Vitamin K is a vital fat-soluble vitamin playing a crucial role in blood clotting and bone health. It exists in two primary forms: phylloquinone, predominantly found in green leafy vegetables, and menaquinones, present in certain animal products and fermented foods. Furthermore, menaquinones can be synthesized by bacteria within the human gut. [1]
Vitamin K is instrumental in producing various proteins essential for blood coagulation and bone development. Prothrombin, a vitamin K-dependent protein, is directly involved in blood clotting. Osteocalcin, another protein requiring vitamin K, is essential for producing healthy bone tissue.
Vitamin K is distributed throughout the body, including the liver, brain, heart, pancreas, and bones. However, it is rapidly broken down and eliminated through urine or stool. This rapid metabolism means that toxic levels are rarely reached, even with high intakes, unlike some other fat-soluble vitamins.
Recommended Daily Intake of Vitamin K
When sufficient evidence is lacking to establish a Recommended Dietary Allowance (RDA), an Adequate Intake (AI) is used. The AI represents an estimated intake level assumed to ensure nutritional adequacy. For adults aged 19 and older, the AI for vitamin K is 120 micrograms (mcg) daily for men and 90 mcg for women, including those who are pregnant or lactating.
The Health Benefits of Vitamin K
Blood Clotting
Vitamin K is essential for producing four of the thirteen proteins required for blood clotting. This process is critical for stopping bleeding from wounds, allowing them to heal properly. Individuals prescribed anticoagulants (blood thinners) to prevent blood clots in the heart, lungs, or legs are often advised about vitamin K intake. Due to its role in blood clotting, vitamin K can potentially counteract the effects of blood-thinning medications. Prothrombin time (PT), which measures how long it takes for blood to clot, is a common way to estimate vitamin K levels. Patients on anticoagulant medication like warfarin (Coumadin) are often advised to maintain a consistent vitamin K intake through diet and supplements. While minor changes in vitamin K intake typically don’t affect PT, significant and sudden variations can alter PT levels and compromise the medication’s effectiveness. Although vitamin K is not a required nutrient listed on Nutrition Facts labels, healthcare providers usually provide information on foods containing vitamin K to patients taking anticoagulant medication.
Close-up of various green leafy vegetables, including kale, spinach, and romaine lettuce, representing food sources rich in Vitamin K.
Bone Health
Vitamin K is involved in producing bone proteins, including osteocalcin, crucial for preventing bone weakening. Research suggests that higher vitamin K intakes are associated with a reduced risk of hip fractures and low bone density. [2] Conversely, low blood levels of vitamin K have been linked to decreased bone density. [2] The Nurses’ Health Study indicated that women consuming at least 110 mcg of vitamin K daily were 30% less likely to experience a hip fracture compared to those with lower intakes. [3] Within the study group, consuming a daily serving of lettuce or other green leafy vegetables halved the risk of hip fracture compared to eating one serving per week. Data from the Framingham Heart Study also demonstrated a correlation between high vitamin K intake and a decreased risk of hip fracture in both men and women, as well as increased bone mineral density in women. [4, 5] However, the results of clinical trials and meta-analyses regarding the effectiveness of vitamin K supplements in reducing bone fractures have been inconsistent. [1] This inconsistency may be attributed to other factors influencing bone health, such as insufficient calcium, vitamin D, and weight-bearing exercise, potentially masking the benefits of vitamin K supplementation.
Heart Disease
Some studies have examined the role of vitamin K in heart health. Vitamin K is involved in the production of matrix Gla proteins (MGP), which help prevent calcification or hardening of heart arteries, a contributing factor to heart disease. Given the limited research in this area, more studies are necessary before recommending a specific vitamin K intake beyond the standard recommendation for this condition.
What Food Contains Vitamin K? Top Food Sources
Vitamin K1 (phylloquinone) is found in:
- Leafy Green Vegetables: Kale, spinach, collard greens, turnip greens, mustard greens, romaine lettuce, and green leaf lettuce are excellent sources.
- Other Vegetables: Broccoli, Brussels sprouts, cabbage, and asparagus.
- Plant-Based Oils: Soybean oil, canola oil, and olive oil contain vitamin K1.
Vitamin K2 (menaquinones) is found in:
- Animal Products: Chicken, egg yolks, and certain cheeses (like hard cheeses) contain varying amounts of vitamin K2.
- Fermented Foods: Natto (fermented soybeans) is exceptionally high in vitamin K2. Other fermented foods like sauerkraut and kefir can also contribute.
Signs of Vitamin K Deficiency
Vitamin K deficiency is rare in adults but can occur in individuals taking medications that interfere with vitamin K metabolism, such as antibiotics, or those with conditions that cause malabsorption of nutrients. Newborn infants are also at risk because vitamin K does not cross the placenta effectively, and breast milk contains low amounts. The following are the most common signs of a deficiency:
- Prolonged prothrombin time (as measured by a physician)
- Excessive bleeding
- Hemorrhaging
- Osteopenia or osteoporosis
Important Facts About Vitamin K
- Antibiotics can disrupt the gut bacteria that produce vitamin K, potentially lowering vitamin K levels, especially with long-term use. Individuals with poor appetite while taking long-term antibiotics may be at higher risk of deficiency and may benefit from a vitamin K supplement.
- Since vitamin K is fat-soluble, consuming vitamin K-rich foods with some fat enhances absorption. For example, drizzle olive oil or add avocado to leafy green salads.
Conclusion
Ensuring adequate vitamin K intake through a balanced diet rich in leafy green vegetables, certain animal products, and fermented foods is crucial for maintaining optimal blood clotting, bone health, and potentially cardiovascular health. If you have concerns about your vitamin K levels or are taking anticoagulant medications, consult with your healthcare provider for personalized recommendations.
References
- National Institutes of Health – Vitamin K Fact Sheet for Health Professionals
- Harvard T.H. Chan School of Public Health – Vitamin K
- Feskanich D, Weber P, Willett WC, et al. Vitamin K and hip fracture in women: a prospective study. Am J Clin Nutr. 1999;69(1):74-9.
- Booth SL, Tucker KL, Chen H, et al. Associations between vitamin K biochemical measures and bone mineral density in elderly men and women. J Clin Endocrinol Metab. 2004;89(10):4904-9.
- Sokoll LJ, Booth SL, O’Brien ME, et al. Changes in serum osteocalcin, gamma-carboxylated osteocalcin, and undercarboxylated osteocalcin concentrations upon vitamin K supplementation in women. Am J Clin Nutr. 1997;65(4):1126-31.
