Vitamin D3 and K2 are taken together because vitamin D helps your body absorb calcium, while vitamin K2 switches on the proteins that help put that calcium into bone. It makes sense on paper, but human trials of the pair are few and the results so far are mixed. For most Australians, the bigger questions are whether you are actually low in vitamin D, and how much you take.
This guide explains the theory, what the research shows, the Australian dose rules and who should talk to their GP first, especially anyone taking warfarin.
Vitamin D3 and K2 at a glance
| Vitamin D3 | Vitamin K2 | |
|---|---|---|
| Main job | Helps your body absorb calcium from food | Switches on proteins, such as osteocalcin in bone, that bind calcium |
| Main source in Australia | Sunlight on your skin (food adds only a little) | Made by bacteria. K1, the plant form, comes from leafy greens |
| Daily guide for adults | Depends on sun, age and blood level: ask your GP | Adequate intake for all vitamin K: 70 mcg (men), 60 mcg (women) |
| Australian dose rule | Above 25 mcg (1,000 IU) a day is a scheduled medicine | No upper level set |
| Main safety point | Too much can raise calcium in the blood | Can change how warfarin works |
| In this guide | Swisse Ultiboost Vitamin D, 1,000 IU a capsule, $39.95 | Horbaach K2 MK-7 100 mcg, $69.95 |
Prices are in AUD at the time of writing. The vitamin D figure is per daily dose, which is what the Australian rule is based on.
Why vitamin D3 and K2 are paired
Vitamin D's main job in this story is calcium absorption. With enough vitamin D, your gut absorbs more of the calcium you eat. Vitamin K comes in next. Your body uses vitamin K to activate a group of proteins, including osteocalcin in bone and matrix Gla protein in blood vessel walls. Once activated, these proteins can bind calcium.
So the idea is simple. D3 brings more calcium in, and K2 helps it end up in bone rather than in places you do not want it, such as artery walls or heart valves. That is why many brands sell the two in one capsule.
Vitamin K comes in two main types. K1 (phylloquinone) comes from plants, and leafy greens such as kale, spinach and salad greens are the main source in our diet. K2 (menaquinones) is made by bacteria. Most K2 supplements use a form called MK-7. Australian nutrient guidelines set an adequate intake for all vitamin K of 70 mcg a day for men and 60 mcg for women, and no upper level, according to the NHMRC nutrient reference values.
What the research actually shows
The theory is stronger than the evidence. Two of the better studies:
- Bones: a 2019 review of randomised trials found no evidence that vitamin K changes bone density or spine fractures in postmenopausal women or people with osteoporosis. It may lower the risk of other fractures, but the authors said the evidence is not strong enough to confirm it.
- Heart valves: the AVADEC trial gave 365 Danish men aged 65 to 74 with calcium on their aortic valve 720 mcg of vitamin K2 plus 25 mcg of vitamin D every day for two years. It did not slow the calcium build-up compared with placebo. The results only apply to older men.
So far there is no strong proof that adding K2 to vitamin D improves bone or heart health in people who are otherwise well. The research is clearer on the basics: get your vitamin D level right if it is low, and eat a varied diet with plenty of vegetables.
Do you need vitamin D3 and K2 in Australia?
For most Australians, sunlight is the main source of vitamin D, and food adds only a little, says healthdirect. In summer, many people need only a few minutes outdoors on most days. In southern states in winter, healthdirect suggests time outdoors around the middle of the day with some skin uncovered. When the UV index is 3 or above, sun protection still matters.
Low levels are common in winter here. The ABS found about one in four Australian adults (23 per cent) had low vitamin D in 2011 to 12, and in winter the figure for Victoria was 49 per cent. People who spend most of the day indoors, cover most of their skin, or have naturally darker skin are more likely to run low.
The only way to know your level is a blood test, so talk to your GP before taking vitamin D every day for months. Vitamin K comes from a normal diet with leafy greens, so a K2 supplement is a personal choice rather than a must.
Safety: dose limits, warfarin and who should check first
- Vitamin D dose: under the Poisons Standard, vitamin D for oral use is a scheduled medicine unless the product has 25 micrograms (1,000 IU) or less per daily dose (TGA). Many imported D3 and K2 capsules contain 5,000 IU, so this guide only links products at 1,000 IU a day or less. Higher doses are for your GP to decide after a blood test.
- Too much vitamin D: toxicity is rare, but too much from supplements can raise the calcium level in your blood (hypercalcaemia). Do not stack two products that both contain vitamin D without adding up the total.
- Warfarin: vitamin K can change how warfarin works, and the nutrient reference values note that warfarin affects vitamin K needs. If you take warfarin or any other blood thinner, talk to your GP or pharmacist before you start or stop K2.
- Other medical conditions: talk to your GP first if you are pregnant or breastfeeding, have a medical condition or take regular medication.
Vitamin D3 and K2 products to consider
These products stay at or under 1,000 IU of vitamin D a day. Cost per day is worked out from the bottle size and the daily dose on each label, so use it as a rough guide.
- Swisse Ultiboost Vitamin D: 25 mcg (1,000 IU) of vitamin D3 per capsule, taken once a day with a meal. A 400-capsule bottle is $39.95, or about $0.10 a day. A simple D3 you can pair with a separate K2.
- Horbaach Vitamin K2 MK7 100 mcg: 100 mcg of K2 as MK-7 per softgel, 250 softgels for $69.95, about $0.28 a softgel. Good value if you want to add K2 to vitamin D you already take.
- Doctor's Best Natural Vitamin K2 MK-7: a lower dose of 45 mcg of K2 as MK-7 (MenaQ7) per capsule, 60 capsules for $49.95.
- Life Extension Bone Restore with Vitamin K2: an all-in-one bone formula. Four capsules a day give 25 mcg (1,000 IU) of vitamin D3 and 200 mcg of K2 as MK-7, plus calcium and magnesium, for about $1.67 a day. Do not take it with another vitamin D product unless your GP advises it.
Our verdict
Vitamin D3 and K2 are paired for a good reason on paper, but trials so far have not shown clear extra benefits from adding K2. Start with the basics: safe sun, a varied diet with leafy greens, and a blood test through your GP if you think you are low. If you do take a supplement, keep vitamin D at 1,000 IU a day or less unless your doctor advises more.
For a simple, low-cost daily vitamin D, Swisse Ultiboost Vitamin D is our pick. If you want K2 as well, Horbaach K2 MK-7 is the value add-on. If you take warfarin or another blood thinner, talk to your GP before taking K2. For other everyday basics, see our best multivitamin guide and our guide to supplements for men over 40.
Shop the products in this guide
Swisse Ultiboost Vitamin D 400 capsules$39.95
Horbaach Vitamin K2 MK-7 100 mcg 250 softgels$69.95
Doctor's Best Natural Vitamin K2 MK-7 60 capsules$49.95
Life Extension Bone Restore with Vitamin K2 120 capsules$49.95
Browse all fish oil, another everyday basic.
FAQs
Should vitamin D3 and K2 be taken together?
They can be, and many people take them as a pair because vitamin D helps you absorb calcium and vitamin K activates proteins that bind calcium in bone. But trials so far, including a two-year study of K2 plus vitamin D in older Danish men, have not shown clear extra benefits from adding K2. If your vitamin D is low, the vitamin D is the part that matters most. K2 is an optional extra, and anyone on warfarin should ask their GP first.
How much vitamin K2 should I take with vitamin D3?
Australian nutrient guidelines do not set a K2 dose to pair with vitamin D. The adequate intake for all vitamin K, from food and supplements, is 70 mcg a day for men and 60 mcg for women, and no upper level has been set. K2 supplements in this guide give 45 to 200 mcg of MK-7 a day. Follow the label, and check with your GP or pharmacist if you take any regular medication.
Can I take vitamin K2 if I am on blood thinners?
Only with your doctor's advice. Vitamin K can change how warfarin works, so starting or stopping a K2 supplement can affect your blood tests and your dose. Australian nutrient guidelines note that warfarin affects vitamin K needs. If you take warfarin or another blood thinner, talk to your GP or pharmacist before you start, stop or change any vitamin K product, including a multivitamin that contains it.
Why are vitamin D supplements in Australia usually 1,000 IU or less?
Under the Poisons Standard, vitamin D for oral use is a scheduled medicine unless the product has 25 micrograms (1,000 IU) or less per recommended daily dose. That is why most vitamin D supplements sold off the shelf in Australia stop at 1,000 IU a day, while many imported D3 and K2 capsules contain 5,000 IU. A higher dose can be right for some people, but that is a decision for your GP after a blood test.
Can I get enough vitamin D from the sun in Melbourne?
Many people can for much of the year. In summer, a few minutes outdoors on most days is often enough. In winter the sun in southern Australia is weaker, and ABS data from 2011 to 12 found about half of Victorian adults had low vitamin D in winter. Healthdirect suggests time outdoors in the middle of the day with some skin uncovered in winter. If you are worried, ask your GP about a blood test.
When is the best time to take vitamin D3 and K2?
Both are fat-soluble vitamins, so labels usually suggest taking them with a meal. The Swisse Ultiboost Vitamin D label, for example, says to take one capsule during or right after a meal. The time of day matters less than taking them consistently, and keeping your total vitamin D at or under the dose on the label unless your GP advises otherwise.
Sources: healthdirect: Vitamin D and your health; ABS: One in four adults are vitamin D deficient; TGA scheduling decision: vitamin D (Poisons Standard entry); NHMRC nutrient reference values: vitamin K; Diederichsen et al. 2022, AVADEC trial, Circulation; Mott et al. 2019, vitamin K and bone meta-analysis, Osteoporosis International; product labels and brand nutrition panels, checked October 2026.
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Prices and stock were checked on 29 October 2026 and can change at any time. Always read the label and follow the directions for use. If you have a medical condition, are pregnant or take regular medication, check with your doctor first.
