Vitamin K is a fat-soluble vitamin essential for blood clotting, bone metabolism, and cellular regulation. Whilst its role in haemostasis is well established, emerging research has explored potential links between vitamin K and cancer prevention or treatment. However, it is crucial to understand that no official connection has been proven, and current evidence remains preliminary. This article examines what vitamin K does in the body, reviews the existing research on vitamin K and cancer, discusses dietary sources and safe intake levels, and highlights important considerations for patients undergoing cancer treatment or taking anticoagulant medications.

What Is Vitamin K and Its Role in the Body

Vitamin K is a fat-soluble vitamin essential for several critical physiological processes in the human body. It exists primarily in two naturally occurring forms: vitamin K1 (phylloquinone), found predominantly in green leafy vegetables, and vitamin K2 (menaquinone), which is produced by bacteria in the gut and present in fermented foods and animal products. Vitamin K3 (menadione) is a synthetic form not licensed for human use due to toxicity concerns, though it is used in some animal feeds.

The most well-established role of vitamin K is in blood coagulation. It acts as a cofactor for the enzyme gamma-glutamyl carboxylase, which activates clotting factors II, VII, IX, and X, as well as proteins C and S. Without adequate vitamin K, the blood clotting cascade cannot function properly, leading to increased bleeding risk. This mechanism is the basis for warfarin therapy, which works by inhibiting vitamin K epoxide reductase (VKORC1) to achieve therapeutic anticoagulation.

Beyond haemostasis, vitamin K plays important roles in bone metabolism and calcium regulation. It activates osteocalcin and other bone Gla proteins that bind calcium to the bone matrix. While this supports bone mineralisation, the clinical benefits of vitamin K supplementation for bone mineral density and fracture prevention remain unproven. Emerging research suggests vitamin K may also be involved in cellular processes including cell growth regulation, apoptosis (programmed cell death), and inflammation modulation. These additional functions have prompted scientific interest in whether vitamin K might influence cancer development or progression.

The UK sets a safe intake (adequate intake) for vitamin K of approximately 1 microgram per kilogram of body weight daily for adults. Deficiency is relatively uncommon in healthy individuals consuming a balanced diet. However, certain medical conditions affecting fat absorption, prolonged antibiotic use, or specific medications may increase deficiency risk. The extent to which colonic menaquinone (K2) production contributes to overall vitamin K status in humans remains uncertain.

Current Research on Vitamin K and Cancer Prevention

Scientific interest in the relationship between vitamin K and cancer has grown over the past two decades, though it is important to emphasise that there is no official link established between vitamin K supplementation and cancer prevention or treatment. The research remains preliminary, with most evidence derived from laboratory studies and observational epidemiological data rather than robust clinical trials.

Laboratory and mechanistic studies have demonstrated that vitamin K compounds, particularly vitamin K2 and certain synthetic analogues, can inhibit cancer cell growth and induce apoptosis in various cancer cell lines, including hepatocellular carcinoma, leukaemia, lung cancer, and prostate cancer cells. The proposed mechanisms include interference with cell cycle progression, modulation of inflammatory pathways, and effects on gene expression related to cell survival. However, it is important to note that effects observed in laboratory settings do not necessarily translate to clinical benefits in humans.

Observational epidemiological studies have produced mixed results. Some population-based research has suggested associations between higher dietary vitamin K2 intake and reduced risk of certain cancers. For example, the EPIC Heidelberg study found an inverse association between vitamin K2 intake and prostate cancer risk, though no such association was observed with vitamin K1. However, other large cohort studies have found no significant protective effect, and observational findings are subject to confounding factors that limit their interpretation.

Critically, randomised controlled trials examining vitamin K supplementation for cancer prevention are lacking. Both Cancer Research UK and the World Cancer Research Fund International indicate there is insufficient evidence to recommend vitamin K supplements for cancer prevention. The existing evidence does not support recommending vitamin K supplements specifically for cancer prevention outside of maintaining adequate nutritional status. Patients concerned about cancer risk should focus on evidence-based prevention strategies including smoking cessation, maintaining healthy body weight, limiting alcohol consumption, and attending national screening programmes.

Dietary Sources and Safe Intake of Vitamin K

Achieving adequate vitamin K intake through diet is straightforward for most individuals, as the vitamin is widely available in common foods. Vitamin K1 sources include dark green leafy vegetables such as kale, spinach, spring greens, broccoli, and Brussels sprouts. A single 80g portion of cooked kale can provide over 400 micrograms of vitamin K1, far exceeding daily requirements. Other sources include vegetable oils (particularly soybean and rapeseed oil), certain fruits like kiwi and avocado, and herbs such as parsley.

Vitamin K2 sources are more limited but include fermented foods like natto (fermented soybeans, extremely rich in K2), certain cheeses (particularly hard and soft ripened varieties), egg yolks, and meat products, especially liver. The gut microbiome also produces some vitamin K2, though the extent to which this contributes to overall vitamin K status remains debated.

For individuals following a balanced diet, deficiency is uncommon. However, certain groups may be at increased risk, including newborn infants (who receive prophylactic vitamin K at birth in the UK), individuals with malabsorption disorders (coeliac disease, Crohn's disease, cystic fibrosis), those with chronic liver disease, and people taking certain medications long-term. These include bile acid sequestrants (cholestyramine), orlistat, prolonged broad-spectrum antibiotics, and those with chronic cholestasis. Very low-fat diets may also impair vitamin K absorption due to its fat-soluble nature.

Supplementation is generally unnecessary for healthy adults eating varied diets. According to NHS guidance, supplements containing up to 1 mg (1000 micrograms) per day are unlikely to cause harm for most adults. However, supplements should be used cautiously, particularly in individuals taking anticoagulant medications such as warfarin. Patients considering supplementation should consult their GP or a registered dietitian, particularly if they have existing health conditions or take regular medications.

Important Considerations and Interactions with Cancer Treatment

Patients undergoing cancer treatment must exercise particular caution regarding vitamin K intake and supplementation due to potential interactions with therapies and the complexity of cancer care. There is no evidence supporting vitamin K supplementation as an adjunct to conventional cancer treatment, and patients should never use supplements as alternatives to evidence-based oncological care.

The most significant concern involves anticoagulant therapy. Many cancer patients require anticoagulation due to increased thrombotic risk associated with malignancy and certain chemotherapy regimens. Patients taking warfarin must maintain consistent vitamin K intake, as fluctuations can significantly affect International Normalised Ratio (INR) levels and anticoagulation control. Sudden increases in vitamin K-rich foods or supplements can reduce warfarin effectiveness, increasing thrombosis risk, whilst sudden decreases may potentiate anticoagulation and bleeding risk. Patients on warfarin should discuss dietary vitamin K with their anticoagulation clinic and avoid vitamin K supplements unless specifically prescribed. Newer direct oral anticoagulants (DOACs) such as apixaban or rivaroxaban do not interact with dietary vitamin K in the same manner.

Other medications that may affect vitamin K status include prolonged courses of broad-spectrum antibiotics (which reduce gut bacteria that produce vitamin K2), certain cephalosporins with NMTT side chains, bile acid sequestrants like cholestyramine, and orlistat. Patients should inform their oncology team about all supplements, including vitamin K, as part of comprehensive medication reconciliation.

When to seek medical advice: Patients should seek urgent medical help (call 999 or go to A&E) if they experience severe or uncontrolled bleeding. For non-urgent concerns, they should contact their GP, oncology team or anticoagulation clinic if they experience unusual bleeding or bruising, as this may indicate coagulation problems. Those considering vitamin K supplementation during or after cancer treatment should discuss this with their oncologist or specialist nurse. Maintaining a balanced diet rich in vegetables remains appropriate and beneficial for overall health during cancer treatment, but dramatic dietary changes should be discussed with the healthcare team. Patients should report any suspected adverse reactions to medicines or supplements via the MHRA Yellow Card scheme. Registered dietitians within oncology services can provide individualised nutritional guidance that considers treatment regimens, drug interactions, and individual patient factors to optimise nutritional status safely throughout the cancer journey.

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