Vitamin K is essential for making proteins to regulate normal blood clotting, which helps wounds heal. It is not a single vitamin, but an umbrella term for a group of fat-soluble compounds.
Vitamin K1 (phylloquinone) is found in plants. Vitamin K2 (menaquinone) is made by bacteria that line the gastrointestinal tract and is found in foods including: soft cheese; egg yolk; chicken liver; and fermented foods like sauerkraut and miso. Vitamin K3 (menadione) is a synthetic man-made form.
Vitamin K2 has recently been singled out for scientific investigation and found to play a role far beyond blood clotting, including the protection of both bone and heart health. Vitamin K2 is now available as a supplement.
Dr Sarah Brewer says: 'Vitamin K is vital for normal blood clotting. Vitamin K2 supplements may play a role in protecting bone health and maintaining cardiovascular health. Vitamin K1 is most important in the liver to make clotting proteins. Outside of the liver, vitamin K2 is the preferred form used by cells and helps to ensure that calcium is deposited in bones rather than in the arteries or other tissues.'
What does vitamin K2 do?
Vitamin K2 is essential for activating proteins that play a role in preventing calcification (furring up) of the arteries, a risk factor for heart attack and stroke. Vitamin K2 also plays a role in producing osteocalcin, one of the proteins used in building bone.
Vitamin K's main function is regulating blood clotting. If your levels of vitamin K aren't high enough you are at greater risk of heavy bleeding, including potentially excessive menstrual bleeding or bleeding after surgery.
Osteoporosis is a fragile bone condition that affects three million people in the UK. A 2006 study concluded taking vitamin K2 with raloxifene, a drug used to treat osteoporosis by preventing bone mass loss, could help to prevent fractures in post-menopausal women with osteoporosis.
A 2015 review of published studies concluded vitamin K2 may play a role in the maintenance and improvement of bone mineral density in spinal vertebrae and prevent fractures in post-menopausal women with osteoporosis.
Vitamin K2 may also help prevent hardening of the arteries which can cause heart attacks and strokes. The Rotterdam Study, a large trial involving more than 7,000 men and women who were followed for three years, found those who ate a diet higher in foods containing vitamin K2 had lower incidence of arterial calcification (hardened arteries) and concluded it may protect against heart disease.
Lack of vitamin K is associated with easy bruising, and vitamin K creams are used to treat rosacea and thread veins on the face. Vitamin K cream can reduce (but not prevent) bruising after cosmetic pulsed dye laser therapy. However, vitamin K applied to skin can cause allergic skin reactions.
High intensity training in female endurance athletes is associated with increased risk of osteoporosis. Vitamin K2 supplements are sometimes tried to reduce this, but one study found they did not prevent bone loss. Although another study found that takin 10mg vitamin K per day for one month increased calcium-binding capacity of the bonge protein, osteocalcin, and other changes suggesting improved bone formation and reduced bone resorption.
Getting vitamin K2 from your diet
Many of us get enough vitamin K1 in our diet from leafy greens, including kale, Swiss chard and broccoli. Vitamin K2 is found in fermented foods like sauerkraut and miso, as well as egg yolks, butter, soft cheese, chicken and salami.
Vitamin K2 supplements may be a useful top-up though, particularly for people on long-term antibiotics, or who have medical conditions which affect gut absorption.
Research has generally shown no problems from consuming high levels of vitamin K in food or supplements. The UK Expert Group on Vitamins and Minerals recommends a safe upper daily limit of 1,000 mcg of vitamin K.
However, if you are taking anticoagulants (blood-thinning) medication, the action of vitamin K can stop them from working. If you are on warfarin, tell your doctor how many vitamin K-rich foods you typically eat, so your warfarin dose can be adjusted and try to keep your levels consistent. Always discuss with a doctor whether vitamin K2 supplements are safe if you are on warfarin.
The EU labelling RDA/NRV for vitamin K is 75mcg a day.
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