Taking vitamin D without K2 calcifies your arteries
You probably already know vitamin D is important. Your doctor may have told you to supplement it. The NHS recommends it every autumn and winter. So you bought a bottle, took the capsule, and assumed you were sorted.
Here’s the part nobody mentioned: vitamin D increases calcium absorption by up to 40%. That’s its primary mechanism. But it doesn’t control where the calcium ends up. Without vitamin K2 directing the traffic, the calcium you’re absorbing more efficiently can deposit in your arteries and soft tissues instead of your bones and teeth.
The advice to supplement vitamin D was correct. The failure to pair it with K2 was an oversight that most of the supplement industry still hasn’t corrected.
Where the “just take vitamin D” advice comes from
The push for vitamin D supplementation is well-founded. In the UK, roughly 1 in 5 adults have serum 25(OH)D levels below 25 nmol/L, the threshold for deficiency (National Diet and Nutrition Survey, 2020). In northern latitudes, UVB exposure drops close to zero from October through March, making endogenous production impossible for nearly half the year.
So the recommendation to supplement was right. The problem is that vitamin D became the entire conversation. GPs test for it. Headlines cover it. Most multivitamins include it (often as the less effective D2 form). Vitamin K2 remained a footnote, if it appeared at all.
That’s not because K2 doesn’t matter. It’s because the research connecting D3 and K2 only gained real traction in the last 15 years, and nutritional guidelines move slowly. The advice you received about vitamin D was based on real data. It was just incomplete.
What the research actually shows
Vitamin D upregulates two proteins that depend on vitamin K2 for activation: osteocalcin (in bones) and matrix Gla protein, or MGP (in arterial walls).
Osteocalcin binds calcium into bone matrix. Without K2 to activate it (a process called carboxylation), calcium doesn’t get incorporated into bone properly. Matrix Gla protein inhibits calcification in arterial walls. Without K2, MGP stays inactive and calcium deposits in soft tissue go unchecked.
The result: more calcium enters the bloodstream thanks to vitamin D, but nothing directs it to where it should go.
The Rotterdam Study, one of the largest population-level investigations into K2, followed 4,807 subjects over 7-10 years. Those with the highest dietary K2 intake had a 52% lower risk of severe aortic calcification and a 57% lower risk of coronary heart disease mortality (Geleijnse et al., Journal of Nutrition, 2004).
A 2015 randomised controlled trial found that three years of K2 supplementation reduced arterial stiffness and improved vascular elasticity in healthy postmenopausal women (Knapen et al., Thrombosis and Haemostasis, 2015). Knapen used MK-7; MK-4 (the form in Fireblood) supports the same carboxylation pathway with different pharmacokinetics — faster, shorter half-life, taken daily. The variable in both studies wasn’t exotic. It was K2 intake.
D3 without K2 creates a calcium trafficking problem. More gets absorbed. Less gets directed. The science on this is consistent.
The form of K2 matters too
Two main forms exist: MK-4 (menaquinone-4) and MK-7 (menaquinone-7). Most supplements default to MK-7 because it has a longer half-life, roughly 72 hours compared to 4-6 hours for MK-4. Longer half-life sounds better on a label. In practice, it’s more nuanced.
MK-4 is the form your body naturally converts from K1. It’s found in animal tissues and it’s the form used in the majority of clinical trials on K2 and bone health, particularly the large Japanese fracture-prevention studies (Cockayne et al., Archives of Internal Medicine, 2006). MK-7 stays in circulation longer, which is genuinely useful for maintaining serum levels, but that persistence also means it can interact less predictably with anticoagulant medications.
Fireblood uses MK-4 at 120ug alongside K1 at 120ug. K1 handles blood clotting. K2 as MK-4 handles calcium distribution. They’re not interchangeable for this purpose.
What this means for you
If you supplement vitamin D (and at northern latitudes, you probably should), check whether your supplement also contains K2. Most don’t. If it does, check the dose and the form. If the label says “vitamin K” without specifying K1 or K2, it’s almost certainly K1 only.
Fireblood includes 50ug (2000 IU) vitamin D3 as cholecalciferol, 120ug vitamin K2 as MK-4, and 120ug vitamin K1 as phylloquinone. The three were formulated together because the synergy between them is the point. Taking D3 alone covers half the equation.
The fix isn’t complicated. It just requires your supplement to have thought about it in advance. Most haven’t. Check the label.
If you want D3, K2, and K1 already paired at effective doses in a single scoop, Fireblood covers all three.
