Vitamin K: functions, foods, deficiency and supplements

Find out all about vitamin K, including what it does, how much you need, where to find it and who might need to supplement their diet

Written by Jack Feeney on December 14, 2018 Reviewed by Fiona Hunter on January 3, 2019

Overview

What is vitamin K? And what does vitamin K do?

This fat-soluble nutrient is known for its ability to help blood clot but its importance goes far beyond this, as vitamin K is also needed for strong bones.¹ There are two types of vitamin K found in our diet:^2,3

• vitamin K1 (phylloquinone) – important for blood clotting, it’s mainly found in plant foods, in particular green leafy veg. About 75-90% of the vitamin K we eat is K1⁴
• vitamin K2 (menaquinone) – needed for blood clotting, but also supports the body’s absorption of calcium. Found in certain animal and fermented foods, the body also produces K2 in the intestines

Vitamin K is often found as part of a multivitamin.

Function of vitamin K

What does vitamin K do in the human body?

Vitamin K has three main functions – supporting blood clotting, bone and heart health. Blood clotting Vitamin K is needed to activate several proteins that are critical for blood-clotting, for example from a wound.⁵ Bone health Vitamin K is also essential to metabolise calcium for normal bone development. Vitamin K2 activates proteins that help take calcium out of the blood, into the bones.⁶ Healthy heart Vitamin K is also important for our hearts. One of the proteins activated by vitamin K2 is matrix GLA protein, which imits the amount of calcium laid down on the walls of blood vessels, including the coronary artery.⁷

How much vitamin K do I need?

Around 1mcg a day for each kilogram of body weight. So if you weigh 75kg, you would need 75mcg.⁸ An 80g serving of broccoli has about 148mcg of vitamin K1, while 80g of spinach contains 315mcg of K1. A skinless chicken breast provides 90mcg of vitamin K2. As it’s a fat-soluble nutrient, you don’t need to eat vitamin K foods every day – any excess is stored in the liver.¹⁰

How much vitamin K do children need?

• 7-11 months: 10mcg a day
• 1-3 years: 12mcg a day
• 4-6 years: 20mcg a day
• 7-10 years: 30mcg a day
• 11-14 years: 45mcg a day
• 15-17 years: 65 mcg a day¹¹

Vitamin K foods

Which foods are high in vitamin K?

The best sources of vitamin K1 are plant sources, and in particular:¹²

• green leafy veg, especially kale, chard, spinach and broccoli
• natto, a Japanese fermented soybean food
• avocado
• green beans
• prunes
• soybeans
• kiwi fruit

Animal sources of vitamin K2 include:¹³

• beef liver
• pork chops
• chicken
• cheese
• egg yolk

Vitamin K deficiency

What are the symptoms of vitamin K deficiency?

Vitamin K deficiency is rare in adults.¹⁴ However, you may be at risk if you:^15,16,17

• are taking antibiotics for longer than 10 days
• have a gastrointestinal disorder such as cystic fibrosis, coeliac disease or ulcerative colitis which interferes with nutrient absorption
• eat a low-fat diet, as vitamin K needs fat for absorption

Symptoms of vitamin K deficiency include:¹⁸

• excessive bleeding
• bruising easily
• blood clots under nails
• black or bloody stool

What happens if I consume too much vitamin K?

There isn’t enough evidence to show how excess vitamin K affects the body, but the NHS recommends taking less than 1mg a day.¹⁹ If you take the medication warfarin, which stops blood clotting, it’s important to keep your vitamin K intake stable as it can interfere with your medication. Ask your GP for advice.²⁰

Vitamin K supplements

When should I take a vitamin K supplement?

Most people get all the vitamin K they need from their diet, but you could consider a supplement if you don’t eat enough vitamin K-rich foods or have a condition that limits absorption of fat-soluble nutrients.

Should children take a vitamin K supplement?

Children should be able to get all the vitamin K they need for a healthy, balanced diet. For babies, the NHS offers all parents a vitamin K injection. Newborn babies are at risk of a vitamin K deficiency because:²¹

• vitamin K doesn’t easily cross the placenta from the mother to baby
• there are low levels of vitamin K in breast milk

Symptoms of deficiency in babies include bleeding – a newborn condition called vitamin K deficiency bleeding – and it can be fatal.²²

Should women take a vitamin K supplement during pregnancy?

Pregnant women don’t need additional vitamin K, so should be able to get all they need from their diet. A 2017 review of studies in the European Food Safety Authority’s EFSA Journal reported that pregnant women had similar concentrations of vitamin K in their blood to non-pregnant women.²³

What are the potential benefits of taking a vitamin K supplement?

As well as supporting cardiovascular health, people take vitamin K for strong bones. A 2017 study by Canada’s University of Alberta reported that vitamin K2 can improve bone density and reduce the number of bone fractures in women with osteoporosis.²⁴ Shop Supplements Advice is for information only and should not replace medical care. Please check with your GP before trying any remedies. Sources 1. European Commission. EU Register of nutrition and health claims made on foods 2. Schwalfenberg GK. Vitamins K1 and K2: The Emerging Group of Vitamins Required for Human Health 3. National Institutes of Health. Vitamin K 4. Keith Pearson. Healthline. Vitamin K1 vs K2: What’s the Difference? 5. As Source 4 6. Maresz K. Proper Calcium Use: Vitamin K2 as a Promoter of Bone and Cardiovascular Health 7. As above 8. NHS. Vitamin K 9. As Source 8 10. European Food Safety Authority. EFSA Panel on Dietetic Products, Nutrition and Allergies. Dietary reference values for vitamin K 11. Atli Anarson. Healthline. 20 Foods that Are High in Vitamin K 12. Bethany Cadman. Medical News Today. Vitamin K deficiency 13. As Source 11 14. As Source 2 15. As Source 3 16. As Source 2 17. As Source 13 18. As Source 7 19. NHS. Warfarin 20. Lippi G, Franchini M. Vitamin K in neonates: facts and myths 21. Guy’s and St Thomas’ NHS Foundation Trust. Vitamin K and your newborn baby 22. As Source 10 23. As Source 2