Vitamin D3 and K2: Why They Are Paired and What UK Guidance Says

Vitamin D3 and vitamin K2 are often sold together as a pair. Each has its own job: vitamin D helps your body absorb calcium, and vitamin K is needed for normal blood clotting and for keeping bones normal. Here is what each one does, what UK guidance says about getting enough, and what to check before you take them together.

What vitamin D does

Vitamin D helps your body absorb and use calcium and phosphate. These keep bones, teeth and muscles healthy, and vitamin D also contributes to the normal function of the immune system.

Your skin makes vitamin D from sunlight. In the UK, most people make enough from late March or early April to the end of September. In autumn and winter there is not enough sunlight, so the NHS advises that everyone should consider a daily supplement of 10 micrograms (400 IU) during those months. Some people are advised to take it all year, for example if they are rarely outdoors or usually cover most of their skin outside.

Do not take more than 100 micrograms (4,000 IU) a day unless a doctor has told you to.

What vitamin K2 does

Vitamin K is needed for normal blood clotting and contributes to the maintenance of normal bones. There are two main forms:

• Vitamin K1, found in green leafy vegetables such as broccoli and spinach, and in vegetable oils
• Vitamin K2, found in smaller amounts in meat, dairy and some fermented foods

Adults need about 1 microgram of vitamin K a day for every kilogram of body weight, and the NHS says most people can get all they need from a varied diet.

Why D3 and K2 are paired

The idea behind the pairing is simple: vitamin D helps you absorb calcium, and vitamin K is involved in the proteins that help build bone. That is why many supplements combine them.

The research on taking the two together is still developing. There is not yet strong evidence that a combined supplement works better than getting enough vitamin D and eating a varied diet with plenty of green vegetables. So treat the pairing as a convenience, not a necessity.

Check before you take vitamin K

Vitamin K affects blood clotting. If you take a blood-thinning medicine such as warfarin, do not start or stop a vitamin K supplement, or change how much you eat of foods rich in vitamin K, without talking to your GP or pharmacist first. It is also worth asking a pharmacist if you take any other regular medicines.

A food-first approach

• Get outdoors in daylight in the spring and summer months
• Consider a 10 microgram vitamin D supplement from October to March, as the NHS advises
• Eat green leafy vegetables most days for vitamin K
• Include calcium sources such as dairy or fortified alternatives, tinned fish with bones, or tofu

When to see your GP

See a GP if you have ongoing bone pain, muscle weakness or tiredness without a clear reason. These have many possible causes, and a supplement is not a substitute for finding out why.

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This article is general information, not medical advice. Speak to your GP or pharmacist before starting a supplement, especially if you take regular medicines.