Vitamin K: A Comprehensive Guide to the Essential Nutrient

Vitamin K is a fat-soluble vitamin essential for various physiological functions, primarily involved in blood clotting and bone metabolism. Discovered in the 1930s by Danish scientist Henrik Dam, the “K” stands for “Koagulation,” the German word for coagulation, due to its key role in the blood-clotting process. While Vitamin K often flies under the radar compared to other vitamins like C, D, or B12, its importance in maintaining overall health is profound and multifaceted.

This article explores the different forms of vitamin K, its biological roles, food sources, deficiency symptoms, recommended intakes, interactions with medications, and its implications in various health conditions.

Forms of Vitamin K

There are two primary natural forms of vitamin K:

1. Vitamin K1 (Phylloquinone)

• Found in green leafy vegetables.
• The predominant dietary source.
• Directly involved in the blood-clotting process.

2. Vitamin K2 (Menaquinone)

• Found in fermented foods and produced by gut bacteria.
• Includes several subtypes (MK-4 to MK-13), each with different lengths of side chains and biological activity.
• Plays roles in cardiovascular health and bone metabolism.

A third form, Vitamin K3 (Menadione), is a synthetic form used in animal feed but not recommended for human consumption due to potential toxicity.

Biological Functions of Vitamin K

1. Blood Clotting

The primary role of vitamin K is to activate proteins involved in coagulation. This process includes:

• Prothrombin (Factor II) and other clotting factors (VII, IX, X) require vitamin K-dependent gamma-carboxylation for activation.
• This carboxylation enables calcium binding, which is essential for the proteins to function in the clotting cascade.

Without adequate vitamin K, the body cannot form blood clots effectively, leading to excessive bleeding from even minor injuries.

2. Bone Health

Vitamin K is essential for synthesizing osteocalcin, a protein that binds calcium to bone matrix, increasing bone mineralization. Its role in bone health includes:

• Enhancing bone strength and reducing fracture risk.
• Supporting bone density, especially in postmenopausal women.

3. Cardiovascular Health

Vitamin K2 activates Matrix Gla Protein (MGP), which inhibits arterial calcification. Low levels of vitamin K2 are associated with:

• Increased arterial stiffness.
• Higher risk of coronary artery disease.
• Accelerated vascular aging.

4. Cellular Functions and Cancer Prevention

Emerging research suggests vitamin K may have roles in:

• Regulating cell growth and apoptosis.
• Reducing inflammation.
• Possibly decreasing the risk of certain cancers, such as liver, prostate, and lung cancer.

Dietary Sources of Vitamin K

Sources of Vitamin K1

• Kale
• Spinach
• Broccoli
• Brussels sprouts
• Parsley
• Green beans
• Soybean and canola oils

Sources of Vitamin K2

• Natto (fermented soybeans) – highest source
• Hard and soft cheeses
• Egg yolks
• Liver and other organ meats
• Chicken
• Fermented foods like sauerkraut

The bioavailability of K2 from animal sources is generally higher than that of K1 from plant sources, and K2 has a longer half-life in the body.

Absorption and Metabolism

Being a fat-soluble vitamin, vitamin K requires dietary fat for optimal absorption. It is absorbed in the small intestine and transported via chylomicrons into the lymphatic system and bloodstream.

Vitamin K1 is preferentially taken up by the liver, whereas K2 is more widely distributed in extra-hepatic tissues such as bones and blood vessels.

Recommended Daily Intake

The recommended daily intake (RDI) for vitamin K varies by age and gender. Here are the general guidelines:

Note: These values refer primarily to K1. No specific daily value has been set for K2, but emerging evidence suggests it is also crucial.

Vitamin K Deficiency

Causes

• Inadequate dietary intake
• Fat malabsorption disorders (e.g., celiac disease, cystic fibrosis, bile duct obstruction)
• Prolonged use of antibiotics (which disrupt gut microbiota that synthesize K2)
• Liver disease
• Use of anticoagulant medications (like warfarin, which interferes with vitamin K recycling)

Symptoms

• Easy bruising
• Excessive bleeding from wounds, punctures, or surgical sites
• Nosebleeds or bleeding gums
• Blood in urine or stool
• Heavy menstrual bleeding
• Osteopenia or osteoporosis (long-term deficiency)

Vitamin K Deficiency Bleeding (VKDB) in Newborns

• Newborns have low vitamin K levels due to poor placental transfer and sterile intestines.
• VKDB can lead to serious, even fatal bleeding.
• Prophylactic vitamin K injection is standard practice after birth.

Vitamin K and Anticoagulant Therapy

Vitamin K plays a central role in the mechanism of action of warfarin and other vitamin K antagonists. These drugs inhibit vitamin K epoxide reductase, which is necessary for recycling vitamin K to its active form.

Important Considerations:

• Patients on warfarin must maintain consistent vitamin K intake.
• Sudden increases or decreases in dietary vitamin K can affect INR levels, leading to increased bleeding risk or thrombosis.

Health Benefits of Vitamin K Beyond Clotting

1. Osteoporosis Prevention

• Higher vitamin K intake is associated with better bone mineral density.
• Supplementation (especially K2) may reduce fracture risk, particularly in postmenopausal women.
• Some countries, like Japan, use K2 therapeutically for osteoporosis.

2. Arterial Calcification and Heart Disease

• Vitamin K2 reduces the risk of calcium buildup in arteries, potentially preventing atherosclerosis.
• Several studies indicate that higher K2 intake is linked to lower cardiovascular mortality.

3. Diabetes and Metabolic Health

• Vitamin K may improve insulin sensitivity.
• Some data suggest a lower incidence of Type 2 diabetes with higher K2 intake.

4. Cancer Prevention

• Anti-inflammatory and cell-regulatory effects may reduce cancer risk.
• Observational studies link higher K2 intake to lower risk of prostate and liver cancers.

5. Cognitive Health

• Vitamin K plays a role in sphingolipid metabolism in the brain.
• Low vitamin K status has been correlated with cognitive decline and Alzheimer’s disease in the elderly.

Supplementation

Vitamin K supplements come in the following forms:

• K1 (Phylloquinone): Generally used for correcting deficiency.
• K2 (MK-4 and MK-7): Used in bone and cardiovascular health supplements.

Dosage

MK-7 is more bioavailable and has a longer half-life than MK-4.

Common doses:

• MK-4: 5 mg–45 mg/day
• MK-7: 90 mcg–360 mcg/day

Supplements should be taken with meals containing fat for optimal absorption.

Safety and Toxicity

Vitamin K is considered safe, with no known toxicity from dietary intake.

Cautions:

• Synthetic K3 (menadione) can be toxic and is not approved for human use.
• Excessive supplementation in those on anticoagulants can counteract medication effects.

Vitamin K and Gut Microbiota

• Gut bacteria produce vitamin K2, particularly MK-7 to MK-13.
• The extent to which intestinally synthesized vitamin K contributes to overall vitamin K status is still under investigation.
• A healthy microbiome may support vitamin K levels.

Interactions with Other Nutrients

• Vitamin D and Calcium: Work synergistically with vitamin K for bone and cardiovascular health.
• Vitamin A: Also involved in bone remodeling, but high doses may compete with vitamin K.
• Vitamin E: High doses may antagonize vitamin K and affect clotting.

Conclusion

Vitamin K is a vital nutrient with roles far beyond coagulation. From supporting strong bones to preventing cardiovascular disease and possibly even cancer, vitamin K deserves greater attention in both public health and clinical settings.

While deficiency is rare in healthy adults with balanced diets, certain populations such as newborns, the elderly, and individuals with malabsorption are at higher risk. Moreover, the growing body of research around K2, particularly its role in bone and heart health, suggests that adequate intake of both forms of vitamin K is essential.

Maintaining a diet rich in leafy greens, fermented foods, and healthy fats, or considering supplementation when necessary, can help ensure optimal vitamin K levels and long-term health.