Combining Vitamin D3 and K2: Effects, Dosage, and What You Should Pay Attention To

Vitamin D3 and K2 – Why They Work Best Together

According to studies, people who regularly take vitamin D supplements benefit from also ensuring they get enough vitamin K2. Vitamin K2 plays a key role in helping the body incorporate calcium into bones and teeth and in reducing unwanted deposits in arteries and soft tissues.

Vitamin D promotes the formation of vitamin K2-dependent proteins that transport calcium in the body. However, vitamin K2 is required for these proteins to perform their function.

Therefore, taking vitamin D3 increases the need for vitamin K2. Both vitamins work synergistically: together, they support bone health and can contribute to normal calcium metabolism.

EFSA-confirmed functions:

• Vitamin D: contributes to normal absorption of calcium and phosphorus and supports normal bone health
• Vitamin K: contributes to normal blood clotting and is important for the maintenance of normal bones

Mechanism of Action

Understanding exactly how vitamin D3 and K2 interact at a molecular level helps explain why taking both together is more effective than either alone.

How Vitamin D3 Works

• In the kidneys, vitamin D3 (cholecalciferol) is converted into the active hormone calcitriol (1,25-dihydroxyvitamin D).
• Calcitriol binds to the Vitamin D Receptor (VDR), a nuclear receptor present in nearly every cell.
• VDR activation upregulates expression of calcium-transport proteins in the intestinal wall — primarily calbindin-D9k and TRPV6 — dramatically increasing calcium absorption from food.
• Calcitriol also stimulates osteoblasts (bone-forming cells) to produce osteocalcin, the primary bone matrix protein that binds calcium into the hydroxyapatite crystal structure of bone.
• Additionally, calcitriol modulates immune-cell differentiation, muscle function, and gene expression across hundreds of target genes.

How Vitamin K2 Works

• Vitamin K2 is an essential cofactor for the enzyme gamma-glutamyl carboxylase (GGCX).
• GGCX carboxylates specific glutamate (Glu) residues on Gla proteins, converting them into gamma-carboxyglutamate (Gca) residues that can bind calcium ions.
• Without K2, these proteins remain in their inactive, undercarboxylated form (ucOC, ucMGP) and cannot bind calcium effectively.

Two key Gla proteins relevant to D3 synergy:

• Osteocalcin (OC): activated by K2, it incorporates calcium into bone mineral. D3 increases OC production; K2 activates it. Both are required.
• Matrix Gla Protein (MGP): activated by K2, it inhibits vascular calcification by chelating calcium in artery walls and preventing crystal nucleation.

The Synergy in Summary

Vitamin K2 – Dosage Information

Since vitamin K is stored in the body only in small amounts, we rely on a regular intake through food or dietary supplements. Evidence suggests that many people in Western populations do not consume enough vitamin K.

According to the scientific literature, people over 50 may have an increased risk of cardiovascular disease and osteoporosis. At the same time, older adults tend to consume less vitamin K through their diet.

Vitamin K2 researcher Prof. Cees Vermeer of Maastricht University has found evidence in several studies that supplementation with vitamin K2 (MK-7) in doses of 100 to 200 µg daily may be beneficial for adults—especially for those over 50.

Forms of Vitamin K2: MK-4 vs MK-7

Not all vitamin K2 supplements are equal. The two main forms available in supplements — MK-4 and MK-7 — differ in structure, origin, half-life, and practical dosing requirements.

What Does 'All-Trans' Mean?

MK-7 exists in two geometric forms: all-trans (the natural, biologically active configuration) and cis (inactive). When choosing a K2 MK-7 supplement, look for 'all-trans MK-7' on the label — this is the form studied in research and the one with verified bioavailability. Some synthetic MK-7 sources contain a mixture of cis/trans isomers; natural fermentation sources (typically from natto) predominantly yield all-trans MK-7.

Which Form Should You Choose?

• For most adults: MK-7 all-trans is the preferred daily supplement form due to its long half-life, lower dose requirement, and strong evidence base for bone and cardiovascular outcomes.
• MK-4 is commonly used in high-dose prescription products in Japan (e.g. 45 mg/day for osteoporosis treatment) under medical supervision — not typical for dietary supplement use in Europe.
• Some premium supplements combine low-dose MK-4 and MK-7 to provide both short- and long-duration K2 activity, though evidence for this combination over MK-7 alone is limited.

Why take Vitamin D3 and K2 together?

Vitamin D3 stimulates the production of so-called Gla proteins—particularly osteocalcin and matrix Gla protein (MGP). However, these proteins can only perform their function if sufficient Vitamin K2 is present to activate them.

• Supports calcium absorption from the intestines
• Contributes to the normal function of the immune system
• Promotes bone health

• Activates Gla proteins, which regulate calcium balance
• Supports the incorporation of calcium into bones and teeth
• May help reduce calcium deposits in arteries and soft tissues

Note: The claim that vitamin K2 “prevents” atherosclerosis has not been conclusively proven scientifically. However, studies show associations between vitamin K2 intake and vascular health.

EFSA Reference Values for Vitamin D and Vitamin K

The European Food Safety Authority (EFSA) has developed reference values for adequate nutrient intake. These apply to healthy individuals and are not individual recommendations. Personal needs may vary depending on age, health status, and lifestyle. If in doubt, consult a doctor.

Reference Values for Vitamin K (EFSA, 2023)

Reference Values for Vitamin D (EFSA, 2023)

Source: EFSA DRV Finder (multimedia.efsa.europa.eu/drvs)

Safety & Upper Limits

Both vitamins are fat-soluble. Unlike water-soluble vitamins, excess amounts accumulate in body fat and the liver rather than being excreted in urine, making upper-limit awareness important — especially for vitamin D3.

Vitamin D3 Toxicity

Vitamin D toxicity (hypervitaminosis D) is rare but clinically significant. It does not result from sun exposure — the skin has a self-regulatory mechanism — but can occur from prolonged supplementation well above 4,000 IU/day without monitoring.

• Early symptoms: nausea, fatigue, loss of appetite, excessive thirst, frequent urination.
• Severe hypercalcaemia: confusion, cardiac arrhythmias, kidney stones, kidney failure.
• The main risk factor is supplementing without periodic 25(OH)D blood-level checks.
• Most safety researchers consider doses of 2,000 IU/day to be conservative and safe for most adults; 4,000 IU/day is the EFSA/IOM upper limit for adults.

Vitamin K2 Safety

Vitamin K2 at standard supplemental doses (100–200 mcg/day) has an excellent safety profile. No upper tolerable intake level has been established by EFSA because excess K2 is not associated with toxicity in healthy individuals.

• K2 does not raise blood coagulation risk in healthy people — it acts on specific Gla proteins, not general clotting pathways at supplement doses.
• The clinically relevant safety issue is the interaction with anticoagulant medications.

Age-Specific Dosage Guidance

Vitamin D3 and K2 needs vary significantly across life stages, driven by changes in sun exposure, bone turnover, calcium metabolism, and risk of deficiency. The table below provides a practical reference; always consult a healthcare professional for personalised dosing.

* Infants generally receive vitamin K1 (phylloquinone) injections at birth and may be prescribed K drops; K2 supplementation for infants should only be undertaken on medical advice.

Key Considerations by Life Stage

• Perimenopause & post-menopause: Oestrogen decline accelerates bone turnover and osteocalcin undercarboxylation. Studies (Knapen et al., 2013) show MK-7 supplementation significantly improves bone mineral density and reduces ucOC in post-menopausal women. A daily dose of 180–200 mcg MK-7 alongside 1,000–2,000 IU D3 is commonly referenced.
• Over-70s: Skin synthesis of vitamin D3 is reduced by ~75% in older adults compared to young adults. Combined with lower dietary intake, supplementation is especially important. EFSA notes an adequate intake of 15 mcg (600 IU) but many practitioners recommend 800–1,000 IU given reduced synthesis efficiency.
• Athletes: High bone turnover, indoor training, and higher sweat losses may increase both D3 and K2 requirements. Performance-focused supplementation should be guided by blood testing.

How do you combine vitamin D3 with vitamin K2?

We recommend taking vitamin K2 daily in an amount that suits you—regardless of whether you also take vitamin D3 supplements. The dosage of vitamin D3 should ideally be adjusted based on your personal 25(OH)D blood level.

Experts generally recommend keeping blood 25(OH)D levels within the optimal range. Have your levels checked regularly by a doctor to determine the right dosage for you.

D3:K2 Dosage Ratio Reference Table

A widely referenced practical guideline, supported by the work of Prof. Cees Vermeer and colleagues at Maastricht University, is a ratio of approximately 100 mcg K2 MK-7 per 1,000 IU of D3. The table below applies this ratio across common supplementation levels.

Note: The 100 mcg per 1,000 IU ratio is a practitioner guideline, not a fixed EFSA recommendation. Individual needs vary by baseline vitamin D status, diet, age, body weight, and health conditions. Have your 25(OH)D level tested before establishing a dosing protocol.

Magnesium: The Third Partner

Magnesium is required for the conversion of vitamin D3 into its active form (calcitriol). Without adequate magnesium, supplementing D3 may be less effective, and magnesium deficiency may mask a functional vitamin D deficiency. Many practitioners recommend ensuring adequate magnesium (200–400 mg elemental magnesium/day from diet and supplements) alongside the D3 + K2 protocol.

How does vitamin K2 work?

Research on vitamin K2 has increased significantly in recent decades. In particular, studies conducted at Maastricht University have helped to better understand the importance of vitamin K2 for calcium metabolism.

Vitamin K2 influences how calcium is distributed and utilized in the body. This occurs via so-called GLA proteins (gamma-carboxyglutamic acid proteins), which can only be activated with the help of vitamin K2.

Important GLA proteins:

• Osteocalcin: This protein plays a role in the incorporation of calcium into the bones. It is produced by vitamin D3 but requires vitamin K2 to be activated.
• Matrix Gla protein (MGP): MGP is involved in regulating calcium in the arterial walls. Studies suggest that an adequate supply of vitamin K2 may be associated with healthy vascular function.

Studies indicate that an insufficient supply of vitamin K2 may be associated with an increased risk of vascular calcification and bone loss. Research in this area is still ongoing.

For this reason, it is recommended to take vitamin D3 together with sufficient vitamin K2 to support the optimal effect of both vitamins on calcium metabolism.

Drug Interactions

Beyond the warfarin interaction, several other medications can affect — or be affected by — vitamin D3 and K2 supplementation. The table below summarises the most clinically relevant interactions.

If You Are on Any Regular Medication

• Always inform your GP, pharmacist, or anticoagulation clinic before starting vitamin D3 or K2 supplementation.
• For patients on warfarin or similar drugs, some clinicians take the approach of maintaining a steady daily K2 dose (rather than avoiding K2 entirely) so that INR remains stable and predictable — but this must only be done under medical guidance with regular INR monitoring.
• Patients with kidney disease (CKD) should exercise particular caution with vitamin D3, as impaired kidneys have reduced ability to hydroxylate D3 and are more susceptible to hypercalcaemia.
• The information above is for general awareness and does not replace individual medical advice. Always consult your prescriber.

Sources & Further Reading

• EFSA (2017): Dietary Reference Values for Vitamin D. EFSA Journal 2017;15(9):4928. https://www.efsa.europa.eu/en/press/news/161028
• EFSA (2017): Dietary Reference Values for Vitamin K. EFSA Journal 2017;15(5):4780.
  ⁠https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2017.4780
• Vermeer C. (2012): Vitamin K: the effect on health beyond coagulation – an overview. Food & Nutrition Research, 56. https://pubmed.ncbi.nlm.nih.gov/22489224/
• Holick MF. (2007): Vitamin D Deficiency. New England Journal of Medicine, 357:266-281. https://pubmed.ncbi.nlm.nih.gov/17634462/
• EU Health Claims Register: EC Regulation No 1924/2006 – authorized health claims for Vitamins D and K. https://pmc.ncbi.nlm.nih.gov/articles/PMC12841469/
• Knapen MH et al. (2013) Osteoporos Int. MK-7 RCT; Holden JL et al. on K2 half-life; EFSA DRV Finder 2023. https://pubmed.ncbi.nlm.nih.gov/23525894/