Vitamin D3 and K2: what the trials show

What is vitamin K2?

Vitamin K2 is the name for a family of compounds called menaquinones. Vitamin K comes in two natural forms. K1 (phylloquinone) comes from plants, mainly green leafy vegetables, and is the main form in most diets. K2 covers the menaquinones, labelled MK-4 to MK-13 after the length of their side chain. Bacteria make most of them, including the bacteria that ferment cheese and natto, and the body makes MK-4 itself from K1.

Every form does the same basic job. Vitamin K helps an enzyme switch on certain proteins by adding a chemical group to them, a step called carboxylation. Those proteins include the clotting factors made in the liver, osteocalcin in bone, and matrix Gla protein (MGP) in the walls of blood vessels. In Great Britain, vitamin K is authorised to say it contributes to normal blood clotting and to the maintenance of normal bones, and those claims cover K1 and K2 alike.

Adults need about 1 µg of vitamin K per kilogram of body weight a day, and the NHS says most people get enough from a varied diet. The UK has no separate figure for K2. When the European Food Safety Authority (EFSA) set reference values in 2017, it set them for K1 only, because the evidence on how much K2 people eat, absorb and use was too thin.

What is vitamin K?

Vitamin K is a group of vitamins the body needs for blood clotting, which helps wounds heal. The NHS also says there is some evidence that vitamin K may help keep bones healthy. K1 and K2, described above, are its two natural forms. Vitamin K the body doesn't need straight away is stored in the liver, so you don't need it in your diet every day. The NHS says a varied, balanced diet should provide enough, and that 1 mg (1,000 µg) or less of vitamin K supplements a day is unlikely to cause harm. It adds that there isn't enough evidence to know the effects of taking high doses every day.

MK-4 or MK-7: which form of K2?

MK-7 stays in the blood far longer than the other forms. EFSA's review notes that K1 clears from the blood within hours, while MK-7 takes 48 to 96 hours. In a small Japanese study, women given a single 420 µg dose of MK-7 with breakfast still had it in their blood 48 hours later. The same dose of MK-4 couldn't be detected at any point, and a week of 60 µg a day raised blood MK-7 in every participant but left MK-4 levels unchanged.

A 2013 study described 45 µg as the usual retail dose of MK-7. The bone trials below used 180 to 375 µg a day, and one heart valve trial used 720 µg.

What do vitamin D3 and K2 do together?

People take vitamin D3 and K2 together because of biology: what each does in the body overlaps around calcium. Vitamin D contributes to the normal absorption and utilisation of calcium. Vitamin K activates osteocalcin, which is involved in binding calcium into bone, and MGP, which some researchers think helps control calcium build-up in soft tissues such as artery walls. The theory is that D brings calcium in and K2 sends it to the right place.

It's a reasonable idea, but UK guidance doesn't ask for it. The NHS advises 10 µg of vitamin D a day in autumn and winter, with no mention of K2; our guide to vitamin D in a UK winter covers who needs it and how much. The NHS warning about vitamin D and calcium concerns taking too much for a long time, which can make calcium build up in the body. That's one reason adults shouldn't take more than 100 µg (4,000 IU) a day.

One analysis has tested part of that idea directly. A 2026 analysis of an Austrian trial in 200 adults with high blood pressure and low vitamin D found that people's starting vitamin K levels made no difference to how vitamin D affected their bone turnover markers or heart and blood vessel measures.

What are the benefits of vitamin D3 and K2?

The benefits of D3 and K2 that can be claimed in Great Britain are the authorised ones. Vitamin D contributes to the maintenance of normal bones, teeth and muscle function, to the normal function of the immune system, and to the normal absorption and utilisation of calcium and phosphorus. Vitamin K contributes to normal blood clotting and the maintenance of normal bones.

Wider claims have been assessed and turned down. The GB register lists "vitamin K2 contributes to vascular health", and a claim that MK-7 helps keep the heart and blood vessels healthy, as not authorised, because the evidence didn't back them up.

What are the benefits of vitamin K2 on its own?

The K2 vitamin benefits you can state are the authorised vitamin K ones, normal blood clotting and the maintenance of normal bones, and they apply to K1 and K2 alike. The NHS adds that there is some evidence vitamin K may help keep bones healthy. Beyond that, the trials of K2 below found small and inconsistent effects on bone density and little effect on calcification outside kidney disease. Taking K2 together with D3 hasn't been shown to add much to taking D3 alone.

What the trials show for bones

The results are mixed, and the effects that show up are small.

• A 2020 meta-analysis pooled eight trials (971 people) and found vitamin K plus vitamin D raised total bone density slightly. Some of its comparisons were against no treatment rather than vitamin D alone, so it can't show what K adds on top of D.
• A 2019 University of York review of 36 trials of vitamin K found no evidence that it changes bone density or spinal fractures in women after the menopause or people with osteoporosis. It may reduce other fractures, but the evidence was too weak to confirm it. A 2020 correction adjusted the estimates after doubts were raised about one Japanese trial.
• The two main MK-7 trials point different ways. In 244 healthy women after the menopause, 180 µg a day for three years slowed bone loss at the lower spine and the neck of the thigh bone, but not at the hip as a whole. In 148 women with low bone density, all also taking calcium and vitamin D, 375 µg a day for a year made no difference to bone density anywhere, though it preserved some of the fine structure of bone in the shin.

What the trials show for arteries

K2 consistently lowers a blood marker of inactive MGP. Whether it slows calcium building up in arteries is much less clear.

• A 2023 meta-analysis of 14 trials (1,533 people, most with kidney disease or type 2 diabetes) found vitamin K slightly slowed the rise in coronary artery calcium scores. The authors called for more rigorous trials.
• A 2023 review limited to people without kidney disease found five vitamin K trials, and none of them slowed calcification.
• In a Danish trial of 365 men with an average age of 71 and existing calcium in the aortic valve, two years of 720 µg MK-7 plus 25 µg vitamin D did no better than placebo at slowing it.

Which foods are high in vitamin K2?

Natto, by a long way, then aged and hard cheeses. Natto is a Japanese dish of fermented soya beans, and two laboratories have measured about 940 to 1,100 µg of K2 per 100 g, almost all of it MK-7. In Western diets, cheese is the main source, but the amount swings with the type of cheese, how long it's ripened, its fat content and where it's made. Meat and eggs contain smaller amounts, mostly as MK-4.

Except where noted, the figures come from one Dutch laboratory that tested a small number of samples in 2018, so treat them as rough guides. UK data is thin: the UK's official food composition tables (CoFID) list vitamin K1 but not K2, and we couldn't find a reliable measured figure for eggs.

Which foods contain vitamin K?

K1 is the easier form to find. The NHS lists green leafy vegetables such as broccoli and spinach, vegetable oils and cereal grains, with smaller amounts in meat and dairy. EFSA cites a study in which eating spinach with butter roughly tripled the K1 absorbed. A varied diet should provide enough. If you take warfarin, the NHS says you can still eat foods high in vitamin K, such as broccoli and spinach, but to talk to your warfarin clinic for advice.

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

With a meal that contains some fat, at whatever time of day you'll remember. Both vitamins are fat-soluble, and vitamin K is absorbed along with dietary fat and bile. Because MK-7 lingers in the blood for days, the hour you take it matters far less than taking it regularly. Our vitamin D guide covers what the research says about meals and vitamin D.

Vitamin K2 supplement: what to look for on the label

A K2 supplement should say which form it contains, how many micrograms are in a serving, and carry a warning for people on warfarin. MK-7 stays in the blood far longer than MK-4, and a 2013 study described 45 µg as the usual retail dose of MK-7, while the bone trials used 180 to 375 µg a day. Add up vitamin K from every product, because combined D3 and K2 products and multivitamins can contain it, and the NHS says 1 mg (1,000 µg) or less a day from supplements is unlikely to cause harm. Take a K2 supplement with a meal that contains some fat. Don't take any K2 supplement on warfarin unless your anticoagulation clinic agrees, and don't expect a label to promise artery or heart benefits: the GB register has not authorised them.

Do vitamin D3 and K2 help with weight loss?

No. Neither vitamin has an authorised claim about body weight, and trials of each show no effect. In the US VITAL trial, 771 older adults who took 50 µg (2,000 IU) of vitamin D3 a day for two years had no change in weight, BMI or body fat compared with placebo. A 2024 meta-analysis found vitamin K made no difference to weight (five trials) or BMI (six trials). We found no trial testing the two together for weight.

Who should not take vitamin K2?

Anyone taking warfarin, unless their anticoagulation clinic agrees. Warfarin works by blocking vitamin K, so extra vitamin K can undo its effect, and the NHS says not to take vitamin K supplements while you're on it. The NHS also says foods rich in vitamin K can affect warfarin; you can still eat them, but talk to your clinic.

If you take a different blood thinner and aren't sure how it works, ask your pharmacist. For everyone else, the NHS says 1 mg (1,000 µg) or less of vitamin K from supplements a day is unlikely to cause harm. If you take several products, our guide to which supplements to take together shows how to add up the totals. If bone health after the menopause is your reason for looking, our guide to perimenopause supplements explains the authorised calcium and vitamin D claim for women over 50.

Where Natura fits

Daily D3 (vitamin D3 from lichen, 10 µg) and Daily K2 (vitamin K2 as MK-7, 75 µg) are separate bottles in the Natura cabinet (coming soon), so you can take D3 without K2; both are listed with the daily bottles, and the Vitamins Pack contains both alongside vitamins C, E and A. Vitamin D contributes to the maintenance of normal bones and normal muscle function, and vitamin K contributes to normal blood clotting and the maintenance of normal bones. If you take warfarin, don't start Daily K2 or the Vitamins Pack unless your anticoagulation clinic agrees.