Vitamin D3 helps your body absorb calcium; vitamin K2 helps direct where that calcium ends up. That is the reason they are formulated together. For adults in the EU, the tolerable upper intake level for vitamin D set by EFSA is 100 µg per day — that is 4,000 IU, and it is a ceiling, not a target. Typical supplement doses sit between 400 and 2,000 IU daily, with K2 (as MK-7) commonly at 45–200 µg.
This guide covers what the two vitamins do, what the EU actually permits you to be told about them, and where the widely repeated "5,000–10,000 IU" advice comes from and why it does not apply here.
Key points
- EFSA upper limit for adults: 100 µg (4,000 IU) per day. Includes food, fortified products and supplements combined
- Common supplement range: 400–2,000 IU D3 daily
- K2 as MK-7 is the longer-lasting form; typical supplements use 45–200 µg
- Authorised EU claims exist for both — and they are narrower than most articles suggest
- Higher doses are a clinical decision made on a blood test, not a shopping decision
- If you take a vitamin K antagonist such as warfarin, talk to your doctor before starting or stopping a K2 supplement
What vitamin D3 does
Cholecalciferol (D3) is produced in skin exposed to UVB light and is also obtained from a small number of foods. It is converted in the liver and then the kidneys into its active form, which is involved in calcium and phosphate handling.
Across most of Europe, UVB is too weak for meaningful vitamin D synthesis in the skin from about October to March, which is why intake becomes a practical question in winter for many people.
What vitamin K2 does
Vitamin K2 (menaquinone) is involved in the activation of vitamin K-dependent proteins, including osteocalcin in bone and matrix Gla-protein in vascular tissue. Activation requires vitamin K to be present; without it these proteins remain partly uncarboxylated.
MK-7 is the form most used in supplements because it stays in circulation considerably longer than MK-4.
Why they are taken together
The reasoning is mechanistic and worth stating carefully. D3 increases calcium absorption from the intestine. Vitamin K-dependent proteins are involved in how that calcium is subsequently handled in bone and vascular tissue. Formulating them together is therefore a mechanistically rational pairing.
What follows from that mechanism in terms of long-term outcomes is still being studied, and you should be sceptical of any product page that presents the mechanism as a demonstrated result.
What may lawfully be said in the EU
Under Regulation (EC) No 1924/2006, only claims listed on the EU Register may be used. For these two vitamins the authorised wordings include:
- Vitamin D contributes to normal absorption and utilisation of calcium and phosphorus
- Vitamin D contributes to the maintenance of normal bones
- Vitamin D contributes to the normal function of the immune system
- Vitamin K contributes to the maintenance of normal bones
- Vitamin K contributes to normal blood clotting
Note what is absent. There is no authorised claim about cardiovascular risk, arterial calcification or osteoporosis for either vitamin. Content that promises those things is not simply enthusiastic — it is outside what EU law permits, whatever the underlying literature may eventually show.
How much, and the number you should not copy
| Vitamin D3 | Vitamin K2 (MK-7) | |
|---|---|---|
| Common supplement dose | 400–2,000 IU / day | 45–200 µg / day |
| EU tolerable upper intake (adults) | 100 µg = 4,000 IU / day | No UL established |
| Applies to | All sources combined | — |
You will frequently see 5,000 or even 10,000 IU recommended. Those figures circulate from contexts that do not transfer: correcting a diagnosed deficiency under medical supervision, short protocols, or guidance written for other regulatory environments.
For routine supplementation in the EU, 4,000 IU per day is the ceiling for total intake from all sources — food, fortified products and supplements combined. Going above it is a decision for a blood test and a doctor, not a larger capsule.
Who should ask a doctor first
Vitamin K interacts with vitamin K antagonist anticoagulants such as warfarin, because those medicines work by interfering with vitamin K-dependent clotting factors. If you take one, a K2 supplement is a conversation with your doctor, not a self-service decision.
Vitamin D needs the same caution if you have kidney disease, a condition affecting calcium handling, or take medication affecting calcium levels.
Frequently asked questions
How much vitamin D3 per day is safe?
EFSA sets the tolerable upper intake level for adults at 100 µg — 4,000 IU — per day from all sources
combined. Most supplements provide 400 to 2,000 IU. Going above the UL should follow a blood test and
medical advice.
Do I need to take K2 with D3?
They are commonly formulated together because vitamin D increases calcium absorption and vitamin
K-dependent proteins are involved in how that calcium is subsequently handled — a mechanistically
sensible pairing. But there is no established requirement to combine them, and no convincing evidence
that the combination outperforms D3 alone.
Is 5,000 IU too much?
It is above the EU tolerable upper intake level for adults. That does not make a single capsule
dangerous, but it is not a dose to take routinely without medical supervision and a blood test.
MK-4 or MK-7?
MK-7 remains in circulation longer, which is why most supplements use it and why once-daily dosing is
typical.
Can I take these with blood thinners?
Ask the doctor managing your anticoagulation before starting a supplement. Vitamin K interacts with
vitamin K antagonists such as warfarin, and the dose of your medicine may need adjusting. Vitamin D
is a separate question and is usually not an issue.
When should I take them?
Both are fat-soluble, so absorption is better with a meal containing fat.
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