What Is Vitamin K? A Practical Guide to Its Role in the Body

Vitamin K is a fat-soluble vitamin best known for its role in normal blood clotting and bone health. In fact, vitamin K contributes to the normal clotting of blood and the maintenance of normal bones under authorised European health claims. ¹

Although vitamin K is essential for several biological processes, it often receives less attention than nutrients like vitamin D or vitamin C. Yet maintaining adequate intake is important for supporting normal physiological functions, particularly those related to bone metabolism and cardiovascular health.

This guide explains what vitamin K is, the different forms it comes in, where it’s found in the diet, and how supplements are typically used.

Understanding Vitamin K

Vitamin K refers to a group of structurally related compounds that help activate proteins involved in blood clotting and calcium regulation.

The two main forms are:

Vitamin K1 (phylloquinone) - found mainly in leafy green vegetables
Vitamin K2 (menaquinones) - found in fermented foods and certain animal products

Both forms ultimately support similar physiological processes in the body, though they differ in dietary sources and how they circulate in the bloodstream. ²

Vitamin K is stored in small amounts in the liver and other tissues, meaning regular intake through food is important.

Vitamin K and Normal Blood Clotting

One of vitamin K’s most important functions is its role in blood coagulation.

When a blood vessel is damaged, a cascade of clotting factors is activated to stop bleeding. Several of these clotting proteins require vitamin K to function properly. ³

Without sufficient vitamin K, the body cannot activate these proteins effectively. Severe deficiency can lead to impaired clotting and increased bleeding risk.

This is why newborn babies are routinely given vitamin K shortly after birth to prevent a rare condition known as vitamin K deficiency bleeding. ⁴

For healthy adults consuming a balanced diet, deficiency is uncommon.

Vitamin K and Bone Health

Vitamin K also contributes to the maintenance of normal bones. ¹

This role is linked to a protein called osteocalcin, which helps bind calcium within the bone matrix. ⁵ Vitamin K is required to activate osteocalcin so that it can perform its function.

Research exploring vitamin K intake and bone density has produced mixed results, but observational studies suggest that diets rich in vitamin K may be associated with better bone health outcomes. ⁶

It’s important to note that bone health is influenced by multiple nutrients, including calcium, vitamin D, magnesium and protein, rather than a single vitamin alone.

Vitamin K and Calcium Regulation

Calcium must be carefully regulated in the body. While it is essential for bone strength, excessive calcium deposition in soft tissues can be undesirable.

Vitamin K helps activate proteins that regulate calcium distribution, ensuring it is used appropriately within the body. ⁷

This mechanism has led researchers to explore vitamin K’s role in cardiovascular health, though definitive conclusions are still under investigation.

At present, authorised health claims in the UK remain limited to blood clotting and bone maintenance.

Dietary Sources of Vitamin K

Vitamin K is widely available in many foods, particularly plant-based sources.

Common dietary sources include:

Kale, spinach and other leafy greens
Broccoli and Brussels sprouts
Vegetable oils such as soybean or rapeseed oil
Fermented foods such as natto (rich in vitamin K2)

Because vitamin K is fat-soluble, absorption may be improved when consumed alongside dietary fats. ⁸

Individuals who eat a varied diet containing vegetables usually obtain adequate vitamin K through food alone.

How Much Vitamin K Do You Need?

In the UK, recommended intake for vitamin K is based on body weight rather than a fixed daily amount.

Guidelines suggest approximately 1 microgram per kilogram of body weight per day. ⁹

For example:

A 60kg adult would need around 60µg daily
A 75kg adult would need around 75µg daily

These amounts are typically achievable through regular consumption of green vegetables.

Unlike some nutrients, vitamin K deficiency in healthy adults is relatively rare.

Who Might Consider Vitamin K Supplements?

While most people obtain adequate vitamin K through diet, supplementation may sometimes be considered in specific situations.

Individuals who might review their intake include those who:

Consume very limited vegetables
Follow highly restrictive diets
Have conditions affecting fat absorption

However, supplementation should be approached carefully, particularly for individuals taking medications that influence blood clotting.

Vitamin K and Blood-Thinning Medications

Vitamin K plays a direct role in blood clotting processes. Because of this, individuals taking anticoagulant medications such as warfarin must maintain consistent vitamin K intake. ¹⁰

Sudden changes in vitamin K consumption can affect how these medications work.

Anyone taking blood-thinning medication should consult a healthcare professional before using vitamin K supplements or significantly altering dietary intake.

Vitamin K1 vs Vitamin K2 - Is One Better?

The distinction between vitamin K1 and vitamin K2 often appears in supplement marketing.

Vitamin K1 is the primary form obtained from vegetables and is responsible for most dietary intake. Vitamin K2 refers to several subtypes known as menaquinones, found in fermented foods and animal products. ²

Some researchers have explored differences in how long these forms remain in circulation within the body. However, both ultimately contribute to vitamin K’s core physiological functions.

At present, health authorities do not specify separate intake recommendations for K1 and K2.

Safety and Upper Intake Levels

Vitamin K from foods is considered safe and does not have an established upper intake level for healthy individuals. ¹¹

Supplement doses typically range from 50µg to 200µg depending on the formulation.

Side effects are rare, but individuals taking anticoagulant medication should avoid self-supplementation unless advised by a healthcare professional.

Why Vitamin K Matters

Vitamin K may not be the most talked-about nutrient, but it plays a vital role in maintaining normal physiological processes.

By contributing to blood clotting and bone maintenance, it supports essential systems that keep the body functioning properly.

Most people can obtain sufficient vitamin K through a balanced diet rich in vegetables. Supplements may offer additional support where dietary intake is inconsistent, but they should be used thoughtfully and in line with professional advice when necessary.

As with all nutrients, vitamin K works best as part of a broader approach to health that includes varied nutrition, physical activity and adequate sleep.