Do You Need K2 With D3? How the Two Vitamins Work Together

Think of vitamin D as the vitamin that brings calcium in, and vitamin K as the one that helps put it to use. Here is how each step works and what studies of the pair measured.

Vitamin D helps your body absorb calcium from food, according to MedlinePlus^[3]. Calcium is needed for normal bone formation.*

Vitamin D also acts directly on bone-building cells. A 2024 review^[1] describes it promoting the maturation of these cells, called osteoblasts, and the making of a protein called osteocalcin.*

The same review says bone cells carry the vitamin D receptor, the docking point that lets the vitamin act, at high levels. It adds that osteoblasts may also convert vitamin D into its active form on the spot.

So vitamin D does two jobs here. It raises the supply of calcium, and it helps build a protein that handles calcium. That second job is where vitamin K comes in.

Osteocalcin and matrix Gla protein (MGP) are made in an inactive form. Vitamin K is a helper for an enzyme that adds calcium-binding groups to them, a step called carboxylation. The 2024 review^[1] describes this process.

Once activated, osteocalcin is thought to bring calcium into bone. The review says activated MGP has five calcium-binding groups and three phosphate-binding groups.*

Vitamin K is better known as the clotting vitamin, since the liver uses it to make clotting factors. MedlinePlus^[2] adds that some studies suggest it helps maintain strong bones in older adults.*

The link between the two vitamins is simple. Vitamin D raises the making of these proteins, and vitamin K activates them. The 2024 review concludes that adequate vitamin K on top of optimal vitamin D seems to add to the benefit.*

It helps to see the two vitamins as steps in one sequence, based on how the 2024 review describes them.

1. Vitamin D helps calcium move from food into the body.
2. Vitamin D helps bone cells make osteocalcin, which starts out inactive.
3. Vitamin K carboxylates osteocalcin and MGP, so they can bind calcium.

If you remove step one or step two, there is less calcium or less protein to work with. If you remove step three, the protein is there but is not switched on. That is the reasoning behind pairing the two vitamins.*

The pooled analysis

Who took part: 971 people across 8 randomized trials, as reported in the 2024 review^[1], which cites a 2020 meta-analysis by Kuang and colleagues.

What changed: vitamin K combined with vitamin D3 significantly increased total bone mineral density, a measure of how much mineral the bone holds.*

The catch: results varied with study design and with who took part. The review notes that benefit is expected mainly where vitamin K status starts out low.

The 3-year trial

Who took part: 142 postmenopausal women with osteopenia, meaning lower than normal bone mass.

What was given: MK-7 (a form of K2) at 375 mcg per day, plus D3 and calcium, compared with D3 and calcium alone. This was a placebo-controlled trial lasting three years.

What changed: osteocalcin carboxylation was higher in the group that also received vitamin K.* That is the activation step described above.

The catch: bone turnover markers were similar in the groups with and without added vitamin K. The review also calls for more research on the best intakes and the ratio of vitamin K to vitamin D3.

Vitamin K comes mostly from food. MedlinePlus^[2] lists leafy greens such as kale and spinach, cruciferous vegetables such as broccoli, and smaller amounts from fish, liver, meat and eggs.

The same page lists Adequate Intake (the amount assumed to be enough for most people) at 90 mcg for adult women and 120 mcg for adult men. The FDA lists^[5] the Daily Value for vitamin K on labels as 120 mcg. Its Daily Value for vitamin D is 20 mcg.

Vitamin D is harder to get from food. MedlinePlus^[4] says few foods contain it naturally, and that the body makes it after sun exposure. It also says supplements taken with a meal that has some fat improve absorption.

Both vitamins are fat-soluble, meaning they dissolve in fat. That is why an oil base suits a D3 plus K2 formula. For more on the K2 forms, see MK-4 vs MK-7.

For reference, our vitamin D3 + K2 spray has 55 mcg (2200 IU) of vitamin D3 and 100 mcg of vitamin K2 per 1 mL serving.

Vitamin D raises calcium absorption and the making of osteocalcin. Vitamin K carboxylates osteocalcin and MGP so they can bind calcium. Each vitamin covers a step the other does not.*

A 2024 review concludes that adequate vitamin K on top of good vitamin D status seems to add to bone-health benefits.* Trials of the combination have also measured higher osteocalcin activation and bone mineral density gains.*

Taken together, the evidence supports treating the two vitamins as a team, with D3 bringing calcium in and K2 helping put it to use. A form that delivers both together fits that logic. Questions about amounts are for your doctor, and a vitamin D blood test is how levels are checked.

Vitamin K affects clotting. MedlinePlus^[2] says it can affect how warfarin works, and that people on warfarin should eat a consistent amount each day because sudden changes can alter the medicine's effect.

Too much vitamin D raises blood calcium. MedlinePlus^[3] says this can lead to calcium deposits in soft tissues, nausea, kidney stones and kidney damage, and that toxicity almost always comes from supplements.

Check with your doctor first if you:

• take blood thinners such as warfarin, or any other medication
• are pregnant or breastfeeding
• are choosing a supplement for a child
• have kidney disease or a history of kidney stones
• have a health condition, or have not had your vitamin D level checked

The upper limit for vitamin D from all sources is 4,000 IU (100 mcg) per day for children and adults 9 years and older, per MedlinePlus^[3].