Vitamin K is a fat-soluble vitamin essential for blood clotting and bone metabolism, yet it remains one of the most overlooked nutrients in women's health. Found primarily in green leafy vegetables and fermented foods, vitamin K activates proteins crucial for coagulation and skeletal integrity. For women, adequate intake becomes particularly important during life stages associated with bone loss, such as the menopause, when oestrogen decline affects bone density. Understanding vitamin K benefits for women can support informed dietary choices and long-term wellbeing, especially regarding bone health and proper blood clotting function.

What Is Vitamin K and Why Do Women Need It?

Vitamin K is a fat-soluble vitamin essential for blood clotting and bone metabolism. It exists in two primary forms: vitamin K1 (phylloquinone), found predominantly in green leafy vegetables, and vitamin K2 (menaquinone), present in fermented foods and animal products. While gut bacteria can produce some vitamin K2, its contribution to overall vitamin K status in humans remains uncertain.

For women, vitamin K plays several critical physiological roles. The vitamin activates specific proteins through a process called gamma-carboxylation, including clotting factors II, VII, IX, X, and proteins C and S, which are essential for proper blood coagulation. Vitamin K also activates osteocalcin, a protein that binds calcium to the bone matrix, thereby supporting skeletal integrity. This function becomes particularly relevant during life stages associated with accelerated bone loss, such as the perimenopausal and postmenopausal periods when oestrogen decline affects bone density. Some observational studies suggest vitamin K-dependent proteins may help regulate vascular calcification, though current UK guidelines do not recommend vitamin K supplementation for cardiovascular health.

Women may face specific challenges in maintaining adequate vitamin K status. Dietary restrictions, malabsorption disorders (such as coeliac disease or inflammatory bowel disease), and certain medications—including warfarin and some antibiotics—can interfere with vitamin K metabolism. Pregnancy and lactation also increase nutritional demands, though current UK guidance does not specify increased vitamin K requirements during these periods.

The UK does not have a Reference Nutrient Intake (RNI) for vitamin K; instead, a safe intake level is recommended. Understanding the importance of this often-overlooked nutrient can help women make informed dietary choices to support long-term health, particularly regarding skeletal wellbeing.

When to seek medical advice: Consult your GP if you experience unexplained bruising, frequent nosebleeds, or blood in urine or stools. Seek urgent medical care for heavy or uncontrolled bleeding.

How Much Vitamin K Do Women Need Daily?

The UK Department of Health recommends a safe intake of approximately 1 microgram of vitamin K per kilogram of body weight per day for adults. For an average woman weighing 70 kg, this equates to roughly 70 micrograms daily. This guidance differs from the Recommended Dietary Allowance (RDA) used in some countries, as the UK employs safe intake levels when insufficient evidence exists to establish precise requirements.

Vitamin K deficiency is relatively uncommon in healthy adults consuming a varied diet, as the vitamin is widely available in foods and partially synthesised by intestinal bacteria. However, certain populations may be at increased risk. Women with fat malabsorption conditions (including Crohn's disease, ulcerative colitis, or chronic pancreatitis) may have impaired absorption of fat-soluble vitamins. Those taking long-term antibiotics may experience reduced bacterial synthesis of vitamin K2 in the gut. Other medications that can affect vitamin K absorption include orlistat (a weight-loss medication) and cholestyramine (used to lower cholesterol).

It is important to note that vitamin K requirements do not officially increase during pregnancy or breastfeeding according to UK guidance, though maintaining adequate intake remains essential. Newborns receive vitamin K supplementation at birth to prevent vitamin K deficiency bleeding (VKDB), as they have limited stores and immature gut flora, in line with NHS and UK Health Security Agency guidance.

Women taking anticoagulant medications, particularly warfarin, require special consideration. Warfarin works by inhibiting vitamin K-dependent clotting factors, and significant fluctuations in dietary vitamin K intake can affect anticoagulation control. These patients should maintain consistent vitamin K consumption rather than avoiding it entirely, and any dietary changes should be discussed with their GP or anticoagulation clinic. Regular INR (International Normalised Ratio) monitoring ensures therapeutic efficacy and safety.

Food Sources and Supplements: Getting Enough Vitamin K

Achieving adequate vitamin K intake through diet is straightforward for most women, as the nutrient is abundant in commonly consumed foods. Green leafy vegetables represent the richest sources of vitamin K1, with particularly high concentrations in:

• Kale (approximately 390 micrograms per 100g cooked)

• Spinach (approximately 380 micrograms per 100g cooked)

• Spring greens (approximately 340 micrograms per 100g cooked)

• Broccoli (approximately 140 micrograms per 100g cooked)

• Brussels sprouts (approximately 140 micrograms per 100g cooked)

These values are approximate and can vary based on growing conditions, preparation methods, and cooking techniques. Other valuable sources include cabbage, lettuce, asparagus, and herbs such as parsley and coriander. Vitamin K2 is found in fermented foods (particularly natto, a Japanese fermented soybean product), cheese, egg yolks, and meat, though typically in lower concentrations than K1 in vegetables.

As a fat-soluble vitamin, absorption is enhanced when consumed with dietary fat. Adding a small amount of olive oil to salads or cooking vegetables with butter can improve bioavailability.

Supplementation is rarely necessary for healthy women eating a balanced diet. However, those with malabsorption disorders or taking medications affecting vitamin K metabolism may benefit from supplementation under medical supervision. Vitamin K supplements are available as K1 (phylloquinone) or K2 (menaquinone-7 being the most common form). According to NHS advice, taking 1 mg or less of vitamin K per day in supplements is unlikely to cause harm, though supplements should not be taken unnecessarily. Most vitamin K supplements in the UK are regulated as food supplements under UK food law (overseen by the Food Standards Agency and Trading Standards), not by the MHRA. Note that vitamin K3 (menadione) is not recommended for human supplementation.

Important safety consideration: Women taking warfarin or other vitamin K antagonists should not start vitamin K supplements without consulting their healthcare provider, as this can significantly reduce anticoagulant effectiveness. Those taking medications that affect fat absorption (such as orlistat or cholestyramine) should seek medical advice about potential vitamin K interactions. Conversely, those on direct oral anticoagulants (DOACs) such as apixaban or rivaroxaban do not have the same dietary restrictions, as these medications work through different mechanisms.

Vitamin K Benefits for Bone Health in Women

The relationship between vitamin K and bone health has garnered increasing research attention, particularly regarding osteoporosis prevention in women. Vitamin K activates osteocalcin, a protein synthesised by osteoblasts that binds calcium ions to the hydroxyapatite matrix of bone. Without adequate vitamin K, osteocalcin remains undercarboxylated and functionally impaired, potentially compromising bone mineralisation.

Epidemiological studies have suggested associations between higher vitamin K intake and improved bone mineral density, as well as potential links to fracture risk in postmenopausal women. Some research indicates that vitamin K2 (particularly the MK-7 form) may be more effective than K1 for bone health due to its longer half-life and better distribution to peripheral tissues. However, the evidence base remains mixed, and NICE guidance on osteoporosis does not currently recommend vitamin K supplementation for prevention or treatment of osteoporosis.

The Royal Osteoporosis Society emphasises a holistic approach to bone health, including adequate calcium and vitamin D intake, weight-bearing exercise, smoking cessation, and moderation of alcohol consumption. Vitamin K contributes to this multifactorial framework but should not be viewed as a standalone solution.

For women at high risk of osteoporosis—including those with premature menopause, long-term corticosteroid use, or previous fragility fractures—assessment through risk calculation tools (FRAX or QFracture) may be appropriate. Based on NICE guidance and National Osteoporosis Guideline Group (NOGG) thresholds, DEXA scanning and consideration of pharmacological interventions (such as bisphosphonates or denosumab) may be recommended. These decisions should be made in consultation with a GP or specialist.

When to seek medical advice: Women concerned about bone health, particularly those with risk factors for osteoporosis or experiencing height loss or fractures from minimal trauma, should contact their GP for assessment. Maintaining adequate vitamin K intake through a varied diet rich in green vegetables represents a sensible component of overall bone health strategy, alongside other evidence-based lifestyle measures and, where indicated, medical treatment.

Frequently Asked Questions