Vitamin D3 and K2: Why They Are Paired and Who Might Consider Both

From October to March, UK sunlight is too weak for most of us to make enough vitamin D in our skin. That is why government advice is for everyone to consider a daily vitamin D supplement through autumn and winter. Look at the shelves this time of year, though, and you will notice many vitamin D products now come paired with something else: vitamin K2.

Why the pairing? Is K2 essential, a sensible extra, or a marketing add-on? This guide explains what each vitamin does, what the evidence and the official wording actually say, and who should be careful before taking K2.

What vitamin D does

Vitamin D is a fat-soluble vitamin your body makes when skin is exposed to UVB light. It is also found in a few foods, such as oily fish, egg yolks, red meat and fortified cereals and spreads, but diet alone rarely provides enough during winter.

Its authorised roles 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 maintenance of normal muscle function.
• Vitamin D contributes to the normal function of the immune system.

UK guidance on vitamin D

The NHS advises that adults and children over four should consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D during autumn and winter. Some groups, such as people who rarely go outside, cover most of their skin, or have darker skin, are advised to consider it all year round. Adults should not take more than 100 micrograms (4,000 IU) a day unless advised by a doctor.

What vitamin K2 does

Vitamin K is actually a family of compounds. K1 (phylloquinone) comes mainly from green leafy vegetables such as kale, spinach and broccoli. K2 (menaquinones) is found in fermented foods, such as the Japanese soybean dish natto, some cheeses and, in smaller amounts, egg yolks and meat. The MK-7 form of K2 is the one most often used in supplements because it stays in the body longer.

The authorised roles for vitamin K are:

• Vitamin K contributes to normal blood clotting.
• Vitamin K contributes to the maintenance of normal bones.

The UK guideline for adults is about 1 microgram of vitamin K per kilo of body weight per day. Most people meet this through a varied diet that includes green vegetables.

Why are D3 and K2 paired?

Both vitamins have a role in bone maintenance, and both are involved, in different ways, in how the body handles calcium. Vitamin D contributes to the normal absorption of calcium from food, while vitamin K is needed for the normal function of certain proteins involved in bone. Because both are linked to bones, many manufacturers combine them in one product for convenience.

You will see a lot of bolder claims online about where K2 "sends" calcium in the body. Research in this area is ongoing, but those claims are not authorised for use on food supplements in Great Britain, and we do not make them. What can be said is simple: vitamin D and vitamin K both contribute to the maintenance of normal bones.

Who might consider a D3 and K2 combination?

• People who want one product instead of two and already plan to take vitamin D through winter.
• People who eat few fermented foods or leafy greens and want to cover vitamin K alongside vitamin D.
• Active adults and people doing strength training, given that vitamin D contributes to the maintenance of normal muscle function and both vitamins contribute to normal bones.

If you already eat plenty of green vegetables and simply want to follow the NHS winter advice, a plain vitamin D supplement is perfectly adequate.

Who should check with a doctor first

This is the most important section of this article. Because vitamin K contributes to normal blood clotting, anyone taking warfarin or other vitamin K antagonist anticoagulants must not start a vitamin K supplement without speaking to their GP or anticoagulation clinic first. Changes in vitamin K intake can affect how these medicines work.

You should also speak to a healthcare professional before taking vitamin D or K if you:

• have kidney disease, high calcium levels or a condition affecting calcium balance;
• are pregnant or breastfeeding;
• take any regular prescription medication.

How to take vitamin D3 and K2

• With a meal containing some fat. Both are fat-soluble, so taking them with food is a sensible habit.
• Once a day, consistently. Daily routine matters more than timing.
• Do not stack products without checking the totals. If your multivitamin already contains vitamin D, add up the amounts so you stay under 100 µg a day.
• Choose a format you will stick with. Tablets, capsules and oil drops all work; drops suit people who dislike swallowing tablets.

Food first: easy winter sources

• Oily fish (salmon, sardines, mackerel) twice a week for vitamin D.
• Eggs and fortified breakfast cereals or spreads.
• Leafy greens, broccoli and cabbage for vitamin K1.
• Hard cheeses and fermented foods for some vitamin K2.

Supplements fill a gap; they do not replace a varied diet.

Products mentioned in this article

FAQs

Do I need to take K2 with vitamin D?

No. The NHS advice is about vitamin D alone, and most people get enough vitamin K from food. A combined product is a convenience choice, not a requirement.

What is the difference between vitamin K1 and K2?

K1 comes mainly from leafy green vegetables; K2 comes from fermented foods and some animal products. Supplements usually use K2 as MK-7.

Is 2,000 IU of vitamin D too much?

2,000 IU equals 50 µg, which is below the 100 µg (4,000 IU) daily upper limit advised for adults in the UK. Do not exceed that limit across all products unless your doctor advises it.

Can I take vitamin K2 if I am on blood thinners?

Not without medical advice. If you take warfarin or a similar anticoagulant, speak to your GP or anticoagulation clinic before taking any vitamin K supplement.

When is the best time to take vitamin D3 and K2?

With a meal that contains some fat, at whatever time of day you will remember consistently.

Food supplements should not be used as a substitute for a varied, balanced diet and a healthy lifestyle.