Vitamin K: blood clotting, bones & K2

You dutifully take your vitamin D and your calcium – and yet the letter K suddenly appears on every nutrient list. Rightly so: vitamin K is the bouncer that decides where the calcium in your body goes. Without it, calcium doesn't reliably end up where it belongs – in the bones.

Short and to the point: vitamin K contributes to normal blood clotting and to the maintenance of normal bones. There are two main forms – K1 from green vegetables and K2 (MK-7) from fermented foods. K2 is the form that is particularly interesting as a team with vitamin D. Anyone who takes coumarin-type blood thinners must keep the intake constant and coordinate it with a doctor.

The most important points in brief

• Vitamin K contributes to normal blood clotting and to the maintenance of normal bones (approved EU claims).
• K1 (phylloquinone) is found in green leafy vegetables, K2 (menaquinone, MK-7) above all in fermented foods.
• K2 activates the protein osteocalcin, which incorporates calcium into the bones – hence the popular combination D3 + K2.
• With coumarin-type blood thinners (Marcumar, warfarin): keep intake constant and clarify any supplement with a doctor.

What is vitamin K – and what does it do in the body?

Vitamin K is a fat-soluble vitamin that performs a very specific chemical task: it activates certain proteins by giving them a kind of "gripping hook" for calcium (experts call the process carboxylation). Only with this hook can these proteins bind calcium and do their work.

Two of these protein families are especially important. The clotting factors ensure that a wound closes rather than continuing to bleed – this is where the approved claim about normal blood clotting applies. And the bone protein osteocalcin stores calcium in the bone tissue – this is where the claim about the maintenance of normal bones applies. So you can picture vitamin K as the key that unlocks both locks.

K1 and K2 – the decisive difference

"Vitamin K" is a collective term for two forms that behave differently in the body:

The practical point: MK-7 stays in the blood significantly longer than K1 or MK-4. That's precisely why it is the form usually used in combination preparations with vitamin D – even one dose a day keeps the level stable.

The trio of vitamin D, K2 and calcium

This is where it gets really interesting, and this is the reason why vitamin K has received so much attention in recent years. Imagine a chain:

1. Vitamin D opens the door for calcium absorption from the gut – more calcium reaches the blood.
2. Vitamin K2 then activates osteocalcin, which directs this calcium specifically into the bones.
3. Calcium itself is the building material in question.

That's why D3 and K2 are so often offered together – they literally mesh with each other. How to sensibly combine the duo can be read in the guides Vitamin D3 + K2 and Vitamin D and K2 together. The basics on the building material can be found under Calcium: effects, daily requirement & sources and Calcium and vitamin D together.

What research is investigating

Established and textbook-solid is the role of vitamin K in activating the clotting factors and osteocalcin – that is the core of the approved claims. Beyond that, research is looking more closely at bone metabolism: it is investigating what role well-activated osteocalcin plays for bone density over the years and how the supply of K2 affects these markers.

A second, more recent field of research is another K-dependent protein called MGP (matrix Gla protein), which occurs in the vessel walls. Research is interested in how MGP regulates calcium in the tissue. This is an exciting mechanism that is being actively researched – concrete health promises cannot be derived from it for a food, and we deliberately word this cautiously. What can be stated: the role of vitamin K as an activator of calcium-binding proteins is an active, well-founded research topic.

Why many people keep an eye on vitamin K today

For a long time, vitamin K was regarded mainly as the "clotting vitamin" – a marginal topic that healthy people hardly gave a thought to. That has shifted since vitamin D became ever more popular. Because the more deliberately people keep an eye on their vitamin D level and their bones, the more obvious the question becomes of where the absorbed calcium is actually directed. It is precisely here that K2 comes into the picture. It isn't a fad without substance, but the logical continuation of the D-calcium idea – the reason why D3/K2 combinations are on almost every shelf today.

Approved claims for vitamin K

These two claims are officially approved in the EU – so you can go by them when looking at the label:

In practice: dosage, form and timing

Vitamin K is fat-soluble – so it's best to take it with a meal containing some fat, which improves absorption. As a supplement form, K2 as MK-7 has become established, because it acts for a long time and once a day is enough. If you're taking vitamin D anyway, a D3/K2 combination preparation is practical, because both fat-soluble ones are absorbed together.

Who is it particularly sensible to keep an eye on vitamin K for? For everyone who supplements vitamin D at a higher dose, for people who eat little green vegetables and no fermented foods, and for everyone focused on nutrients for the bones. Further bone building blocks are put into context by the guide Cell renewal.

Important safety note: blood thinners

Because vitamin K directly influences clotting, one point is non-negotiable: anyone who takes anticoagulant medication of the coumarin type (e.g. phenprocoumon/Marcumar, warfarin) should keep vitamin K intake constant and clarify any supplement with a doctor beforehand – both through diet and through supplements. These medications work precisely by blocking vitamin K. Newer blood thinners of the DOAC type are generally not affected, but that too should be clarified with a doctor in case of doubt. More on this in the guide Dietary supplements and medication.

When you might be interested in a supplement

Two everyday scenarios make the topic tangible. First: over the winter you take higher-dose vitamin D to maintain your level – then K2 comes into the picture as a logical partner, because both accompany the calcium pathway together. Second: you rarely eat green leafy vegetables and practically never fermented foods such as natto. In both cases, K2 intake through diet tends to be scarce, even though the need is there. How to gauge the amount of vitamin D over the winter is covered in the guide The dark season & vitamin D.

Food sources

Rich in K1 are kale, spinach, chard, broccoli and parsley – green leaves are practically always a good bet. Because K1 is fat-soluble, you get more out of it with some oil in the salad. K2 is provided above all by natto (fermented soybeans), as well as matured cheese and other fermented foods. Since natto rarely ends up on the table in Germany, K2 intake through diet tends to be scarce for many people – one reason why supplementing is so popular. The body can partly convert K1 into K2, but only to a limited extent, which is why the two forms don't fully replace each other.

Quality features: what you can watch out for when buying

Anyone buying a K2 preparation should watch out for one detail: with MK-7 there are two spatial variants, and only the all-trans form is fully biologically active. Good products state a high all-trans proportion. Further points that speak for quality: a clear declaration of the form (K1, MK-4 or MK-7), a laboratory test and a traceable origin. With combination preparations containing vitamin D, it's worth looking at a sensible ratio of the amounts of the two. General criteria are explained by the guide Lab-tested – what does that mean?.

Honestly put into context

Established and approved are the contributions to normal blood clotting and normal bones – that is solid nutritional science, not hype. The exciting topics around vessels and MGP are fields of research, not proven promises of effect. Vitamin K is not a miracle cure, but a logical, often overlooked partner of vitamin D and calcium. If you have this trio in view anyway, K2 sensibly belongs to it.

Matching products from Scheunengut

Vitamin K products – individually and as a D3/K2 combination – can be found in the Vitamin K category. The matching partners are available under Vitamin D and Calcium. Further reading: Combining D3 + K2, Nutrients for the bones, Vitamin D dosage and the overview All 13 vitamins at a glance.

Frequently asked questions (FAQ)

What does the body need vitamin K for?

Vitamin K activates calcium-binding proteins. That's why it contributes to normal blood clotting and to the maintenance of normal bones – these are the two claims approved in the EU. Put simply, it ensures that calcium ends up where it is needed.

What is the difference between vitamin K1 and K2?

K1 (phylloquinone) comes from green vegetables and is used preferentially for blood clotting. K2 (menaquinone, especially MK-7) comes from fermented foods, stays in the blood longer and is the form that is interesting for bone metabolism in interplay with vitamin D.

Should you always combine vitamin D with K2?

Both mesh together: vitamin D promotes calcium absorption, K2 helps to direct this calcium into the bones. That's why D3/K2 combination preparations are popular. The combination isn't mandatory, but especially with higher-dose vitamin D it is a sensible, logical partner.

May I take vitamin K with blood thinners?

With coumarin-type medications (e.g. Marcumar, warfarin), vitamin K directly influences the effect. Keep intake constant and clarify any supplement with a doctor beforehand. Newer DOAC blood thinners are usually not affected – in case of doubt, please also clarify with a doctor.