Vitamin D3 K2: Why They Are Paired, What Studies Show and Who Should Be Careful

Vitamin D3 K2 supplements combine two fat-soluble vitamins that work on the same system. Vitamin D helps your gut absorb calcium, and vitamin K activates proteins that hold calcium in bone and may keep it out of blood vessels. That is the logic behind taking them together, but the logic has run ahead of the evidence: the NIH Office of Dietary Supplements (ODS) describes vitamin K’s role in bone and heart health as unclear, and large vitamin D trials in healthy adults have not lowered fracture risk. The two safety points matter more than the marketing. Vitamin K can interfere with the blood thinner warfarin, and too much vitamin D can raise blood calcium to harmful levels. Here is what each vitamin does, how MK-4 and MK-7 differ, and what to check before choosing a combination.

Peptide Labs sells products that contain vitamin D3, but none that contain vitamin K2, and this article does not recommend any of them. It is general information, not medical advice.

What is vitamin D3 K2?

Vitamin D comes in two supplement forms, D2 (ergocalciferol) and D3 (cholecalciferol). They differ only in one side chain and both are well absorbed, but ODS notes that D3 raises blood levels of vitamin D more, and keeps them higher for longer, than D2. Your body converts either form in the liver to 25-hydroxyvitamin D, written 25(OH)D, which is what a vitamin D blood test measures.

Vitamin K is a family. Vitamin K1, or phylloquinone, comes mainly from green leafy vegetables and is the main form in the diet. Vitamin K2 is a group of menaquinones, named MK-4 through MK-13 by the length of their side chain. Most are made by bacteria, including bacteria in your own gut, and they appear in modest amounts in animal foods and fermented foods. Natto, a Japanese fermented soybean food, is especially rich in them.

MK-4 vs MK-7

Form	Where it comes from	What ODS notes
MK-4	Made in the body from vitamin K1; found in meat, eggs and dairy	Used in most Japanese bone trials, at amounts far above food intake
MK-7	Bacterial; natto is the richest food source	Well absorbed; longer half-life than vitamin K1
Vitamin K1 (phylloquinone)	Leafy greens and vegetable oils	Main dietary form of vitamin K

ODS says few data compare how well the supplement forms are absorbed. One study found both K1 and MK-7 well absorbed, with MK-7 staying in the blood longer, which is not proof of a better health outcome.

Why take D3 and K2 together?

The case for taking D3 and K2 together is based on how the vitamins work, not on a body of trials. Vitamin D increases calcium absorption from the gut. Vitamin K is needed to activate osteocalcin, a protein in bone, and matrix Gla-protein (MGP), a protein in blood vessel walls that may help limit abnormal calcification. A hypothesis based largely on animal data suggests that low vitamin K leaves MGP inactive and could allow more calcium to build up in arteries. Supplement makers extend this to argue that anyone taking vitamin D should add K2, but we could not find a large trial showing that the pair improves bone or heart outcomes compared with vitamin D alone.

Vitamin K2 benefits: what the research shows

Bone

A 2006 meta-analysis summarized by ODS pooled 13 trials, mostly from Japan and mostly in postmenopausal women. Twelve found better bone mineral density with vitamin K1 or MK-4, and the seven with fracture data, all using MK-4 at amounts far beyond what food provides, found fewer fractures. Later trials were less encouraging, finding no effect on bone density in older men or women. In Japan and parts of Asia, high amounts of MK-4 are prescribed as an osteoporosis medicine, which is different from an over-the-counter supplement. The European Food Safety Authority accepts a claim that vitamin K contributes to normal bone; the FDA has not authorized a vitamin K health claim in the US.

Heart and arteries

Two observational studies from the Netherlands, one in 564 postmenopausal women and one in 4,807 adults aged 55 and older, linked higher dietary menaquinone intake with less calcium in the arteries and lower rates of death from coronary heart disease. Such links cannot show cause and effect. The one randomized trial ODS describes, in 388 adults aged 60 to 80, found that adding vitamin K1 for three years did not change coronary artery calcification. ODS calls the role of the different forms of vitamin K in arterial calcification unclear and an active research area.

What about vitamin D on its own?

In VITAL, a large US trial, 25,871 generally healthy older adults took vitamin D3 or a placebo for a median of 5.3 years. Vitamin D did not lower total, hip or nonvertebral fractures; most participants were not deficient at the start. The US Preventive Services Task Force also concluded that vitamin D does not reduce falls in adults 65 and older living at home who are not deficient. The open question is whether extra helps people who already have enough.

Reference intakes for vitamin D and vitamin K

Value (adults)	Vitamin D	Vitamin K
Recommended intake	RDA 600 IU (ages 19 to 70); 800 IU (71 and older)	Adequate Intake only; no RDA set
Daily Value on labels	800 IU	Listed only when vitamin K is added
Tolerable Upper Intake Level	4,000 IU from all sources	None set; low potential for toxicity

These are official Food and Nutrition Board reference values reported by ODS, not a suggestion of how much to take. The vitamin D upper limit counts food, multivitamins and every other supplement together.

Vitamin D3 K2 side effects and who should be careful

Vitamin D3 K2 side effects are rare at label amounts, but both vitamins carry real cautions:

• Warfarin: ODS describes a serious and potentially dangerous interaction. Warfarin works by blocking vitamin K, so a sudden change in vitamin K intake can raise or lower its effect. People on warfarin need a consistent vitamin K intake, and should not start or stop a K2 product without talking to their prescriber.
• Too much vitamin D: toxicity, almost always from supplements, causes high blood calcium, with nausea, vomiting, muscle weakness, loss of appetite, dehydration, heavy urination, thirst and kidney stones. Extreme cases have led to kidney failure and heart rhythm problems.
• Calcium on top: in a large trial of 36,282 postmenopausal women, calcium plus vitamin D raised kidney stone risk by 17% over seven years.
• Thiazide diuretics: combined with vitamin D, they may raise blood calcium, especially in older adults and people with reduced kidney function.
• Statins and steroids: ODS lists both as medicines that can interact with or lower vitamin D.
• Orlistat: the weight loss drug in alli and Xenical reduces absorption of fat-soluble vitamins, including D and K, and with warfarin it may change clotting time. Our guide to alli and orlistat covers how it works.

Pregnancy, kidney disease, past kidney stones, high calcium or a bleeding disorder are all reasons to check with your clinician first.

Should you test your vitamin D first?

Serum 25(OH)D is the main marker of vitamin D status. The Food and Nutrition Board says people are at risk of deficiency below 30 nmol/L (12 ng/mL), that 50 nmol/L (20 ng/mL) or more is enough for most people, and that levels above 125 nmol/L (50 ng/mL) can be linked with adverse effects. Lab methods vary, so results can be misleading. The Endocrine Society does not recommend routine testing in healthy people, so it is a decision to make with your clinician.

ODS lists groups more likely to run low: people with little sun exposure, darker skin, a BMI of 30 or more, a past gastric bypass, conditions that limit fat absorption, a milk allergy or lactose intolerance, or a vegan diet. Testing beats guessing here, much as it does for vitamin B12. Vitamin K status is not routinely tested except in people on anticoagulants, and ODS notes that deficiency in adults is very rare.

Labels we carry that list vitamin D3

None of the products we sell contain vitamin K2. A few list vitamin D, which counts toward your total:

• Bragg Apple Cider Vinegar Capsules add vitamin D3 from organic algae at 150% of the Daily Value per serving, plus zinc. The directions are for adults, with food and water.
• Metamucil Fiber Gummies with Vitamins C, D, B12 list vitamin D as cholecalciferol at 20% of the Daily Value per serving, alongside inulin and soluble corn fiber.
• alli is an OTC drug, not a supplement. Its Drug Facts label notes that orlistat can reduce the absorption of some vitamins and advises a multivitamin at bedtime.

ODS notes that fat in a meal helps vitamin D absorption, although some is absorbed without it. If you are also sorting out other supplements, see our guides to the when to take magnesium and magnesium glycinate vs citrate. A testing seal confirms a label matches the contents, not that a product works; our guide to third-party tested supplements explains each seal.

Frequently Asked Questions

Questions about a label? Contact us or read our Buyer’s Guide. Follow the label and ask a healthcare provider before starting any supplement, especially if you take medicines. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Last reviewed October 2026.

Related guides: B12 Benefits: What Vitamin B12 Does, Who Runs Low and Which Form, Best Time to Take Magnesium: Morning or Night and Medicine Timing, Third Party Tested Supplements: What USP, NSF and Other Seals Mean, Boron: Benefits, Supplement Facts, Side Effects and the Borax Trend.