Vitamin D and K are both fat-soluble vitamins essential for human health, yet they serve distinct physiological roles. Vitamin D regulates calcium absorption and bone mineralisation, whilst vitamin K activates proteins involved in blood clotting and calcium regulation. Emerging evidence suggests these vitamins work synergistically, particularly in bone and cardiovascular health, with vitamin K helping to direct calcium absorbed through vitamin D's action to appropriate tissues. Understanding how vitamin D and K interact provides insight into optimising skeletal health and preventing vascular calcification, though UK guidance does not currently recommend routine combined supplementation for the general population.

What Are Vitamins D and K?

Vitamin D and vitamin K are both fat-soluble vitamins essential for human health, yet they serve distinct physiological roles. Vitamin D exists primarily in two forms: vitamin D2 (ergocalciferol), found in some plant sources and fortified foods, and vitamin D3 (cholecalciferol), synthesised in the skin upon exposure to ultraviolet B (UVB) radiation and also obtained from animal-based foods. Once absorbed or synthesised, vitamin D is converted to 25-hydroxyvitamin D (25(OH)D) in the liver, which is the main circulating form used to assess vitamin D status. This is further converted in the kidneys to calcitriol (1,25-dihydroxyvitamin D), the active hormone that regulates calcium and phosphate metabolism, supports bone mineralisation, and modulates immune function.

Vitamin K comprises a group of structurally related compounds, the most clinically significant being vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). Vitamin K1 is predominantly found in green leafy vegetables and plays a crucial role in hepatic synthesis of clotting factors (II, VII, IX, and X) and anticoagulant proteins C and S. Vitamin K2, produced by gut bacteria and present in fermented foods and animal products, is increasingly recognised for its role in activating proteins involved in calcium regulation beyond the liver, particularly in bone and vascular tissue, though evidence for these extrahepatic roles continues to evolve.

Both vitamins are absorbed in the small intestine alongside dietary fats, and their bioavailability can be impaired in conditions affecting fat absorption, such as coeliac disease, Crohn's disease, or following bariatric surgery. Deficiency in either vitamin can lead to significant health consequences: vitamin D deficiency is associated with rickets in children and osteomalacia in adults, while vitamin K deficiency can result in impaired blood clotting and increased bleeding risk. Understanding the individual roles of these vitamins provides the foundation for appreciating their relationship in maintaining skeletal and cardiovascular health.

How Vitamin D and K Work Together in the Body

The interplay between vitamins D and K centres on calcium metabolism and bone health. Vitamin D enhances intestinal absorption of calcium and phosphate, increasing circulating levels of these minerals necessary for bone mineralisation. However, simply increasing calcium availability is insufficient for optimal bone health—the body must direct calcium to appropriate tissues (bones and teeth) whilst preventing its deposition in soft tissues such as arteries and kidneys. This is where vitamin K becomes essential.

Vitamin K activates specific proteins through a process called carboxylation. Two key proteins in this system are osteocalcin and matrix Gla protein (MGP). Osteocalcin, produced by osteoblasts in bone tissue, requires vitamin K-dependent carboxylation to bind calcium effectively and incorporate it into the bone matrix. Without adequate vitamin K, osteocalcin remains undercarboxylated and functionally impaired, potentially reducing bone mineralisation despite adequate calcium and vitamin D levels.

Matrix Gla protein serves a complementary protective function by inhibiting calcium deposition in arterial walls and soft tissues. Vitamin K activates MGP, enabling it to prevent vascular calcification—a process implicated in cardiovascular disease. Emerging evidence suggests that vitamin D may upregulate the production of these vitamin K-dependent proteins, but without sufficient vitamin K to activate them, the intended benefits may not be fully realised.

This biochemical synergy suggests that adequate vitamin K status may be necessary to optimise the skeletal benefits of vitamin D supplementation. Whilst the evidence base continues to evolve, the mechanistic rationale for combined supplementation is biologically plausible. However, it is important to note that no UK guidance (NICE, SACN, NHS) currently recommends routine vitamin K co-supplementation with vitamin D, and clinical guidance remains focused on addressing documented deficiencies of each vitamin individually.

Benefits of Taking Vitamin D and K Together

The potential benefits of combined vitamin D and K supplementation are primarily related to bone health and cardiovascular protection, though the strength of evidence varies. For bone health, several observational studies and some intervention trials suggest that adequate intake of both vitamins may support bone mineral density more effectively than either vitamin alone. This is particularly relevant for postmenopausal women and older adults at increased risk of osteoporosis and fractures. The proposed mechanism involves vitamin D increasing calcium absorption whilst vitamin K ensures proper calcium utilisation in bone tissue through osteocalcin activation.

Regarding cardiovascular health, the hypothesis centres on preventing arterial calcification. Vitamin K2, in particular, has been studied for its role in activating matrix Gla protein, which inhibits calcium deposition in blood vessel walls. Some epidemiological studies have associated higher vitamin K2 intake with reduced cardiovascular risk, though causality has not been definitively established. Vitamin D also influences cardiovascular health through effects on blood pressure regulation, endothelial function, and inflammation, though evidence for cardiovascular benefits from supplementation remains inconsistent.

It is important to emphasise that high-quality randomised controlled trials specifically examining combined vitamin D and K supplementation are limited. Cochrane and other reviews show insufficient or low-certainty evidence that vitamin K (alone or with vitamin D) reduces fractures, and cardiovascular outcome data are primarily observational and inconsistent. UK guidance does not recommend vitamin D (with or without vitamin K) for cardiovascular disease prevention.

Patient safety considerations include ensuring that any supplementation addresses documented or likely deficiency rather than pursuing speculative benefits. Individuals should be advised that whilst combined supplementation appears safe for most people, it should not replace a balanced diet or evidence-based medical treatments for osteoporosis or cardiovascular disease. Those with existing health conditions should discuss supplementation with their GP or healthcare provider before commencing.

Recommended Doses and Food Sources

Vitamin D recommendations in the UK are guided by the Department of Health and Social Care (DHSC) and NHS. The reference nutrient intake (RNI) is 10 micrograms (400 IU) daily for the general population aged 4 years and above. For infants and young children, the recommendations are:

• Birth to 1 year: 8.5-10 micrograms daily (unless consuming more than 500ml of infant formula)

• Children 1-4 years: 10 micrograms daily

Certain groups may require supplementation: individuals with limited sun exposure, those with darker skin, care home residents, and people with malabsorption disorders. The NHS recommends that everyone consider taking a daily vitamin D supplement during autumn and winter months (October to March) when UVB exposure is insufficient for cutaneous synthesis.

The UK maximum daily intake levels that should not be exceeded are:

• Adults and children 11+ years: 100 micrograms (4,000 IU)

• Children 1-10 years: 50 micrograms (2,000 IU)

• Infants under 1 year: 25 micrograms (1,000 IU) unless medically advised

For vitamin K, the UK does not have a formal RNI, but adequate intake is estimated at approximately 1 microgram per kilogram of body weight daily (roughly 65–80 micrograms for adults). Most individuals obtain sufficient vitamin K1 from dietary sources without supplementation. There is no established recommended dose for vitamin K2 specifically, though supplements typically provide 45–200 micrograms daily.

Dietary sources of vitamin D include:

• Oily fish (salmon, mackerel, sardines)

• Egg yolks

• Fortified foods (breakfast cereals, fat spreads, some plant-based milk alternatives)

• Red meat and liver (in smaller amounts)

Vitamin K1 food sources include:

• Dark green leafy vegetables (kale, spinach, broccoli, Brussels sprouts)

• Vegetable oils (soybean, rapeseed)

• Some fruits (kiwi, avocado)

Vitamin K2 sources include:

• Fermented foods (natto, sauerkraut, some cheeses)

• Egg yolks

• Meat and dairy products

• Produced by gut bacteria (though contribution to overall status is uncertain)

A varied, balanced diet incorporating these foods can help maintain adequate status of both vitamins. Individuals considering supplementation should discuss appropriate doses with a healthcare professional, particularly if taking medications or managing chronic conditions.

Who Should Consider Vitamin D and K Supplements?

Vitamin D supplementation is recommended for specific population groups at higher risk of deficiency. According to NHS and NICE guidance, the following individuals should consider daily supplementation:

• All adults and children over 4 years during autumn and winter (October to March)

• Year-round supplementation for those with limited sun exposure (housebound, care home residents, those who cover their skin for cultural or religious reasons)

• Individuals with darker skin (African, African-Caribbean, or South Asian backgrounds), as increased melanin reduces cutaneous vitamin D synthesis

• Pregnant and breastfeeding women

• Infants and young children (8.5-10 micrograms daily from birth to 1 year; 10 micrograms daily for ages 1-4, unless taking more than 500ml formula daily)

• People with malabsorption conditions (coeliac disease, Crohn's disease, cystic fibrosis) or following bariatric surgery

• Individuals taking medications affecting vitamin D metabolism (some anticonvulsants, glucocorticoids)

• People with obesity (BMI ≥30), chronic kidney or liver disease

Vitamin K supplementation is less commonly required, as dietary intake is usually adequate and gut bacteria contribute to vitamin K2 production. However, certain groups may benefit:

• Newborn infants receive vitamin K prophylaxis at birth to prevent vitamin K deficiency bleeding (VKDB)

• Individuals with fat malabsorption disorders affecting vitamin K absorption

• People taking long-term antibiotics that may disrupt gut bacteria

• Postmenopausal women and older adults concerned about bone health, particularly if vitamin D supplementation is already being taken

Importantly, people taking warfarin should not start vitamin K supplements unless advised by their anticoagulation clinic. Warfarin users should maintain consistent dietary vitamin K intake rather than avoiding it.

Combined vitamin D and K supplementation may be considered by individuals seeking to optimise bone health, particularly postmenopausal women, older adults with osteoporosis risk factors, or those with documented deficiency in either vitamin. However, there is no official recommendation from NICE or the NHS for routine combined supplementation in the general population. Individuals should consult their GP before commencing supplementation, particularly if they have existing medical conditions, take regular medications, or are considering doses above standard recommendations.

Potential Side Effects and Interactions

Both vitamins D and K are generally well-tolerated when taken at recommended doses, but potential adverse effects and drug interactions warrant consideration.

Vitamin D side effects are rare at the recommended daily intake of 10 micrograms. However, excessive intake (typically above the maximum daily amounts) can lead to hypercalcaemia, characterised by:

• Nausea, vomiting, and poor appetite

• Excessive thirst and frequent urination

• Weakness and fatigue

• Kidney stones or impaired renal function

• Cardiac arrhythmias in severe cases

The maximum daily intake levels that should not be exceeded are:

• Adults and children 11+ years: 100 micrograms (4,000 IU)

• Children 1-10 years: 50 micrograms (2,000 IU)

• Infants under 1 year: 25 micrograms (1,000 IU) unless medically advised

Individuals should not exceed these amounts without medical supervision.

Vitamin K side effects are uncommon, and no tolerable upper limit has been established due to low toxicity. However, vitamin K can interact significantly with anticoagulant medications, particularly warfarin. Vitamin K antagonises warfarin's anticoagulant effect, potentially reducing INR (international normalised ratio) and increasing thrombosis risk. Patients taking warfarin should:

• Maintain consistent vitamin K intake rather than avoiding it entirely

• Not start vitamin K supplements unless advised by their anticoagulation clinic

• Have regular INR monitoring if dietary or supplement intake changes

Novel oral anticoagulants (DOACs such as apixaban, rivaroxaban) do not interact with vitamin K in the same manner, but patients should still inform their healthcare provider about any supplementation.

Other potential interactions include:

• Vitamin D may interact with thiazide diuretics, increasing hypercalcaemia risk

• Some medications (orlistat, cholestyramine) reduce fat-soluble vitamin absorption

• Corticosteroids may increase vitamin D requirements

When to seek medical advice: Patients should contact their GP if they experience symptoms of hypercalcaemia whilst taking vitamin D, notice unusual bleeding or bruising (potential vitamin K deficiency), or have concerns about interactions with prescribed medications. Individuals with kidney disease, sarcoidosis, or hyperparathyroidism require specialist guidance before supplementing with vitamin D due to altered calcium metabolism.

If you suspect side effects from any supplement, report them through the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk). As with all supplements, it is essential to inform healthcare providers about all vitamins and supplements being taken to ensure safe, coordinated care.

Frequently Asked Questions