Vitamin D3 or Vitamin K2: what's the difference

Vitamins D3 and K2 are increasingly sold in a single capsule, which is why they are seen as a “pair” that works only together. In reality these are two different vitamins with their own functions, sources and risks. Our editorial team explains what sets them apart and where their paths really do cross.

The nature of the two vitamins

Vitamin D3, or cholecalciferol, is by its nature closer to a hormone than to a classic vitamin. Most of it is formed in the skin under the action of ultraviolet B from 7-dehydrocholesterol. It is then converted in the liver to 25-hydroxyvitamin D — the form measured in tests — and in the kidneys to active calcitriol.

Calcitriol acts through the nuclear vitamin D receptor (VDR), which is present in the cells of the intestine, bones, kidneys, muscles and immune system. In other words, D3 works as a signalling molecule that changes the activity of hundreds of genes.

Vitamin K is a group of fat-soluble compounds with a naphthoquinone core. Phylloquinone (K1) is found in green leafy vegetables, while the menaquinones (K2) have a side chain of varying length and are designated MK-n. The best known of them are MK-4 and MK-7.

Unlike D3, vitamin K does not act through nuclear receptors as a hormone. It is a cofactor of the enzyme gamma-glutamyl carboxylase, which “activates” certain proteins by adding carboxyl groups to them. Without this, the proteins are synthesised but cannot bind calcium.

So D3 is a hormone-like regulator, whereas K2 is an enzyme cofactor. Even at this level it is clear that there is no basis for talking about “interchangeability”.

Functions: calcium absorption versus its “targeted delivery”

The main classic function of vitamin D is maintaining the level of calcium and phosphorus in the blood. Calcitriol enhances calcium absorption in the intestine, affects its reabsorption in the kidneys and its turnover in bone. With severe deficiency, children develop rickets and adults develop osteomalacia.

Vitamin K is responsible for activating the so-called Gla proteins. In bone this is osteocalcin, synthesised by osteoblasts; in the vessel wall it is matrix Gla protein (MGP), which inhibits calcium deposition in soft tissues. In the liver, vitamin K activates blood-clotting factors II, VII, IX and X.

Interestingly, it is vitamin D that stimulates the synthesis of osteocalcin and MGP. That is, D3 raises the “order” for these proteins, while K2 ensures their activation. It is precisely this connection that underlies the popular idea that D3 “brings” calcium and K2 “directs” it into the bones rather than the vessels.

Still, one should distinguish biochemical logic from clinical outcome. The fact that the mechanism is plausible does not yet prove that additional K2 lowers the risk of vascular calcification in a person taking ordinary doses of D3. This hypothesis is still being tested in studies.

Forms and food sources

Vitamin D comes in two forms: D3 (of animal origin and from lanolin; a vegan D3 from lichens also exists) and D2 (ergocalciferol, of plant-fungal origin). Most studies show that D3 raises blood 25(OH)D levels more effectively. From food, D3 is obtained from oily fish, cod liver and egg yolks, but for many people the main source remains the sun.

Vitamin K1 predominates in the diet: spinach, kale, broccoli, lettuce. K2 is found in fermented and animal products: traditional Japanese natto is the record-holder for MK-7 content, and it is also present in some cheeses, egg yolks and liver. Some of the menaquinones are synthesised by the gut microbiota.

The forms of K2 differ substantially in pharmacokinetics. Schurgers and colleagues (2007) showed that MK-7 has a much longer half-life than K1 and therefore gives more stable blood levels with daily intake. MK-4 is eliminated quickly, so studies used it in much higher, milligram doses.

Another difference is diagnostics. Vitamin D status is easily assessed with a standard test, whereas for K2 there is no routine clinical test.

What research says

The benefit of vitamin D in cases of deficiency is undeniable: correcting a low level is necessary for bone health. However, large randomised trials in people without pronounced deficiency have given more restrained results. For example, in the VITAL trial, taking 2000 IU of D3 per day did not reduce fracture rates in the general population of middle-aged and older people.

The 2024 clinical guidelines of the Endocrine Society recommend empirical vitamin D intake for specific groups (children, pregnant women, people aged 75+, individuals with high-risk prediabetes), but do not advise routinely prescribing it to all healthy adults under 75 and do not recommend routine screening of 25(OH)D levels.

For K2 the evidence base is smaller. In the three-year study by Knapen and colleagues (2013), taking 180 mcg of MK-7 per day in postmenopausal women slowed the loss of bone mineral density in some regions. The observational Rotterdam study linked higher dietary menaquinone intake to a lower risk of ischaemic heart disease, but such data do not prove a cause-and-effect relationship.

In sports medicine, vitamin D has been studied much more broadly: the review by Owens and colleagues (2018) describes its role in muscle function, immunity and bone health in athletes, especially those who train indoors or live at northern latitudes. For K2 there is almost no sports data.

Safety and drug interactions

Vitamin D is one of the few vitamins that you can genuinely be poisoned by. Long-term intake of very high doses leads to hypercalcaemia: nausea, thirst, frequent urination, kidney-stone formation, rhythm disturbances. EFSA has set an upper tolerable intake level for adults of 100 mcg (4000 IU) per day.

For vitamin K2 at ordinary dietary doses no toxicity has been described, and no upper intake level has been set. The main risk is linked not to the vitamin itself but to treatment with vitamin K antagonists — warfarin or acenocoumarol. Additional vitamin K weakens their effect, so such patients may take K2 only with a doctor's permission and under INR monitoring.

Direct oral anticoagulants (apixaban, rivaroxaban, dabigatran) are not affected by vitamin K in the same way as warfarin, but any supplements during anticoagulant therapy should still be agreed with a doctor.

For D3, diseases in which the body produces excess calcitriol (for example, sarcoidosis), hyperparathyroidism and kidney stones are also important. In these situations, taking vitamin D on your own is unacceptable.

Editorial conclusions

Vitamin D3 and vitamin K2 are substances of different nature: the first acts as a hormone and regulates calcium absorption, the second activates proteins that bind calcium in bones and vessels.

Their paths cross through osteocalcin and MGP, which makes the idea of a combination biologically plausible, but convincing clinical evidence that the combination is superior to D3 alone is so far scarce.

The most important things are to assess vitamin D status with a test, to remember the upper safe level and not to take vitamin K while on warfarin without a doctor.

Next we recommend reading our practical article “Vitamin D3 vs Vitamin K2: what to choose and for whom”, as well as articles on the 25(OH)D test and on calcium in an athlete's diet.

References

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3. LeBoff MS, Chou SH, Ratliff KA, et al. Supplemental vitamin D and incident fractures in midlife and older adults. N Engl J Med. 2022;387(4):299–309.
4. Schurgers LJ, Teunissen KJF, Hamulyák K, et al. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood. 2007;109(8):3279–3283.
5. Knapen MHJ, Drummen NE, Smit E, et al. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporos Int. 2013;24(9):2499–2507.
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7. Owens DJ, Allison R, Close GL. Vitamin D and the athlete: current perspectives and new challenges. Sports Med. 2018;48(Suppl 1):3–16.
8. EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on the Tolerable Upper Intake Level of vitamin D. EFSA J. 2012;10(7):2813.