optimal level, measuring the 25(OH)D concentration in the blood is recommended.
Who needs the combination of vitamin D and K2 most?
The combination may be especially useful for:
- Postmenopausal women, whose rate of bone loss is higher
- Older adults, who are at higher risk of both vitamin D deficiency and vascular calcification
- People taking a high dose of vitamin D (over 2000 IU a day)
- Residents of the Nordic countries, who have long winters and limited sun exposure
As with Vitamin C, it is important to remember here that supplements complement balanced nutrition, not replace it. Before using higher doses, it is advisable to consult your family doctor.
References
- Holick MF (2007). Vitamin D Deficiency. New England Journal of Medicine. 357(3):266–281. PMID: 17634462
- Maresz K (2015). Proper Calcium Use: Vitamin K2 as a Promoter of Bone and Cardiovascular Health. Integrative Medicine (Encinitas). 14(1):34–39. PMID: 26770129
- Knapen MH, Drummen NE, Smit E, et al. (2013). Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International. 24(9):2499–2507. PMID: 23525894
- van Ballegooijen AJ, Pilz S, Tomaschitz A, Grübler MR, Verheyen N (2017). The Synergistic Interplay between Vitamins D and K for Bone and Cardiovascular Health: A Narrative Review. International Journal of Endocrinology. 2017:7454376. PMID: 29138634
Frequently asked questions
Do you have to take vitamin D and K2 together?
Taking them together is not a strict requirement, but it is scientifically justified, especially if you take vitamin D in doses above 1000 IU a day. K2 helps ensure that the increased calcium absorption goes to the bones rather than the blood vessel walls.
Which form of K2 is better — MK-4 or MK-7?
Based on the research, MK-7 is preferable: its half-life in the blood is longer (over 24 hours), and clinical studies have shown a measurable benefit for bone density at a dose as low as 180 μg a day.
When is the best time to take vitamin D and K2?
Both are fat-soluble vitamins, so the most sensible approach is to take them with a meal that contains fat (e.g. lunch or dinner), which improves absorption.
Is it OK to combine vitamin D and K2 with calcium supplements?
Yes — in fact, this is one of the main reasons the combination is recommended. If you take calcium, K2 helps direct it to the bones and reduces the risk of it depositing in the arteries.
Can you cover a vitamin D and K2 deficiency through food?
Vitamin D is found in fatty fish, egg yolks and whole milk, but in Estonia sun exposure in winter is minimal and the amounts obtained from food are mostly insufficient. K2 is found in fermented foods and liver, but rarely in sufficient amounts. That is why supplements are a sensible choice for most residents of the Nordic countries.
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