vitamin k

Vitamin K refers to a family of fat‑soluble vitamins best known for their essential roles in blood coagulation and bone health. The most common dietary form is phylloquinone (vitamin K1), found primarily in green leafy vegetables and some plant oils. Other forms, collectively referred to as menaquinones (vitamin K2), are present in fermented foods, certain animal products, and are also synthesized by intestinal bacteria. These distinct forms share a core naphthoquinone ring structure but differ in their side chains, influencing their absorption, distribution, and biological activities. Phylloquinone was first identified as a vital nutrient for blood clotting—its name derives from the German word "Koagulation" for coagulation—and scientists later discovered that it is a cofactor for the enzyme vitamin K‑dependent carboxylase. This enzyme catalyzes the carboxylation of specific glutamic acid residues on several proteins, enabling them to bind calcium and perform their physiological functions. In the liver, vitamin K is indispensable for the gamma‑carboxylation of clotting factors II (prothrombin), VII, IX, and X, as well as anticoagulant proteins C and S. Without sufficient vitamin K, these proteins remain inactive, leading to impaired hemostasis and a propensity for bleeding. Beyond coagulation, vitamin K‑dependent gamma‑carboxylated proteins also include osteocalcin, which is produced by osteoblasts and plays a role in bone mineralization; and matrix Gla‑protein (MGP), present in vascular smooth muscle and cartilage, which may help prevent abnormal calcification in arteries. While deficiency severe enough to cause clinical bleeding is rare in healthy adults, it is a critical concern in newborns due to poor transplacental transfer and low levels in breast milk, making prophylactic vitamin K administration standard practice in many countries. Despite its critical functions, vitamin K stores in the body are relatively low compared with other fat‑soluble vitamins because it is rapidly metabolized and excreted. Plasma concentrations of phylloquinone among healthy adults range broadly, and normal values are not universally agreed upon due to variability in dietary intake and measurement challenges. Because dietary intake varies widely and deficiency is uncommon outside of specific clinical conditions, health authorities set Adequate Intake (AI) levels rather than Recommended Dietary Allowances (RDAs). These guidelines aim to ensure nutritional adequacy in healthy populations based on observed intakes and functional markers.

Vitamin K is perhaps best known for its indispensable role in the blood clotting cascade but its biological actions extend to multiple systems within the body. Its primary function is as a cofactor for vitamin K‑dependent carboxylase enzymes that catalyze the post‑translational modification of specific proteins, converting glutamic acid residues into gamma‑carboxyglutamic acid (Gla). This modification is required for a family of clotting factors including prothrombin (factor II) and factors VII, IX, and X, enabling them to bind calcium ions and interact with phospholipid surfaces during the coagulation response. Without adequate vitamin K, these clotting factors remain under‑carboxylated, leading to impaired hemostasis and increased bleeding risk. In skeletal health, vitamin K is essential for modifying osteocalcin, a protein secreted by osteoblasts that binds calcium and hydroxyapatite in bone matrix. Systematic reviews and meta‑analyses suggest that adequate vitamin K status, and particularly supplementation with forms such as menaquinone‑4, is associated with improvements in bone mineral density (BMD) and reductions in fracture risk. One meta‑analysis of randomized controlled trials found that supplementation with phytonadione and menaquinone‑4 produced significant reductions in fracture incidence and slowed bone loss compared with controls, indicating potential benefits for populations at risk of osteoporosis, particularly older adults at high fracture risk. However, the evidence is nuanced and may vary by form, dose, and baseline nutritional status. Beyond hemostasis and skeletal maintenance, research indicates roles for vitamin K in cardiovascular health, metabolic regulation, and inflammation. Observational studies link higher circulating vitamin K concentrations or dietary intakes with favorable vascular outcomes, including lower coronary heart disease risk, possibly due to vitamin K‑dependent matrix Gla‑protein inhibiting vascular calcification. Another systematic review suggests modest improvements in markers of insulin resistance with vitamin K supplementation, though impacts on lipid profiles and broader metabolic outcomes are less consistent across trials. Meta‑analyses examining associations between vitamin K status and all‑cause mortality or cardiovascular events find mixed results, underscoring the complexity of nutrient interactions and the need for high‑quality large‑scale trials. Vitamin K’s involvement in diverse health processes has prompted exploration of its potential roles in immune support, cancer biology, and neurological health, though evidence in these areas remains emergent and less conclusive. Dietary guidelines emphasize obtaining vitamin K through nutrient‑dense foods, noting that gut microbial synthesis of menaquinones may contribute to overall status, though the clinical significance of this endogenous production remains to be fully quantified. Overall, vitamin K’s well‑established functions in coagulation and bone health are supported by both mechanistic and clinical evidence, while its broader health effects warrant continued investigation.

Public health authorities determine nutrient intake recommendations to ensure sufficient levels for physiological needs in healthy populations. For vitamin K, the U.S. Institute of Medicine (IOM) has established Adequate Intake (AI) levels rather than specific RDAs, reflecting limited evidence to define an Estimated Average Requirement (EAR). AI values represent intake amounts observed in healthy groups that are associated with normal functional outcomes such as effective clotting and maintenance of bone health. For adults aged 19 and older, current AIs are set at 120 micrograms (mcg) per day for men and 90 mcg per day for women, including during pregnancy and lactation. Infants from birth to 6 months require about 2 mcg per day, increasing slightly to about 2.5 mcg for ages 7–12 months. Children from ages 1–3 and 4–8 require 30 mcg and 55 mcg per day, respectively, with adolescents aged 9–18 targeting 60–75 mcg per day depending on age. These values apply to total vitamin K intake from both dietary sources and any supplements. Factors affecting vitamin K needs include age, sex, physiological state, health conditions, and medications. For example, newborn infants typically have low vitamin K stores because placental transfer of this nutrient is limited and breast milk contains relatively low concentrations, prompting routine prophylactic vitamin K administration at birth to prevent vitamin K deficiency bleeding (VKDB). Older adults may have altered absorption or dietary patterns that influence their intake adequacy. People with gastrointestinal conditions that impair fat absorption—such as celiac disease, Crohn’s disease, or cystic fibrosis—may require careful monitoring because vitamin K is fat‑soluble and dependent on bile acids and pancreatic enzymes for effective absorption. Although most healthy adults achieve adequate vitamin K status from a balanced diet rich in dark leafy greens, cruciferous vegetables, and certain plant oils, subgroups with restricted diets or malabsorption disorders may benefit from targeted nutritional guidance to meet intake goals. There are no established tolerable upper intake limits (ULs) for vitamin K because adverse effects from high intakes have not been observed in healthy populations and toxicity appears extremely rare. However, individuals taking medications that influence clotting, such as warfarin, should strive for consistent daily vitamin K intake to minimize fluctuations that could affect medication efficacy. Healthcare providers may recommend monitoring or adjustments for specific clinical contexts, but general recommendations prioritize dietary adequacy through foods and, if necessary, supplementation under supervision.

Vitamin K deficiency is uncommon in healthy adults consuming a varied diet, but it can occur in specific clinical situations or life stages where intake or absorption is impaired. Severe deficiency profoundly impacts the body’s ability to produce functional vitamin K‑dependent proteins, leading to impaired clotting factor activation and an increased bleeding tendency. The hallmark clinical manifestation of deficiency is hemorrhage, which may present as easy bruising, epistaxis (nosebleeds), bleeding gums, or prolonged bleeding from wounds and surgical sites. In severe cases, bleeding can occur internally—such as gastrointestinal bleeding, hematuria (blood in urine), or melaena (black, tarry stools)—reflecting bleeding within the digestive tract. Laboratory investigations typically reveal prolonged prothrombin time (PT) and international normalized ratio (INR) due to ineffective activation of vitamin K‑dependent clotting factors. Newborn infants constitute a high‑risk group because placental transfer of vitamin K is limited and breast milk provides low levels of this nutrient, placing infants at risk for vitamin K deficiency bleeding (VKDB), formerly called haemorrhagic disease of the newborn. VKDB can manifest early (within 24 hours of birth), classically between day two and day seven, or late (after 2 weeks up to 6 months) with signs such as bruising, petechiae, bleeding from the umbilical stump or circumcision site, intracranial hemorrhage, or other serious bleeding events. Prophylactic administration of vitamin K at birth dramatically reduces VKDB incidence. Adults with fat malabsorption disorders—such as cystic fibrosis, celiac disease, cholestasis, or inflammatory bowel disease—or those with chronic antibiotic use that disrupts gut microbiota are at heightened risk because vitamin K is fat‑soluble and partly derived from bacterial synthesis. Malnutrition and very low‑fat diets can also impair absorption. In these populations, deficiency signs may include not only bleeding complications but also subtle effects on bone health, with under‑carboxylated osteocalcin indicative of insufficient vitamin K for optimal bone matrix formation, potentially contributing to osteopenia or osteoporosis over time. Because plasma phylloquinone measurements do not always reflect total body status, clinicians often assess vitamin K deficiency through functional tests such as PT/INR and levels of under‑carboxylated Gla proteins rather than nutrient concentrations alone. Timely recognition and treatment—often with oral or injectable vitamin K—are essential to prevent serious sequelae. Education on dietary sources and attention to risk factors helps minimize deficiency risk in vulnerable individuals.

Vitamin K is abundant in a variety of whole foods, particularly dark leafy greens and certain vegetables. Phylloquinone (vitamin K1) contributes most dietary vitamin K in Western diets and is concentrated in chloroplast‑rich plant tissues. Some foods provide exceptionally high amounts per serving, often exceeding daily intake goals. Dark leafy greens are premier sources: for example, raw parsley provides around 984 mcg per cup, and cooked spinach can exceed 880 mcg per cup. Other cooked greens such as mustard greens, collard greens, beet greens, and Swiss chard also supply several hundred micrograms per cup. Cruciferous vegetables contribute meaningfully; cooked broccoli and Brussels sprouts each provide over 200 mcg per cup, while raw cabbage and kale supply significant portions of daily intake. Salad greens like romaine lettuce and watercress are rich sources in their raw forms, suitable for salads, wraps, and sandwiches. Beyond greens, other vegetables and plant foods provide vitamin K: asparagus, okra, green beans, and plantains are examples with moderate content. Certain fruits such as kiwifruit, blueberries, and grapes offer vitamin K in smaller amounts but can contribute when consumed regularly. Plant oils like soybean and canola oil contribute modest amounts when used in cooking or dressings. Vitamin K2 forms—menaquinones—are found in fermented foods and animal products. Natto, a fermented soybean dish, is among the richest sources of menaquinone, supplying hundreds of micrograms per serving. Fermented dairy products such as cheese contain variable amounts depending on bacterial strains and production methods, while meat, egg yolks, and liver contain lower but meaningful amounts of menaquinones such as MK‑4. The contribution of gut bacterial synthesis of vitamin K, particularly MK‑7 and other long‑chain variants, to total vitamin K status remains an area of research, but endogenous production likely supplements dietary intake in healthy individuals. Because vitamin K is fat‑soluble, consuming these foods with dietary fat—such as olive oil, nuts, seeds, or avocado—enhances absorption. Variability in food preparation affects vitamin K content: cooking often concentrates nutrients per serving but may reduce volume consumed, so a combination of raw and cooked vegetables can maximize intake. Incorporating a variety of these nutrient‑dense foods across meals supports meeting daily vitamin K needs and confers additional nutrients like fiber, antioxidants, and minerals.

Vitamin K is a fat‑soluble nutrient, meaning it requires dietary fat and proper digestive processes to be effectively absorbed. Ingested phylloquinone from green leafy vegetables and other plant sources is incorporated into mixed micelles in the small intestine through the action of bile acids and pancreatic enzymes. Once solubilized, it is taken up by enterocytes and transported via chylomicrons into lymphatic circulation before reaching the liver and peripheral tissues. Because of this reliance on lipid transport mechanisms, conditions that impair fat digestion or bile flow—such as cholestatic liver disease, cystic fibrosis, or prolonged use of bile acid sequestrants—can significantly reduce vitamin K absorption. Bioavailability varies markedly among food sources. Vitamin K in free form, such as in supplements or oils, can be absorbed at an approximate rate of 80%. However, the phylloquinone bound within plant chloroplasts is less accessible; for example, absorption from spinach and similar leafy greens is significantly lower, with body uptake estimates showing only a fraction compared with supplemental forms. Consuming these vegetables with added fats such as olive oil, nuts, seeds, or dairy can enhance micelle formation and improve absorption. Menaquinones (vitamin K2 variants) differ structurally from phylloquinone and may follow similar digestive uptake but also display different tissue distribution and half‑lives, potentially leading to prolonged circulatory presence compared to K1. Long‑chain menaquinones such as MK‑7 are increasingly studied for their bioavailability and effects on extra‑hepatic vitamin K‑dependent proteins, though comprehensive comparisons remain an active area of research. Factors that hinder absorption include low dietary fat intake, gastrointestinal disorders, surgical resection of the small intestine, and certain medications that interfere with lipid metabolism. Individuals with obesity or metabolic syndrome may exhibit altered vitamin K kinetics, though the clinical implications of these differences are under investigation. In practice, promoting balanced meals containing healthy fats alongside vitamin K‑rich foods is a practical strategy to optimize absorption in most individuals.

Supplementation with vitamin K is generally unnecessary for most healthy adults who consume a varied diet rich in green leafy vegetables, cruciferous vegetables, and other nutrient‑dense foods. Because deficiency is rare in the general population and the vitamin is widely distributed in common foods, achieving adequate intake through diet is feasible. Exceptions exist for specific clinical situations where supplements may be appropriate. For example, newborn infants are routinely given a vitamin K injection shortly after birth to prevent vitamin K deficiency bleeding (VKDB), a condition characterized by hemorrhage due to low stores at birth and limited transfer across the placenta. This intervention has dramatically reduced the incidence of VKDB in neonatal populations. Adults with malabsorption disorders, liver disease affecting bile production, or prolonged antibiotic use that disrupts gut microbial synthesis of vitamin K may benefit from monitored supplementation. In these cases, oral vitamin K supplements—often in the form of phylloquinone (vitamin K1) or menaquinone variants such as MK‑7—can help restore adequate levels and support normal physiological function. The choice of form may depend on the clinical context: vitamin K1 is most closely tied to hepatic clotting factor activation, while vitamin K2 forms like MK‑7 may have longer half‑lives and preferential effects on extra‑hepatic proteins such as osteocalcin and matrix Gla‑protein, though definitive clinical superiority remains under study. When considering supplementation, dosing should be individualized and guided by healthcare providers, particularly for individuals on anticoagulant medications like warfarin, where unmanaged increases in vitamin K intake can antagonize drug effects. Routine blood tests, including prothrombin time or INR, help assess functional vitamin K status in at‑risk patients. Supplements are widely available in multivitamins or as single‑ingredient products, and quality varies by manufacturer. When used, vitamin K is typically taken with meals containing dietary fat to enhance absorption. Consultation with a clinician ensures that supplementation aligns with individual health needs, existing medications, and overall nutritional status.

Vitamin K toxicity is extremely rare because the body rapidly metabolizes and excretes excess amounts and because there is no established tolerable upper intake level (UL) for vitamin K; evidence does not indicate adverse effects from high intakes of vitamin K‑rich foods or typical supplemental doses. Most cases of excessive vitamin K intake from food sources do not lead to harmful outcomes, even at levels far exceeding daily intake goals. This safety profile is partly attributable to efficient physiological regulation, including limited storage and swift clearance. However, caution is warranted in specific clinical contexts. High supplemental doses of synthetic forms, particularly menadione (vitamin K3), have been associated with toxicity in the past and are not commonly used today due to potential adverse effects such as hemolytic anemia and jaundice in susceptible individuals. Modern supplements typically provide phylloquinone (K1) or menaquinone forms (K2 variants) at doses considered safe for the general population when used as directed. Although general toxicity is not a concern at typical supplemental doses, vitamin K can have clinical implications when interacting with medications that influence clotting pathways. Individuals using vitamin K antagonists such as warfarin may experience reduced anticoagulant efficacy with increased vitamin K intake, leading to a higher risk of clot formation if not managed carefully. Thus, rather than toxicity, the main risk associated with high vitamin K intake in these patients is a pharmacodynamic interaction rather than a direct nutrient toxicity. Clinicians often advise maintaining consistent vitamin K intake to stabilize anticoagulant response rather than drastic fluctuations. For most people not on anticoagulants, enjoying a variety of vitamin K‑rich foods poses minimal safety concerns. Rare gastrointestinal symptoms such as nausea or upset stomach have been reported with supplemental forms, but these are typically mild. Individuals with specific health conditions or those taking drugs affecting lipid absorption should discuss appropriate vitamin K intake strategies with healthcare providers. In summary, vitamin K has a wide safety margin with no established UL, with toxicity predominantly a theoretical concern outside usual dietary or supplemental ranges.

The most clinically significant drug‑nutrient interaction involving vitamin K occurs with anticoagulant medications that act as vitamin K antagonists, primarily warfarin (Coumadin). Warfarin exerts its anticoagulant effect by inhibiting vitamin K epoxide reductase, an enzyme required to regenerate active vitamin K for gamma‑carboxylation of clotting factors. Because vitamin K and warfarin have opposing actions, increased intake of vitamin K—especially abrupt changes—can antagonize warfarin’s effect, reducing anticoagulation and increasing the risk of thrombotic events. Conversely, decreased vitamin K intake can enhance warfarin’s effect and increase bleeding risk. For these reasons, individuals on warfarin are advised to maintain consistent daily vitamin K intake and to have regular INR monitoring when dietary intake changes. Interactions are not limited to warfarin; other vitamin K antagonists used in Europe also share similar mechanisms. Direct oral anticoagulants (DOACs) such as apixaban, rivaroxaban, and dabigatran do not rely on vitamin K pathways and typically do not interact with vitamin K in the same manner, so dietary fluctuations of vitamin K are less likely to affect their anticoagulant activity. However, patients should still discuss intake with clinicians if they have specific concerns. Antibiotics that significantly disrupt gut microbiota can impair endogenous vitamin K2 production, potentially lowering vitamin K status, particularly in individuals with already marginal intakes or malabsorption disorders. Medications that interfere with fat absorption—such as bile acid sequestrants—also affect vitamin K uptake due to its fat‑soluble nature. Healthcare providers may adjust dosing or recommend nutritional monitoring when such interactions are likely. Professional guidance ensures that nutrient–drug interactions are managed safely, with attention to both therapeutic outcomes and overall nutrient status.

Common Forms: phylloquinone (K1), menaquinone‑4 (MK‑4), menaquinone‑7 (MK‑7)

Typical Doses: 90–120 mcg per day dietary equivalent; therapeutic doses individualized

When to Take: with meals containing fat

Best Form: menaquinone‑7 for longer half‑life

⚠️ Interactions: warfarin, bile acid sequestrants, broad‑spectrum antibiotics