Vitamin D3 and K2: What the Research Actually Shows

Many people, when they think of vitamins, first think of the letters. Vitamin A, vitamin C, vitamin D; and some have numbers attached like vitamin B12. All vitamins have more than one form, similar in chemical structure but different enough that they can have different biological reactions and uptake.

When considering supplements, understanding not just the vitamin, but its specific form — and how that form interacts with other vitamins — can be an important part of making sure supplementation goals are actually being met.

Two of the more popular vitamin supplements on the market today are Vitamin D3 and Vitamin K2. These specific forms of vitamins D and K are purported to have a handful of beneficial effects, but the question is: is there research to back it up? If so, should they be taken, and should they be taken together, separately, or one or the other?

This article reviews the current evidence on both vitamins, how they work, and what the science actually says about supplementing them — individually and in combination.

All About Vitamin D

Most people know vitamin D as the "sunlight vitamin." It is the vitamin the body synthesizes naturally when exposed to sunlight, and it is a common supplement in the colder months when the skies are overcast and more time is spent indoors. Many people have also chosen to megadose vitamin D to try to make up for any perceived deficiency.

Note: Megadosing any vitamin or supplement has inherent risks. The body needs various vitamins and minerals, but too much of a good thing can be harmful. Unless a deficiency has been diagnosed and a doctor has recommended a large dose, it is important to be aware of the potential side effects before starting.

Vitamin D comes in at least five identified forms, appropriately named D1 through D5. The version most relevant here, D3, is also known as cholecalciferol. It is absorbed in the small intestine, primarily alongside dietary fat, and absorption is generally maintained throughout life (unlike some other vitamins and minerals, it does not decline significantly with age.)

Vitamin D deficiency can lead to long-term problems. In children, it can cause rickets; in adults, it causes osteomalacia or a softening of the bones. Normal levels are relatively easy to maintain, however, and global diets tend to be fortified already. Overly high levels of vitamin D can also cause side effects, including:

• Hypercalcemia
• Hypercalciuria (and kidney stones)
• Nausea, vomiting, and muscle weakness
• Dehydration, polyuria, and excessive thirst

In very extreme overdose levels, vitamin D toxicity can even lead to renal failure, cardiac arrhythmia, and even death.

Why do people supplement vitamin D?

Over the years, numerous studies into the effects of vitamin D have shown a variety of potential benefits. Other studies, though, have either failed to replicate those benefits or have been statistically insufficient to draw a real conclusion.

The benefits commonly cited in the literature include:

• Bone support. Since one of the primary functions of vitamin D is to help transport calcium throughout the body, supplementing vitamin D is thought to help ensure an appropriate level of calcium in the bones, particularly with age. A meta-analysis of supplementation studies found inconclusive evidence as to whether or not this works.
• Cancer prevention. One study seemed to indicate an inverse association between vitamin D levels and cancer rates — in other words, decreased cancer incidence with higher levels of vitamin D. Unfortunately, further studies found no similar correlation.
• Cardiovascular health. Much like with cancer prevention, there was one study that seemed to suggest higher levels of vitamin D could reduce the risk of cardiovascular disease; however, further studies were unable to find similar correlations and saw no difference between those who supplemented and those who did not.
• Brain and mood. Vitamin D is involved in various brain processes, so studies have examined the association between vitamin D levels and depression. Observational studies indicate some correlation, but clinical trials have not supported the same conclusions and show no significant reduction in symptoms.

There are other proposed benefits as well, but the pattern is largely the same: promising observational signals that clinical trials frequently fail to replicate.

People who work indoors, live in higher-latitude regions, or regularly wear sunscreen may have reduced sun-driven vitamin D synthesis and could be at greater risk of deficiency. For those at risk, supplementing 1,000–2,000 IU of Vitamin D3 is a common and well-tolerated approach supported by the available evidence.

As for why D3 over another form of vitamin D, it is simply the most common and most readily bioavailable form of the vitamin. A common problem in supplementation is that the form of the vitamin taken may not be usable as-is by the body, so taking more of it provides no benefit without the appropriate form for absorption.

All About Vitamin K

Vitamin K is an entirely different group of molecules from vitamin D. Vitamin K is another fat-soluble vitamin and is primarily used in blood coagulation and in calcium control throughout the body. This calcium control is a significant part of why vitamins D and K are often discussed together, though they are chemically very different and operate in different ways.

Vitamin K2, the form examined here primarily, is one of three main types of vitamin K. It is common in animal products and fermented foods. The most commonly supplemented form today is MK-7, a long-chain menaquinone, most often found in fermented foods as a bacterial byproduct. MK-7 is preferred because of its longer half-life compared to other forms, meaning it remains active in the body for longer after each dose.

Why do people supplement vitamin K?

Like vitamin D, there have been studies to indicate the benefits of vitamin K and others that fail to replicate it. In fact, most of the studies that show strong benefits from vitamin K supplementation are decades old and have since been retracted for being problematic in some way.

That said, a more recent study looked at two possible effects of vitamin K: overall blood vessel health and myocardial infarction. While the first showed no conclusive change, the latter showed a possible significant risk reduction.

Vitamin K2 is preferred over other forms of vitamin K for similar reasons as D3: it is the most bioavailable form readily available as a supplement.

On the bone health side, vitamin K2's role in activating osteocalcin — a protein that helps bind calcium into bone — is a key mechanism researchers have focused on. Osteocalcin requires vitamin K2 to function properly; without adequate vitamin K2, undercarboxylated osteocalcin (ucOC) accumulates and calcium may not be directed to bone as effectively.

Supplementing Vitamin D and K Together

Now we come to the crux of the issue: a study published in November 2023 in JACC: Advances. This study looked at the combined supplementation of both vitamin K2 and vitamin D3. The goal was to examine if there is any benefit to arterial valve calcification. The theory is straightforward: these two vitamins help the body transport calcium to the bones. Since calcium building up in blood vessels and arteries is what causes coronary artery calcification, perhaps supplementing these vitamins could keep more calcium in the bones and away from the arteries.

In this study, a total of 389 participants were randomized, with some given a placebo and some given the vitamins. They were tracked over two years and the process of coronary artery calcification was monitored. In total, there was no significant change in the progression of CAC, but there was a potential reduction in worsening obstructions.

All of this sounds minimal and cautious because it is. The study authors themselves say:

"Although the primary endpoint is neutral, differential responses to supplementation […] and in safety endpoints are hypothesis-generating for future studies."

In other words, this particular study did not conclusively prove a benefit but opens up additional options to test for other beneficial results that may exist.

On the bone side, the combination story is more developed. A meta-analysis of 8 RCTs involving 971 participants found that combined K and D supplementation increased bone mineral density (BMD) (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 Funct, 2020). A 2025 meta-analysis of 9 RCTs involving 2,570 postmenopausal women confirmed that K2 added on top of D3 and calcium shifts bone turnover markers toward formation: osteocalcin +1.86 ng/mL (95% CI 1.17–2.56), ucOC −1.54 ng/mL (95% CI −2.44 to −0.64) (Zhang et al., Front Endocrinol, 2025; doi:10.3389/fendo.2025.1703116). However, most of these factorial studies use vitamin K2 as MK-4 at pharmacological doses (45mg/day), and the largest modern MK-7 + D3 RCT (Rønn et al., 2020) was null on BMD. The combination is biologically rational, but synergy specifically using MK-7 at nutritional doses is not yet definitively proven.

Some of the additional cardiovascular studies are currently ongoing. Results are expected in coming years and are worth watching.

What the Evidence Actually Says

Reviewing the full body of research on these two vitamins leads to the same general conclusion: the evidence is mixed, and the strength of any benefit depends heavily on the specific outcome being measured.

Does vitamin D support bone health and strength? Not clearly. As long as there is no deficiency, there is no conclusive evidence to suggest that taking more vitamin D will strengthen bones. Most studies examining this impact find no significant difference between those who supplement and those who do not, in terms of incidence rates for fractures and osteoporosis.

Does vitamin D help reduce the incidence of cancer? Results are mixed. It is important to remember that "cancer" is an entire class of diseases, and different types of cancer respond in different ways to different things. As it stands, current studies and evidence are inconclusive. It is possible that maintaining adequate levels of vitamin D could reduce cancer mortality rates, but further research is needed.

Does vitamin D reduce the incidence of cardiovascular disease? This is where things get nuanced. A large observational study in Denmark found that both too-low and too-high levels of vitamin D may be associated with a greater risk of mortality from cardiovascular disease and related issues. On the other hand, clinical trials are more mixed, with some showing a reduction in cardiac failure, but not other forms of cardiovascular disease. In short: staying within a normal range — neither too high nor too low — appears most important.

Does vitamin K support bone health and strength? Results are mixed and inconclusive. Some studies found a link between vitamin K supplementation and bone health; others did not. Vitamin K has been authorized as a treatment for osteoporosis in Japan, parts of Asia, and the EU, but not in the USA or elsewhere around the world. Most such studies also supplemented with calcium at the same time, making it difficult to isolate whether vitamin K or calcium was responsible for the observed effects.

Does vitamin K reduce the incidence of cardiovascular disease? This one is complex to analyze. There are a variety of ways that cardiovascular disease can manifest, from arterial hardening to congestive heart failure to stroke to myocardial infarction. Different studies have found different impacts of vitamin K on these different cardiac diseases. As of now, there is no conclusive evidence to support vitamin K as a cardiovascular benefit; however, certain studies have had promising results that warrant further study, and those further studies are ongoing.

If cardiovascular concerns are a primary motivator, it is possible that these vitamins could have some beneficial effects, but more studies are ongoing. It is also possible that they might do the opposite for some people. Additionally, anyone currently on medications such as Warfarin, antibiotics, bile acid sequestrants, orlistat, statins, steroids, or certain diuretics should be aware that these vitamins can have negative interactions, and a healthcare provider should be consulted before starting supplementation.

The choice of whether or not to take a supplement depends on individual health circumstances and advice from a qualified healthcare provider. The current evidence supports ensuring adequate — but not excessive — levels of both vitamins, particularly for those with limited sun exposure, dietary gaps, or specific health concerns around bone density or calcium metabolism.

Sources

1. Vitamin D, Calcium, or Combined Supplementation for the Primary Prevention of Fractures in Community-Dwelling Adults: Evidence Report and Systematic Review for the US Preventative Services Task Force: https://pubmed.ncbi.nlm.nih.gov/29677308/
2. Circulating 25-hydroxyvitamin D serum concentration and total cancer incidence and mortality: a systematic review and meta-analysis: https://pubmed.ncbi.nlm.nih.gov/24036014/
3. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease: https://pubmed.ncbi.nlm.nih.gov/30415629/
4. Serum 25-hydroxyvitamin D and the risk of cardiovascular disease: dose-response meta-analysis of prospective studies: https://pubmed.ncbi.nlm.nih.gov/28251933/
5. Vitamins K1 and K2: The Emerging Group of Vitamins Required for Human Health: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5494092/
6. Effects of Vitamin K2 and D Supplementation on Coronary Artery Disease in Men: A RCT: https://www.jacc.org/doi/10.1016/j.jacadv.2023.100643
7. Vitamin D Fact Sheet for Health Professionals: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
8. Vitamin K Fact Sheet for Health Professionals: https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional/