Vitamin K2 and D3: Evidence-Based Guide to Bone and Cardiovascular Health

Vitamin K2 supplements are exploding in popularity, with claims of transforming bone and heart health—particularly when paired with Vitamin D3. What does the evidence actually show? Are these supplements worth the investment?

The History of Vitamin D and K

The excitement for Vitamin K2 and D3 began when doctors were puzzled by a problem many children started having since the Industrial Revolution: rickets. This condition causes children's bones not to form properly. Scientists suspected it had something to do with diet. Rates of rickets were especially high in Scotland, so one researcher fed the common Scottish diet to dogs. They developed rickets, too.

He found he could cure the condition by giving the dogs cod liver oil. It was known at the time that this oil was high in vitamin A, so he concluded that too little vitamin A in the diet caused rickets. But he was wrong.

Another researcher wasn't so sure. He decided to try cod liver oil with the vitamin A removed. It still cured rickets, indicating something else was at work. Scientists gave this other factor the name "vitamin D."

Simultaneously, scientists elsewhere discovered that exposing children with rickets to sunlight could cure the condition. Eventually, it was understood that sunlight stimulates our skin to produce vitamin D. By the early 1930s, researchers knew that vitamin D could be obtained from our diet or sun exposure and that it cured rickets.

But why? What was the connection? Further discoveries revealed that vitamin D plays a crucial role in helping our bodies absorb calcium in the digestive tract and stimulates bone formation. Without vitamin D, we may be taking in enough calcium, but our bodies cannot utilize it effectively.

1. How Does Vitamin K Fit In?

Vitamin K's main role relates to coagulation—the ability of blood to form clots to stop bleeding.

However, vitamin K also plays a role in how our bodies use calcium. It activates proteins that help keep calcium in bones, promoting bone strength. Without adequate vitamin K, calcium may be directed to the wrong tissues—a mechanism that has significant implications for both bone density and cardiovascular health.

2. Bringing It All Together

Three nutrients work together to promote healthy bones: calcium, vitamin D, and vitamin K. Bones are approximately 40% calcium, an essential structural ingredient. But for the body to make proper use of calcium, vitamins D and K are also required. This is the biological basis for taking vitamin D and K together and the purported benefits for bone health.

But what about the benefits for cardiovascular health? Here is the intriguing possibility: if vitamin K keeps calcium in bones, it might also help keep it out of blood vessels, potentially reducing calcium deposition in arteries. Calcium in blood vessels can contribute to harmful deposits that form in the heart and arteries—a process central to atherosclerosis.

The key protein here is osteocalcin. Vitamin K activates osteocalcin, which binds calcium and directs it to bone tissue. When vitamin K status is low, osteocalcin remains undercarboxylated and cannot perform this function properly—a marker that researchers now use to assess vitamin K adequacy in clinical trials.

The Science

That's the theory, anyway. What does the research actually show?

There has been some controversy around this topic. A meta-analysis published in 2006 claimed that vitamin K2 helps prevent fractures and strengthen bones [1]. However, many of the individual trials included in that analysis have been retracted due to methodological issues [2]. Later trials have also been retracted because of problems with study methods [3]. This makes it hard to connect the theory that vitamin K2 should help with bone strength to actual clinical evidence.

Luckily, there are some robust trials. A 2009 randomized double-blind placebo-controlled trial found that vitamin K2 supplements did not improve bone strength [4]. This study used the MK-4 version of vitamin K2 at a high pharmacological dose of 45 mg daily, with all participants also receiving daily calcium and vitamin D supplements.

Does this show the theory about vitamin K2 and bone strength is flawed? Not necessarily. The study only ran for 12 months, which is a short duration when considering bone strength. In contrast, a 2013 study lasting three years used MK-7, a different and longer-acting form of vitamin K2, at a dose of 180 micrograms. The results were different, showing significant improvements in bone strength, especially in the lower back and hip bone [5]. Consequently, the European Food Safety Authority approved a health claim for vitamin K, noting that "a cause and effect relationship has been established between the dietary intake of vitamin K and the maintenance of normal bone" [6].

Two more rigorous analyses strengthen this picture. A meta-analysis of 8 RCTs (971 participants) found that combined vitamin K plus vitamin D supplementation increased bone mineral density (pooled effect size 0.316, 95% CI 0.031–0.601) and reduced undercarboxylated osteocalcin—a marker of bone vitamin K deficiency (−0.945, 95% CI −1.113 to −0.778) [Kuang et al., Food & Function, 2020]. A 2025 meta-analysis of 9 RCTs in 2,570 postmenopausal women confirmed that adding K2 on top of vitamin D3 plus calcium shifts bone turnover markers toward formation: osteocalcin increased by 1.86 ng/mL (95% CI 1.17–2.56) and undercarboxylated osteocalcin decreased by 1.54 ng/mL (95% CI −2.44 to −0.64) [Zhang et al., Frontiers in Endocrinology, 2025].

3. Cardiovascular Health Claims

Does this mean vitamin K can promote heart health by preventing calcium buildup in blood vessels? Researchers have looked into this in relation to heart valves and blood vessels.

Heart Valves

As we age, calcifications can develop around heart valves, hardening them and interfering with their function. In 2022, a randomized double-blind placebo-controlled trial published in the Journal of Circulation aimed to see whether vitamin K2 supplements reduce calcification around the aortic valve [7]. Conducted at four Danish hospitals, the study compared 720 micrograms of vitamin K2 (MK-7) plus vitamin D to a placebo over 24 months. Surprisingly, there was no treatment effect—no difference between the placebo and treatment groups, even in smaller subgroups [7].

Blood Vessel Health

However, the story is different for blood vessel health. Observational studies, such as one that followed nearly 5,000 people for seven years, found that higher intakes of vitamin K2 were associated with lower rates of all-cause mortality and heart disease, particularly coronary heart disease [8]. Though observational, these findings prompted randomized controlled trials. In 2015, a double-blind, placebo-controlled trial showed that MK-7 supplementation improved arterial stiffness in healthy postmenopausal women [9].

A study by the same group that conducted the aortic valve trial measured calcium buildup in blood vessels using the Coronary Artery Calcium (CAC) score. Participants were split into placebo and vitamin K2 plus vitamin D groups. After two years, there was no significant difference in CAC progression between the groups overall [10]. However, among participants with a baseline CAC score of ≥400 AU—indicating significant pre-existing calcium burden—there was a significant reduction in calcium progression in the vitamin K2 group [10].

4. Safety Considerations

Calcium buildup in blood vessels is a late-stage finding of atherosclerosis, which involves blockages developing in blood vessels. If vitamin K2 stops calcium from building up, there is a theoretical concern that existing blockages might become less stable.

The study addressed this concern by also examining non-calcified plaque buildup. In the placebo group, plaque grew, while in the vitamin K2 group, it slightly decreased [10]. Additionally, safety events related to blood vessel blockages were reduced in the vitamin K2 group compared to placebo [10]. These findings suggest that rather than destabilizing plaques, vitamin K2 may help stabilize them.

5. Summary of Findings

For individuals with significant calcium buildup in their blood vessels (CAC score ≥400 AU), vitamin K2 supplementation appears to reduce calcium progression.

Furthermore, vitamin K2 may stabilize vulnerable plaques in blood vessels, potentially reducing cardiovascular safety events. However, these findings are novel and hypothesis-generating, requiring further research before firm clinical recommendations can be made. The evidence for bone health—particularly the K1 vs K2 mechanism and the 2013 three-year MK-7 trial—is more established, and the European Food Safety Authority has formally recognized the relationship between dietary vitamin K intake and normal bone maintenance.

6. How Much Vitamin K Do We Need?

There are two main forms of vitamin K: K1 and K2. While K1 stays primarily in the liver and is involved mainly in blood clotting, K2 circulates throughout the body and is the form involved in the bone and cardiovascular studies discussed here. The recommended daily intake for vitamin K1 is 120 micrograms, but no official recommendations exist for vitamin K2 due to the more limited research base [6].

Based on the available evidence, a dose of 90 micrograms per day of MK-7 is a reasonable starting point while further research accumulates. While vitamin K1 is found in leafy green vegetables, vitamin K2 is primarily found in fermented products like natto (fermented soybeans) and aged cheeses, making it challenging to obtain meaningful amounts from diet alone for many people in Western countries.

7. Choosing the Right Vitamin K2 Supplement

Several forms of vitamin K2 exist, but MK-7 is considered the preferred form for supplementation because it is absorbed more efficiently and has the greatest bioavailability compared to MK-4 [11].

MK-7 also has a significantly longer half-life than MK-4, meaning it stays active in the body longer and can be taken once daily. The key practical consideration when choosing a supplement is bioavailability and stability: vitamin K2 is oil-soluble and can degrade with exposure to light and oxygen. Encapsulated forms protect against these degradation pathways, helping ensure the dose on the label reflects what reaches the body.

8. Safety of Vitamin K2 Supplements

According to the NIH supplement database, no adverse effects have been associated with vitamin K consumption from food or supplements [6].

However, there is an important interaction to be aware of with blood thinners such as warfarin (Coumadin). Warfarin works by inhibiting vitamin K-dependent clotting factors, so changing vitamin K intake—whether through diet or supplements—can alter the drug's effect. Anyone taking anticoagulants should consult their doctor before starting a vitamin K2 supplement.

9. What About Vitamin D?

As discussed, vitamin D and vitamin K work through complementary mechanisms, and maintaining adequate levels of both is important for bone and vascular health. The combination of D3 and K2 has been shown in direct factorial trials to increase lumbar bone mineral density more than either nutrient alone [Iwamoto et al., Journal of Orthopaedic Science, 2000]. The mutual dependence makes sense mechanistically: vitamin D increases the production of osteocalcin, and vitamin K2 activates it.

The newest Endocrine Society Guidelines offer updated recommendations on vitamin D supplementation. For a detailed review of the current evidence on vitamin D dosing, see the related write-up at https://youtu.be/PxvXKF0zLzo.

Conclusion

Vitamin K2 supplements, particularly when taken as MK-7 in combination with Vitamin D3, show meaningful evidence for promoting bone health and early but promising evidence for reducing calcium progression in blood vessels—especially in those with already-elevated CAC scores.

The evidence base for bone health is robust—the European Food Safety Authority has formally recognized the dietary vitamin K–bone relationship, and multiple RCTs and meta-analyses support the MK-7 + D3 combination for improving bone turnover markers and bone mineral density, particularly in postmenopausal women. The cardiovascular evidence is more preliminary and hypothesis-generating, but the plaque-stabilization signal in the 2023 JACC trial is noteworthy. Overall, the combination of vitamin K2 (as MK-7, 90 mcg) and vitamin D3 represents a biologically rational, well-tolerated approach supported by a growing evidence base. Anyone considering supplementation—especially those taking blood thinners—should consult a healthcare provider first.

References

1. https://pubmed.ncbi.nlm.nih.gov/16801507/
2. https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2674866
3. https://pubmed.ncbi.nlm.nih.gov/23346882/
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC2683650/
5. https://pubmed.ncbi.nlm.nih.gov/23525894/
6. https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional
7. https://www.ahajournals.org/doi/full/10.1161/CIRCULATIONAHA.121.057008?af=R
8. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5494092/
9. https://pubmed.ncbi.nlm.nih.gov/25694037/
10. https://www.jacc.org/doi/10.1016/j.jacadv.2023.100643
11. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6413124/