Vitamin D3 + K2: Why the Combination Matters

You bought a high-dose vitamin D to get through the winter in good shape – and now you read everywhere that you absolutely need to take "K2 alongside it". Rightly unsure? Here you get the clear answer, without half-truths and without scaremongering.

In short: vitamin D3 ensures that your body absorbs calcium from food – vitamin K2 is involved in activating those proteins that put this calcium to use in the body. Both act at different points of the same calcium metabolism. That is why they are sensibly combined, especially when you take vitamin D at a higher dose. Everything else – from the best K2 form and the ratio to how to take it – you can read here.

The most important points at a glance

• Vitamin D3 contributes to the normal function of the immune system, to the maintenance of normal bones and teeth, and to the normal absorption and utilisation of calcium.
• Vitamin K2 contributes to the maintenance of normal bones and to normal blood clotting.
• D3 supports calcium absorption, K2 is involved in its utilisation – which is why the two vitamins complement each other.
• The high-quality K2 form is called all-trans MK-7. Both vitamins are fat-soluble and are best absorbed with a meal that contains some fat.

Why vitamin D plays a special role among the vitamins

Vitamin D is the only vitamin that your body can produce itself in the skin with the help of sunlight. When UV-B radiation hits the skin, the body's own production begins. The problem in our latitudes: from October to March the sun is so low that the UV-B radiation is simply not sufficient to kick-start the body's own synthesis – even on a clear winter's day. The German Nutrition Society (DGE) points out that a considerable proportion of the population does not reach the vitamin D levels regarded as desirable all year round.

In the body, vitamin D is converted into its active form via the liver and the kidney and then acts like a hormone: it docks onto receptors in countless tissues. Officially authorised is, among other things, that vitamin D contributes to the normal function of the immune system, to the maintenance of normal bones and teeth, to normal muscle function and to the normal absorption and utilisation of calcium. It is precisely this last point – calcium absorption – that is the key to why K2 comes into play.

What vitamin K2 does in the body

Many people know vitamin K only from one context: blood clotting. Indeed, vitamin K contributes to normal blood clotting – hence the name (K as in "coagulation"). For the topic of bones, however, a second, officially authorised function is decisive: vitamin K contributes to the maintenance of normal bones.

The mechanism behind this is elegant. Your body contains special proteins that can bind calcium – in the bone, for example, osteocalcin. These proteins are not switched on by themselves, however: they first have to be activated by a chemical step called carboxylation, and this is precisely where vitamin K is needed as a cofactor. Without sufficient vitamin K, these calcium-binding proteins remain, so to speak, in standby mode. For years, research has been examining in detail how K2 helps to put absorbed calcium to good use.

Why D3 and K2 are a logical team

Now the picture comes together. Imagine calcium metabolism like a supply chain:

• Vitamin D3 opens the door so that calcium from food can enter the body in the first place – so it increases the supply.
• Vitamin K2 activates the proteins that direct this calcium to the right address in the body – so it takes care of utilisation.

Both work at different stations of the same chain. Anyone who supplements vitamin D at a higher dose noticeably increases the calcium supply – and many people therefore want to make sure that at the same time there is enough vitamin K2 available so that the utilisation side can keep up. That is the whole basic idea behind a combination product: bringing the two partners together in a sensible ratio. Important for context: this is about the normal physiological function of both vitamins, not about treating diseases.

What research is investigating

Textbook knowledge today is the role of the vitamin K cycle in activating calcium-binding proteins – this mechanism is well described biochemically. Considerably more recent and still under discussion is the question of how relevant an additional K2 intake actually is in everyday life, at what dose, and how D3 and K2 optimally complement each other in interplay. What is examined here above all are markers such as the ratio of activated to non-activated osteocalcin. Honestly: not every advertising claim around K2 is backed by robust data, but the basic mechanism and the sensibleness of the combination with higher-dose D3 are readily understandable. That is exactly why so many high-quality products rely on both vitamins together.

MK-7 all-trans: why the K2 form is so important

Vitamin K2 comes in various variants, the so-called menaquinones (abbreviated MK, with a number for the chain length). For supplementation, MK-7 has become established because it remains available in the body longer than short-chain forms such as MK-4. Decisive, however, is a detail that many overlook: what matters is the proportion of all-trans MK-7. This is the biologically active spatial variant of the molecule. Cheap raw materials often contain a higher proportion of the inactive cis form. High-quality products transparently state a high all-trans proportion (ideally above 99%).

Dosage & intake in practice

In the absence of the body's own production, the DGE gives an estimated value of 20 µg of vitamin D per day (equivalent to 800 IU). The German Federal Institute for Risk Assessment (BfR) considers a daily intake of up to 20 µg via food supplements to be harmless. Higher dosages are common but should – especially with long-term intake – best be chosen based on your individual vitamin D blood level and with medical guidance. How exactly to proceed in winter and what the blood level tells you can be read in Vitamin D: Dosage, Blood Level & Winter.

Because vitamin D and K2 are fat-soluble, you best take them with a meal that contains some fat. Oil-based drops or capsules already bring the necessary fat with them. The timing – morning or evening – is secondary; more important is regularity. Whether drops or a capsule suit you better is clarified in Vitamin D: Drops or Capsules?. When fat-soluble vitamins are best taken is explored in more depth in Nutrients Before or After Eating.

Food sources: where do D and K come from naturally?

Before you reach for a supplement, it is worth looking at your plate – even though vitamin D in particular is hard to cover through diet. Vitamin D3 is found above all in oily sea fish (herring, salmon, mackerel), in smaller amounts in egg yolk; the main source, however, remains the sun during the summer half of the year. Vitamin K is more widely available: vitamin K1 you find plentifully in green leafy vegetables such as kale, spinach and broccoli, the long-chain K2 (MK-7) above all in fermented foods, first and foremost in Japanese natto made from fermented soybeans.

It is precisely because the MK-7-rich natto barely features in our cuisine and D formation stalls in winter that many people reach for a combination product during the winter half of the year.

Honestly assessed: what is established and what is not

Established and officially authorised are the contributions of vitamin D to bones, teeth, muscles, the immune system and calcium utilisation, as well as of vitamin K to bones and blood clotting. Biochemically well documented is the mechanism by which vitamin K activates calcium-binding proteins. Less clear-cut – and in part still the subject of research – is how great the additional benefit of K2 supplementation is in everyday life for each individual. Sensible and widely recognised is the combination above all when you take vitamin D at a higher dose. Anyone who supplements only a little D and at the same time eats a varied diet often already has a certain basic K supply. An important safety point: vitamin K can influence the effect of anticoagulant medication (e.g. coumarins, colloquially "blood thinners"). If you take such medication, be sure to clarify the intake with your doctor beforehand.

Matching products from Scheunengut

In the Vitamin D category you will find vitamin D3 products as drops and capsules, many of them already combined with tested all-trans MK-7. Pure vitamin K we carry in the Vitamin K category, further fat-soluble vitamins in the Vitamins overview. Anyone who wants to systematically build up the basics around nutrients will find the right answers in these guides:

• Vitamin D: Dosage, Blood Level & Winter – how much makes sense in winter
• Vitamin D Deficiency: Recognising the Signs – warning signals and context
• Vitamin D3 or D2? – the difference between the two forms
• Vitamin D: Drops or Capsules? – the right dosage form
• Calcium and Vitamin D Together – the interplay with calcium
• Vitamin K: Blood Clotting & Bones – the two functions of K in detail

Frequently Asked Questions (FAQ)

Do I always have to take K2 alongside vitamin D3?

It is not mandatory, but especially with higher-dose vitamin D the combination makes sense: D3 increases calcium absorption, K2 is involved in its utilisation. Anyone who supplements only at a low dose and eats a varied diet often already has a certain basic K supply. Many, for the sake of simplicity, reach for a ready-made combination product.

Which K2 form is the best?

All-trans MK-7. MK-7 remains available in the body longer than short-chain forms, and the all-trans variant is the biologically active one. Look for a high all-trans proportion (ideally above 99%) that is transparently stated on the label.

When is it best to take D3 and K2?

With a meal that contains some fat, because both vitamins are fat-soluble. Oil-based drops or capsules already bring the fat with them. Whether morning or evening is secondary – what is decisive is regularity.

Is vitamin K2 dangerous if I take blood thinners?

Vitamin K can influence the effect of anticoagulant medication of the coumarin type (e.g. phenprocoumon). If you take such medication, do not take K2 on your own initiative but clarify the intake with your doctor beforehand.