Vitamin K: Types, Sources, and Functions
Vitamin K is a fat-soluble vitamin that exists in the forms of phylloquinone (Vitamin K1) and menaquinone (Vitamin K2). It is primarily found naturally in green leafy vegetables (Vitamin K1) and in foods such as eggs and dairy products (Vitamin K2). In modern times, it is also used as a nutritional supplement.
[1]. Vitamin K plays a crucial role in carboxylation reactions involving specific proteins in the body. These proteins are mainly associated with blood clotting, bone metabolism, and various physiological responses, making Vitamin K an essential substance for regulating important metabolic processes in the body.
[2] [3].
What Is Vitamin K, and How Do Its Different Forms Differ?
Vitamin K comprises Vitamin K1 (phylloquinone, K1 Phylloquinone) and Vitamin K2 (a series of menaquinones, K2 Menaquinone).
[4].
Vitamin K1 (phylloquinone) is primarily synthesized by plants, green algae, and certain cyanobacteria, where it serves as an essential electron carrier during photosynthesis and is involved in protein structure. In humans and other vertebrates, it plays a vital role in blood clotting, as well as the metabolism of bones and blood vessels.
[6].
Vitamin K2 exists in different forms known as MK-n (K2 Menaquinone-n), which vary in side chain length. Research has shown that various K2 Menaquinone-n forms are produced by bacteria in the human colon. Among these, MK-4, MK-7, and MK-9 are the most extensively studied menaquinones.
[4].
MK-4 (K2 Menaquinone-4) stands out from other menaquinones because it is the only one that doesn’t require bacterial conversion from phylloquinone in the human body. It plays a significant role in blood clotting, preventing osteoporosis, reducing cognitive diseases, preventing diabetes, alleviating atherosclerosis, and inhibiting inflammatory responses. Most commercially available nutritional supplements of Vitamin K2 are derived through microbial metabolism.
[7].
On the other hand, MK-7 (K2 Menaquinone-7) can only be produced through bacterial metabolism. It is currently believed to have higher bioavailability and a longer half-life compared to MK-4. Consequently, it may have greater efficacy compared to MK-4. Additionally, traditional Japanese food “natto,” which is fermented using specific strains of Bacillus subtilis, contains MK-7.
[8].
The Four Major Functions of Vitamin K
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Preventing Osteoporosis
Osteocalcin is one of the major proteins in bones. One of the physiological roles of Vitamin K is to participate in the carboxylation of osteocalcin, which regulates bone mineralization and supports bone strength and maintenance.
[4]. Analytical studies have found that subjects who supplemented with Vitamin K experienced a significant increase in lumbar spine bone density.
[9].
💡Research targeting postmenopausal women with osteoporosis found that additional intake of Vitamin K2 significantly improved mid-term and long-term vertebral bone density. Integrated research analyses have also shown a significant reduction in fracture rates with Vitamin K supplementation.
[10].
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Improving Blood Clotting
Vitamin K plays a crucial role in the carboxylation reactions of clotting factors, influencing blood clotting. It has been clinically observed that Vitamin K deficiency in newborns can lead to bleeding disorders.
[3]. Vitamin K has been used in medications for the past few decades and Vitamin K antagonists are commonly used as oral anticoagulants for the treatment and prevention of thrombosis.
[11]. Clinical studies on patients taking oral anticoagulant warfarin revealed that low-dose (150 micrograms) Vitamin K supplements can help improve patients’ anticoagulant balance.
[12]. -
Improving Cardiovascular Health
Atherosclerosis is a risk factor for coronary heart disease, as it increases aortic and arterial hardening. Vitamin K can prevent vascular calcification.
[4]. Clinical research administering Vitamin K2 supplements to postmenopausal women found a significant negative correlation between the supplement dosage and coronary artery calcification, indicating the importance of Vitamin K2 intake for cardiovascular health.
[13]. Another clinical study found that long-term supplementation of MK-7 (especially for those with pre-existing atherosclerosis) improved the degree of arterial hardening in postmenopausal women.
[14]. -
Preventing Diabetes
Many studies indicate that Vitamin K has a positive role in reducing the risk of type 2 diabetes and improving insulin sensitivity.
[15].
💡A 10-year prospective study found that a higher intake of Vitamin K2 was associated with a lower risk of developing type 2 diabetes. The study revealed that with every 10 micrograms increase in Vitamin K2 intake, the risk of type 2 diabetes decreased by 7%.
[16]. Another clinical study found that supplementing with Vitamin K2 significantly improved insulin sensitivity.
[17].
Recommended Vitamin K Intake
Currently, there is not enough data to establish a Recommended Dietary Allowance (RDA) for vitamin K. Therefore, in the United States, Adequate Intake (AI) recommendations are provided, and no upper intake limit is set
[4]. Taiwan follows a similar approach, and you can refer to the table below for the adequate intake of vitamin K for the Taiwanese population
[18]:
| Age | Adequate Intake (micrograms) | |
| 0 months – 6 months | 2.0 | |
| 7 months – 12 months | 2.5 | |
| 1 year – 3 years | 30 | |
| 4 years – 6 years | 55 | |
| 7 years – 9 years | 55 | |
| 10 years – 12 years | 60 | |
| 13 years – 15 years | 75 | |
| 16 years – 18 years | 75 | |
| 19 years and older | Male: 120 | Female: 90 |
| Pregnancy | +0 | |
| Lactation | +0 | |
Foods High in Vitamin K
Food sources of vitamin K1 include vegetables, especially green leafy vegetables (such as spinach, broccoli, and lettuce), plant oils (such as soybean oil and canola oil), and some fruits (like blueberries and grapes). Meat, liver, dairy products, and eggs contain small amounts of vitamin K1 but also contain moderate amounts of vitamin K2. The primary source of vitamin K2 is from other fermented foods, including cheese
[4]. Traditional Japanese food, such as natto, also contains vitamin K2
[8].
Who Is at Risk of Vitamin K Deficiency?
In general, the probability of vitamin K deficiency is very low in individuals with a balanced diet. However, specific groups, such as newborns, may be at risk of vitamin K deficiency due to poor placental transport of vitamin K. People with certain malabsorption syndromes and other gastrointestinal diseases are also at risk of vitamin K deficiency, which can lead to conditions like osteoporosis and abnormal blood clotting
[4]. Modern diets that lack diversity or are nutritionally deficient may also lead to inadequate vitamin K intake
[5].
How to Effectively Supplement Vitamin K? When Is the Best Time to Take Vitamin K?
In general, there is limited data on the bioavailability of different forms of vitamin K from food
[19]. While vitamin K1 is the primary form in plants (>90%), its retention in the body is low, whereas vitamin K2 is the main form in animals (>90%). Research has shown that the absorption rate of vitamin K2, especially MK-4, MK-7, and MK-9, is better compared to vitamin K1, with MK-7 being the best-absorbed form
[20].
The bioavailability of vitamin K from vegetables is lower due to its close association with chloroplasts. The absorption rate of vitamin K from vegetables is only 4% to 17% compared to supplements. To enhance vitamin K absorption, it is recommended to consume vegetables with fat or lipid-rich foods to improve absorption efficiency
[1]
[4]. Fermented products, especially those rich in vitamin K2, offer even better absorption rates and bioavailability
[8].
What Are the Symptoms of Vitamin K Deficiency?
Bleeding is a typical symptom of severe vitamin K deficiency, but it only occurs in severe cases and requires testing of prothrombin activity in the blood. Severe vitamin K deficiency can also reduce bone mineralization and lead to osteoporosis. In a diverse and healthy diet, it is almost impossible to reach the standard clinical indicators for blood clotting due to vitamin K deficiency
[4].
In clinical practice, special attention is given to hospitalized patients who may be at risk of vitamin K deficiency, such as those with inadequate dietary intake, malabsorption syndromes, antibiotic treatment, kidney dysfunction, and newborns at risk of vitamin K deficiency due to placental insufficiency and low vitamin K concentration in breast milk
[21]. However, individuals with a normal and balanced diet do not need to be overly concerned about vitamin K deficiency
[4].
Can Excess Vitamin K Have Side Effects?
Vitamin K from food sources has not been found to have any toxicity
[20]
[4]. Supplementing vitamin K up to recommended levels does not produce side effects. Even the form of vitamin K considered to have the highest bioavailability, MK-7, has not been associated with adverse events
[22]. If you have concerns about vitamin K supplementation, it is recommended to discuss them with your doctor or nutritionist.
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