Vitamin D3 helps your body absorb calcium and keep normal bones, while vitamin K2 activates proteins that put calcium to work in bone. That is the reason the two are often sold together. Vitamin D itself has well-established intake guidelines and large clinical trials behind it, but the research on the D3 + K2 combination is still mostly mechanistic and observational, with only a small number of trials.
Key Takeaways
- Adults need 600 IU (15 mcg) of vitamin D a day, or 800 IU (20 mcg) after age 70. The upper limit for adults is 4,000 IU (100 mcg) a day from all sources.[1]
- Vitamin D3 (cholecalciferol) raises blood levels of vitamin D more than D2 in most studies, especially when taken in large, infrequent doses.[1,4]
- Vitamin K2 comes mainly as MK-4 and MK-7. MK-7 stays in the blood much longer than vitamin K1.[2,5]
- Large trials (VITAL and D-Health) in adults who mostly had adequate vitamin D levels found little or no difference from placebo for the outcomes they measured.[9,10,11,12]
- Pairing D3 with K2 makes biological sense, but few trials have tested the pair directly, and results are mixed.[2,8]
- Vitamin K can change how blood-thinning medicines of the warfarin type work. Talk to your doctor before taking any vitamin K supplement if you take one.[2,7]
Evidence grade: Emerging. For the D3 + K2 combination specifically, the research is mostly mechanistic and observational with few small trials; vitamin D on its own has established NIH intake values and several large randomized trials.
Vitamin D3 and K2 Forms at a Glance
| Form | Where it comes from | What to know |
|---|---|---|
| Vitamin D3 (cholecalciferol) | Made in skin exposed to UVB light; found in fatty fish and egg yolks. Supplements are usually made from lanolin (sheep's wool); a lichen-sourced D3 is available for people who avoid animal products.[1] | Raises blood 25(OH)D more, and keeps it higher longer, than D2 in most studies.[1,4] |
| Vitamin D2 (ergocalciferol) | Made by UV irradiation of ergosterol from yeast; UV-exposed mushrooms also contain it.[1] | Well absorbed and raises 25(OH)D, but usually less than D3.[1,4] |
| Vitamin K1 (phylloquinone) | Green leafy vegetables and vegetable oils; the main form in the diet.[2] | Less absorbed from vegetables than from oils or supplements.[2] |
| Vitamin K2 as MK-4 (menaquinone-4) | Meat, eggs and dairy in small amounts; the body also makes MK-4 from K1.[2] | Studied at high doses (15–45 mg a day) in Japanese bone trials.[2] |
| Vitamin K2 as MK-7 (menaquinone-7) | Fermented foods, especially natto (fermented soybeans).[2] | Long half-life and steadier blood levels than K1; studied at about 180 mcg a day.[5,6] |
What Vitamin D3 and Vitamin K2 Are
Vitamin D
Vitamin D is a fat-soluble vitamin found naturally in only a few foods, added to others, and made by your skin when ultraviolet B (UVB) rays from sunlight reach it.[1] It comes in two main forms. D2 and D3 differ only in a small part of their chemical structure, and both are well absorbed in the small intestine.[1]
Whichever form you take, the liver converts it to 25-hydroxyvitamin D, or 25(OH)D, the form measured in blood tests. The kidneys then turn it into the active form, calcitriol.[1] Labels list vitamin D in micrograms (mcg) and often in International Units (IU); 1 mcg equals 40 IU.[1]
Vitamin K
Vitamin K is a family of fat-soluble compounds. Vitamin K1 (phylloquinone) comes mostly from green leafy vegetables. Vitamin K2 is a group of menaquinones, labeled MK-4 through MK-13 by the length of their side chain; MK-4, MK-7 and MK-9 are the best studied.[2] Most menaquinones are made by bacteria, including bacteria in your gut, while MK-4 is also made by the body from K1.[2]
How Vitamin D and Vitamin K Work in the Body
Vitamin D promotes calcium absorption in the gut and keeps blood calcium and phosphate at the levels needed for normal bone mineralization. It is also needed for bone growth and remodeling.[1] It also plays roles in normal muscle, nerve and immune function.[1]
Vitamin K acts as a helper (coenzyme) for an enzyme that activates certain proteins by a step called carboxylation. These include clotting factors such as prothrombin, osteocalcin in bone, and matrix Gla-protein (MGP) in blood vessel walls, bone and cartilage.[2] MGP is under study because it may help limit calcium build-up in soft tissue.[2]
Why D3 and K2 are paired
The two vitamins meet at the same proteins. Vitamin D increases the production of vitamin K-dependent proteins such as osteocalcin and MGP, and those proteins then need vitamin K to become active.[8] The reasoning behind the combination is that more vitamin D creates more of these proteins, so enough vitamin K helps make sure they are switched on. Whether it translates into better results for people than vitamin D alone has not been settled by large trials.[2,8]
Sunlight, Latitude and Winter
How much vitamin D your skin makes depends on season, time of day, cloud cover, smog, skin pigment and sunscreen. Older adults and people with darker skin make less vitamin D from the same sun. UVB does not pass through window glass, so sitting in a sunny room does not produce vitamin D.[1]
Latitude matters most in winter. In a classic 1988 experiment, sunlight in Boston (about 42 degrees north) produced no previtamin D3 in skin samples from November through February. In Edmonton, Canada (52 degrees north), the gap ran from October through March, while locations at 34 and 18 degrees north still made vitamin D in mid-winter.[3]
Some experts suggest 5 to 30 minutes of midday sun on the face, arms, hands and legs a few times a week is usually enough, but because UV also damages skin, the NIH advises limiting exposure.[1] For more on eating well when it is cold out, see our guide to nutrition in cold climates.
Food Sources of Vitamin D and Vitamin K
Few foods contain much vitamin D naturally. In the U.S., fortified foods such as milk and breakfast cereals supply most of it.[1] Vitamin K1 is easy to get from leafy greens, while K2 is concentrated in natto and present in smaller amounts in cheese, eggs and meat.[2] For more meal ideas, see our list of foods high in vitamin D.
| Food (serving) | Vitamin D | Vitamin K |
|---|---|---|
| Cod liver oil (1 tablespoon) | 1,360 IU (34 mcg) | – |
| Rainbow trout, farmed, cooked (3 oz) | 645 IU | – |
| Sockeye salmon, cooked (3 oz) | 570 IU | 0.3 mcg (MK-4) |
| White mushrooms, UV-exposed (½ cup) | 366 IU | – |
| Milk, 2%, vitamin D fortified (1 cup) | 120 IU | 1 mcg (MK-4) |
| Egg, 1 large | 44 IU (in the yolk) | 4 mcg (MK-4) |
| Cheddar cheese (1.5 oz) | 17 IU | 4 mcg (MK-4) |
| Natto (3 oz) | – | 850 mcg (MK-7) |
| Collards, frozen, boiled (½ cup) | – | 530 mcg (K1) |
| Spinach, raw (1 cup) | – | 145 mcg (K1) |
| Broccoli, boiled (½ cup) | 0 | 110 mcg (K1) |
Values are from the NIH fact sheets; a dash means no value is listed.[1,2]
What Your 25(OH)D Level Means
The blood test for vitamin D measures 25(OH)D. The ranges below are from the Food and Nutrition Board, as summarized by the NIH.[1]
| nmol/L | ng/mL | What it generally means |
|---|---|---|
| Below 30 | Below 12 | Deficient; can lead to soft, weak bones |
| 30 to below 50 | 12 to below 20 | Generally considered inadequate for bone and overall health |
| 50 or higher | 20 or higher | Generally considered adequate for bone and overall health in healthy people |
| Above 125 | Above 50 | Linked to potential adverse effects, especially above 150 nmol/L (60 ng/mL) |
Expert groups differ: the Endocrine Society has not defined cut-offs and does not recommend routine testing in healthy people.[1] There is no routine blood test for vitamin K status; prothrombin time is checked mainly in people who take anticoagulants.[2]
What Human Research Has Measured
| Outcome | What trials found | Certainty |
|---|---|---|
| Raising blood vitamin D (25(OH)D) | D3 raised levels more than D2 in a meta-analysis of randomized trials; the gap was clear with bolus doses and smaller with daily dosing.[4] | High |
| Fractures with vitamin D alone, in people with mostly adequate levels | VITAL (2,000 IU a day) and D-Health (60,000 IU a month) found no overall difference from placebo.[10,12] | High for this population |
| Bone density with MK-7 | 180 mcg a day for 3 years slowed age-related loss at the spine and hip neck, but not the total hip, in 244 postmenopausal women.[6] | Moderate (single trial) |
| Bone density with K1 or MK-4 added to D3 and calcium | Lower inactive osteocalcin but no difference in bone density after 12 months in 381 postmenopausal women.[2] | Moderate; mixed across trials |
| Arterial calcium build-up | Higher dietary menaquinone linked to less calcification in Dutch cohorts; a 3-year K1 + D3 trial found no overall difference.[2] | Low (mostly observational) |
| The D3 + K2 pairing itself | Few trials compared the pair with D3 alone; most combined trials studied bone in postmenopausal women.[8] | Low |
Large vitamin D trials: VITAL and D-Health
The Vitamin D and Omega-3 Trial (VITAL) gave 2,000 IU of vitamin D3 a day or placebo to 25,871 U.S. men aged 50 and older and women aged 55 and older for a median of 5.3 years. Most participants already had adequate vitamin D levels. The supplement did not lower the rates of the trial's main outcomes, invasive cancer and major cardiovascular events, compared with placebo.[9] In a related analysis, vitamin D3 did not reduce total, nonvertebral or hip fractures (hazard ratio for total fractures 0.98).[10]
Australia's D-Health trial gave 21,315 adults aged 60 and older either 60,000 IU of vitamin D3 once a month or placebo for five years. Average blood levels rose to about 115 nmol/L in the vitamin D group, versus 77 nmol/L with placebo. Death rates did not differ (hazard ratio 1.04).[11] Fracture rates also did not differ overall (hazard ratio 0.94), although the authors noted the effect appeared to grow with longer follow-up and called for more research.[12]
Both trials mostly enrolled people who were not low in vitamin D, so they tested whether extra vitamin D adds a benefit on top of adequate status, not whether meeting your needs matters.
Vitamin K2 trials
In a three-year Dutch trial, 244 healthy postmenopausal women took 180 mcg of MK-7 or placebo daily. MK-7 improved vitamin K status and slowed the age-related decline in bone mineral density at the lumbar spine and femoral neck, but not at the total hip. The authors noted the results may not apply to men or children.[6]
An earlier systematic review of 13 trials, most from Japan and most in postmenopausal women, found that K1 or MK-4 improved bone density in 12 of them. The fracture data came from trials using very high MK-4 doses of 15 or 45 mg a day, far above the amounts in typical D3 + K2 supplements.[2] Later trials found no effect on bone density in older adults.[2]
Research on the combination
A 2017 review of the D and K partnership concluded that the biology supports joint use and that combined supplementation might work better than either alone. It also noted that most combined trials studied bone in postmenopausal women and that few looked at heart and blood vessel outcomes.[8] One complication is study design: when vitamin D or calcium is given to the treatment group but not the placebo group, it is hard to tell how much of any benefit comes from vitamin K.[2]
In short, D3 + K2 is a reasonable pairing based on how the vitamins work, but there is little direct trial evidence that the combination gives better results than D3 alone. For the bigger picture on bone nutrition, read our article on calcium and bone health in older adults.
Doses Used in Studies and Reference Intakes
| Nutrient | Amount | Context |
|---|---|---|
| Vitamin D | 600 IU (15 mcg) a day | RDA, ages 1–70, including pregnancy and breastfeeding[1] |
| Vitamin D | 800 IU (20 mcg) a day | RDA, over age 70; also the adult Daily Value on labels[1] |
| Vitamin D | 4,000 IU (100 mcg) a day | Upper limit from all sources, age 9 and older[1] |
| Vitamin D3 | 2,000 IU a day | VITAL, median 5.3 years[9,10] |
| Vitamin D3 | 60,000 IU once a month | D-Health, up to 5 years[11,12] |
| Vitamin K | 120 mcg (men), 90 mcg (women) a day | Adequate Intake for adults; no upper limit has been set[2] |
| Vitamin K2 (MK-7) | 180 mcg a day | Three-year bone trial in postmenopausal women[6] |
| Vitamin K2 (MK-4) | 15–45 mg a day | Japanese bone trials; a pharmacological dose[2] |
Most people in the U.S. get enough vitamin K from food; average intakes are about 122 mcg a day for women and 138 mcg for men.[2] Vitamin D is different. Most Americans eat less than the recommended amount, although survey data suggest most still have adequate blood levels.[1]
If you prefer a multivitamin route, OPA Women's Daily Multivitamin lists 10 mcg (400 IU) of vitamin D3 per serving. That is below the 600 IU RDA and well below the 2,000 IU used in VITAL, and the formula does not list vitamin K2, so it is a base of vitamin D rather than a D3 + K2 product. Count it toward your daily total if you also take a separate vitamin D supplement.
How and When to Take Vitamin D3 and K2
- Take it with a meal. Both vitamins are fat-soluble. Fat in the gut improves vitamin D absorption, though some is absorbed without it, and vitamin K is absorbed along with dietary fat.[1,2]
- Time of day is up to you. Pick the meal you are least likely to skip.
- Daily or less often. Both daily and monthly vitamin D schedules have been studied. Large, infrequent doses should be used only on a clinician's advice.[4,11]
- Add up every source. Multivitamins, calcium supplements with vitamin D, fortified foods and stand-alone products all count toward the 4,000 IU upper limit.[1]
Vitamin D is one of several nutrients involved in normal immune function. For how it fits with others, see our overview of essential vitamins for immune function, and for a broader plan, our guide to nutrition for healthy aging.
Safety and Interactions
Too much vitamin D
Vitamin D toxicity almost always comes from taking too much in supplements, not from food or sun. Because vitamin D raises calcium absorption, excess can cause hypercalcemia (high blood calcium), which may lead to nausea, vomiting, muscle weakness, neuropsychiatric changes, pain, loss of appetite, dehydration, frequent urination, excessive thirst and kidney stones. In extreme cases it can cause kidney failure, calcium deposits in soft tissue and abnormal heart rhythms.[1] Toxicity has occurred from supplements that contained excess vitamin D because of manufacturing errors, or that were taken in excessive amounts.[1] Taking calcium supplements together with vitamin D may increase the risk of some adverse effects.[1]
Stay at or below 4,000 IU (100 mcg) a day from all sources unless your doctor recommends more and monitors you.[1]
Vitamin K safety
No upper limit has been set for vitamin K, because no adverse effects from vitamin K in food or supplements have been reported in people or animals at usual intakes.[2] The main concern is medication interactions.
Medication interactions
- Warfarin (Coumadin) and similar anticoagulants: this is the most important interaction. Vitamin K directly counters these drugs, and sudden changes in vitamin K intake can raise or lower their effect.[2] In a study of anticoagulated healthy volunteers, as little as 10–20 mcg of MK-7 a day changed blood-clotting test results in many participants, and the authors advised avoiding MK-7 supplements during this therapy.[7] Do not start or stop a K2 supplement without talking to your doctor if you take one of these drugs.
- Corticosteroids (such as prednisone) can reduce calcium absorption and impair vitamin D metabolism.[1]
- Thiazide diuretics reduce calcium loss in urine. Combined with vitamin D supplements, they might lead to high blood calcium, especially in older adults or people with reduced kidney function.[1]
- Orlistat (Xenical, alli) can reduce absorption of vitamin D and vitamin K.[1,2]
- Statins: high intakes of vitamin D might reduce the potency of atorvastatin, lovastatin and simvastatin.[1]
- Antibiotics and bile acid sequestrants (such as cholestyramine) can lower vitamin K status, mainly with long-term use.[2]
Talk to your doctor or pharmacist before taking vitamin D3 + K2 if you take any of these medicines.
Who should talk to a clinician first
Check with a healthcare professional before using D3 + K2 if you are pregnant or breastfeeding, have kidney problems, have a condition that affects calcium levels or fat absorption, or have a bleeding or clotting disorder. During pregnancy and breastfeeding the vitamin D RDA stays at 600 IU and the upper limit at 4,000 IU, and the vitamin K Adequate Intake is 90 mcg.[1,2]
Frequently Asked Questions
Is it better to take vitamin D3 with K2?
It may be, but that is not proven. The pairing makes biological sense because vitamin D increases proteins that need vitamin K to work, yet few trials have compared D3 + K2 with D3 alone. Most U.S. diets already supply enough vitamin K.
How much vitamin D3 and K2 should I take per day?
Adults need 600 IU of vitamin D a day up to age 70 and 800 IU after that, with an upper limit of 4,000 IU from all sources. The vitamin K Adequate Intake is 120 mcg for men and 90 mcg for women; the main MK-7 trial used 180 mcg. Your healthcare provider can tailor this to you.
Is MK-7 or MK-4 better?
Neither has been shown to be better in head-to-head trials. MK-7 has a much longer half-life than vitamin K1 and gives steadier blood levels, and it was studied at about 180 mcg a day. MK-4 was mostly studied at milligram doses, 15 to 45 mg a day, in Japanese trials. The interaction cautions in the Safety section apply to every form of vitamin K.
Can I get enough vitamin D from the sun in winter?
Often not, far from the equator. Winter sun in Boston produced no vitamin D in skin from November through February, and sunlight through a window does not count, so food and supplements matter more in those months.
What foods are high in vitamin D and K2?
For vitamin D: cod liver oil, trout, salmon, UV-exposed mushrooms, and fortified milk, plant milks and cereals. For vitamin K2: natto by far, with small amounts in cheese, eggs and meat. Leafy greens supply K1.
Should I get my vitamin D level tested?
The Endocrine Society does not recommend routine testing for healthy people. The NIH lists groups more likely to have low vitamin D: older adults, breastfed infants, people with limited sun exposure or darker skin, and people who have trouble absorbing fat or have had gastric bypass surgery. Ask your healthcare provider whether a test is right for you.
Sources
- NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals.
- NIH Office of Dietary Supplements. Vitamin K: Fact Sheet for Health Professionals.
- Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. J Clin Endocrinol Metab. 1988.
- Tripkovic L, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012.
- Schurgers LJ, et al. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood. 2007.
- Knapen MH, et al. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporos Int. 2013.
- Theuwissen E, et al. Effect of low-dose supplements of menaquinone-7 (vitamin K2) on the stability of oral anticoagulant treatment: dose-response relationship in healthy volunteers. J Thromb Haemost. 2013.
- van Ballegooijen AJ, et al. The synergistic interplay between vitamins D and K for bone and cardiovascular health: a narrative review. Int J Endocrinol. 2017.
- Manson JE, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019.
- LeBoff MS, et al. Supplemental vitamin D and incident fractures in midlife and older adults. N Engl J Med. 2022.
- Neale RE, et al. The D-Health Trial: a randomised controlled trial of the effect of vitamin D on mortality. Lancet Diabetes Endocrinol. 2022.
- Waterhouse M, et al. The effect of monthly vitamin D supplementation on fractures: a tertiary outcome from the population-based, double-blind, randomised, placebo-controlled D-Health trial. Lancet Diabetes Endocrinol. 2023.
Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.
Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: · How OPA researches and writes
