Vitamin D3 vs Vitamin K2: what to choose and for whom

On the shelves of pharmacies and sports shops there is “pure” vitamin D3, standalone K2, and combined D3+K2 products. How do you work out what you actually need? Our editorial team has put together a practical guide: where to start, in which cases vitamin D alone is enough, when K2 is worth considering and for whom both should be taken only under a doctor's supervision.

The first step is a test, not a purchase

The choice between D3 and K2 should start with understanding your vitamin D status. For this a serum 25-hydroxyvitamin D (25(OH)D) test is used. It reflects the vitamin stores obtained from the sun, from food and from supplements.

Interpretation of the results differs between organisations. The US Institute of Medicine (IOM, 2011) considered a level below 30 nmol/L a risk of deficiency, and 50 nmol/L and above sufficient for bone health in the vast majority of people. The Endocrine Society, in its 2011 guidelines, proposed a higher target threshold — 75 nmol/L (30 ng/mL).

For vitamin K2 there is no comparable routine test. So the decision about it is based not on laboratory figures but on diet, age, bone status and the medications a person takes.

It is worth remembering that the 2024 Endocrine Society guidelines do not recommend routine 25(OH)D screening for all healthy people. The test is appropriate when there are risk factors for deficiency or symptoms, and it is precisely for such people that it helps to select a dose.

When vitamin D3 alone is enough

For most people who are found to have a low or insufficient 25(OH)D level, the standard solution is vitamin D3 itself. It is for D3 that the main evidence base has been accumulated regarding the prevention of rickets, osteomalacia and support of bone tissue, and it is included in clinical guidelines.

Typical groups who more often need D3: people who get little sun (office work, gym training, winter at temperate latitudes), people with dark skin, older people, individuals who are overweight, and those who cover their face and body with clothing. Athletes who train mainly indoors are also advised by reviews to check their vitamin D status.

If your diet contains green vegetables, cheeses and fermented products, and you have no bone problems, adding K2 to D3 is unlikely to change anything noticeable. There is currently no convincing evidence that ordinary preventive doses of D3 without K2 harm the vessels.

For a young, healthy athlete with a D deficiency, the basic choice is a D3 monopreparation, the effect of which is assessed with a repeat test after about 2–3 months.

For whom K2 or a combination may make sense

Most research on K2 has been carried out in postmenopausal women. In the three-year randomised study by Knapen and colleagues (2013), MK-7 at a dose of 180 mcg per day slowed the loss of mineral density in the lumbar spine and femoral neck. So K2 is sometimes considered an addition to the main therapy for osteopenia, but not a replacement for it.

The second scenario is people who eat almost no green vegetables, cheeses and fermented products and at the same time take vitamin D long-term. Here a combined D3+K2 product may be a convenient way to cover both needs with a single capsule.

The third scenario is the idea of preventing vascular calcification. It is biologically plausible, but evidence with “hard” endpoints is so far insufficient. People with cardiovascular disease would do better to discuss this topic with a cardiologist rather than rely on advertising.

• postmenopause and osteoporosis risk — K2 as an addition, after consultation;
• a diet poor in vitamin K during long-term D3 intake — a possible combination;
• young, healthy people with a normal diet — K2 is usually not necessary.

Forms, doses and how to read the label

Vitamin D3 is produced in capsules, tablets and drops; for better absorption it should be taken with food that contains fat. Preventive doses for adults are usually 600–2000 IU per day (15–50 mcg). Higher, therapeutic doses are prescribed by a doctor based on test results. Do not exceed 4000 IU (100 mcg) per day without a specialist's supervision — this is the upper tolerable level according to EFSA.

Vitamin K2 is most often available in the form of MK-7 (doses usually 45–200 mcg per day) or MK-4. MK-7 has a longer half-life, so it is well suited for daily intake in microgram doses.

In combined products, check whether the D3 dosage matches your needs: it is often 5000 IU or more, which does not suit everyone and exceeds the upper tolerable level for long-term unsupervised intake.

Who needs special caution

For people taking warfarin or other vitamin K antagonists, any K2 supplements are contraindicated without agreement from a doctor: they reduce the drug's effect and can cause dangerous swings in INR.

High-dose vitamin D is dangerous in hypercalcaemia, sarcoidosis and other granulomatous diseases, hyperparathyroidism, kidney-stone disease and severe kidney disease. For people with such conditions the dose is determined only by a doctor.

Pregnant and breastfeeding women are often recommended vitamin D, but the dose and form should be agreed with a doctor, and combined “sports” products with high doses are inappropriate here.

Finally, people who already take multivitamins should add up all sources of vitamin D so as not to accidentally exceed safe limits.

Editorial conclusions

For most people the correct sequence of actions is this: assess the risk of vitamin D deficiency, get a test if needed and start with a D3 monopreparation at a reasonable dose.

K2 is an addition for specific groups, primarily postmenopausal women and people with a diet poor in vitamin K; the D3+K2 combination is convenient but not obligatory.

The main limitations are warfarin for K2 and the upper safe level for D3.

We also recommend our articles “Vitamin D3 or Vitamin K2: what's the difference”, “How to read a 25(OH)D test” and a piece on bone health in athletes.

References

1. Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press; 2011.
2. Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911–1930.
3. Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2024;109(8):1907–1947.
4. 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.
5. 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.
6. Owens DJ, Allison R, Close GL. Vitamin D and the athlete: current perspectives and new challenges. Sports Med. 2018;48(Suppl 1):3–16.
7. 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.