Vitamin K benefits relate to its role in blood coagulation, and the regulation of calcium to ensure it is deposited where it is needed, such as in the bones, and not in the arteries or other tissues.
Vitamin K is the collective term for a group of fat soluble vitamins:
- Phylloquinone, or vitamin K1, is found in plant-based foods.
- Menaquinones, collectively knwon as vitamin K2, are found in animal-based foods. There are 9 menaquinones, of which menoquinone-4 is most common as it can be made from vitamin K1 within artery walls and within the internal organs. Menaquinone-7 is not naturally produced in the body, but is made from vitamin K1 by E.coli bacteria within the large bowel, some of which is absorbed into the circulation. Menaquinone-4 and menoquinone-7 are commonly found in vitamin K2 supplements.
- Menadione is a synthetic form that is sometimes called vitamin K3. This acts as a provitamin that can be metabolised to form active vitamin K2.
Vitamin K benefits
Vitamin K was initially recognised as essential for normal blood clotting, and gained its name from the German word, koagulation. Vitamin K acts as a co-factor for an enzyme (gamma-glutamyl caboxylase) which activates a series of Gla proteins that form strong bonds with calcium.
In the liver, this activation is needed to produce the clotting proteins, thrombin, Factor VII, Factor IX, Factor X, protein C, protein S and protein Z, all of which are needed for healthy blood coagulation. Vitamin K is used medically as an antidote for the blood thinning drug, warfarin.
Outside of the liver, vitamin K2 is the preferred form used by cells, and this can be produced (in the form of menaquinone-4) by the natural conversion of vitamin K1.
In bone, vitamin K2 is needed for the activation of osteocalcin – a calcium-binding Gla protein which stimulates bone-building cells called osteoblasts. This promotes the formation of new bone tissue. Vitamin K2 also acts together with vitamin D3 to inhibit the re-absorption of bone by cells called osteoclasts, which are involved in bone remodelling. These two actions mean that vitamin K2 is at least as important for bone health as calcium and helps to protect against the bone-thinning disease, osteoporosis.
In the circulation and other tissues, vitamin K2 activates Matrix Gla Protein (MGP) which inhibits unwanted calcification of soft tissues, such as artery walls, to reduce calcification and hardening of the arteries. This may provide some protection against coronary heart disease, stroke and dementia.
Other less well understood interactions between vitamin K2 and Gla proteins are thought to regulate cell proliferation, survival and adhesion, tissue repair, the transportation of thyroxine hormone (T4), and to play a role in insulin secretion within the pancreas.
Lack of vitamin K2 (and of vitamin K1 from which it is produced) is a key risk factor for atherosclerosis and ensuring good intakes may prevent – and even reverse a build up of calcification within the circulation – although this requires more research to confirm absolutely.
Within the EU, the European Food Safety Authority have so far only authorised health claims that vitamin K contributes to:
- normal blood clotting
- the maintenance of normal bones.
Food sources of vitamin K
Ninety percent of dietary vitamin K is in the form of vitamin K1, and just 10% in the form of K2.
Vitamin K1 is mainly found in plant-based foods such as:
- dark green leafy vegetables such as spinach and lettuce.
Some lettuces provide more than 100 mcg vitamin K1 per 100g. Useful amounts of vitamin K1 are also found in yoghurt (produced by the bacteria present), alfalfa, safflower, rapeseed, soya and olive oils, tomatoes, potatoes, pulses and wholegrains.
Vitamin K2 is mainly obtained from animal products such as:
- cod liver oil
- egg yolk
There is no vitamin K in skimmed milk as, being fat-soluble, it is removed along with the skimmed fat. Fermented or aged hard cheeses, made with whole milk, however, are a useful source.
Natto – fermented soy beans – is one of the few vegetarian sources of vitamin K2 (in the form of MK-7) which is synthesised by the strain of bacteria used for fermentation (Bacillus subtilis var. natto).
Other sources of vitamin K2 include fermented foods such as sauerkraut and kefir.
Probiotic bacteria in the gut also produce some vitamin K2 which we can absorb and use – but not in sufficient quantities to prevent deficiency.
NB Significant changes to dietary intakes of vegetables such as broccoli or cauliflower can affect blood clotting control in people taking warfarin. If you are on warfarin, you therefore need to maintain a fairly constant intake of these foods to ensure blood clotting control remains stable.
Vitamin K absorption
As a fat-soluble nutrient, vitamin K is absorbed along with dietary fats within the small intestines. It first enters the lymphatic system and is transported into the main circulation within lipoprotein spheres (chylomicrons) along with triglycerides and cholesterol.
Vitamin K1 and K2 (menaquinone-4) are extracted and stored within the liver for its own use. The longer-chain forms of vitamin K2 (eg menaquinone-7) are redistributed by the liver into the circulation (within LDL-particles) for use by other tissues. This makes menaquinone-7 particularly useful in food supplements designed to help strengthen bones and maintain healthy arteries.
Vitamin K and newborn health
A single dose of vitamin K is recommended for all newborn infants, at birth, to prevent a condition known as haemorrhagic disease of the newborn. This can arise during the first few days of life when vitamin K deficiency can lead to bleeding into the brain.
Vitamin K and osteoporosis
Research suggests that vitamin K2 (but not K1) can reduce loss of bone calcium in post-menopausal women and strengthen bones that are already weakened. Analysis of data from 19 clinical trials, involving 6759 people, found that postmenopausal women with osteoporosis who took vitamin K2 supplements gained a significant improvement in bone mineral density of the spine compared with placebo.
Vitamin K2 deficiency is associated with an increased risk of hip fracture. According to the Nurses’ Health Study, in which the diet of 72,327 women aged 38-63 was assessed, those with the lowest intake of vitamin K (less than 109 mcg per day) were 30% more likely to experience a hip fracture over the following 10 years than those with intakes of more than 109 mcg per day (of which lettuce was a major source). This risk was independent of other known risk factors for osteoporosis, including calcium and vitamin D intakes and has prompted calls to increase the recommended daily intake for vitamin K.
Vitamin K2 is already an approved treatment for osteoporosis in Japan.
Vitamin K and heart disease
Vitamin K2 (but not vitamin K1) may reduce the risk of coronary heart disease in older people, especially postmenopausal women. Researchers looked at diet of 16,057 women, aged 49-70 years, who were initially free of cardiovascular disease, and found that every 10mcg increased intake of dietary vitamin K2 intake was associated with a 9% lower risk of developing coronary heart disease during the following 8 years.
An ongoing 2 year trial looking at whether vitamin K2 supplements (menaquinone-7) can reduce vascular calcification in people with coronary artery disease are awaited. The researchers hope that treatment with MK-7 will slow down or arrest the progression of calcification in the coronary arteries, and lead to a new treatment option to prevent vascular calcification, and subsequent heart attacks and stroke. I’ve certainly started taking vitamin K2 supplements myself.
Vitamin K and peripheral arterial disease
A study involving 36,629 people compared dietary intakes of phylloquinone (K1) and menaquinones (K2) with the risk of developing peripheral arterial disease due to vascular calcification. After 12 years of follow-up, people with the highest vitamin K2 intake were 29% less likely to develop peripheral arterial disease than those with the lowest intakes. The protection was even greater for those with hypertension (41% reduction) or diabetes (44% reduction). No protective effect was found for dietary vitamin K1, however.
The long-term effects of vitamin K2 (menaquinone-7) on arterial stiffness were also investigated in 244 healthy postmenopausal women who took either 180 mcg vitamin K2 or placebo for three years. After three years, the thickness and stiffness of their carotid arteries was assessed and the vitamin K2 group showed a significant decrease in arterial stiffness. In those whose arterial stiffness was higher than normal initially, this decreased by 50% in those taking vitamin K2, compared to placebo.
Vitamin K and diabetes
Vitamin K2 is involved in the secretion of insulin hormone from the pancreas and may reduce the development of insulin resistance to protect against type 2 diabetes. One study found that taking vitamin K1 supplements (500mcg per day for 3 years) reduced the progression of insulin resistance in older men but not in women. I believe the study was poorly designed, however, as the researchers should have used vitamin K2 rather than vitamin K1 if they wanted to see effects outside of the liver.
Vitamin K and skin
Lack of vitamin K has been associated with easy bruising. This may be a factor in the formation of dark circles under the eyes when red blood cells leak from fine capillaries into the surrounding, thin tissues to cause low-grade bruising. This is compounded by fatigue and lack of sleep, especially if you keep rubbing your eyes to help stay awake.
Visible, red, dilated blood vessels are common on the cheeks and nose. Known as telangiectasia, they are believed to result from hypersensitivity of facial blood capillaries in the skin which do not constrict back down after dilation (for example in response to heat or alcohol). Over time, they can increase in size causing capillary walls to weaken.
Dark circles and thread veins can respond to vitamin K creams and gels, especially for people with rosacea.
Vitamin K deficiency
Deficiency of vitamin K1 is unusual, but can occur in older people as a result of poor diet or poor absorption. This can lead to lower levels of clotting factors with prolonged bleeding time, easy bruising, recurrent nose bleeds and, in menstruating women may contribute to heavy periods (although genetic variations in the ability to produce clotting factors are more likely as a diagnosis).
Deficiency of vitamin K2 is more common as it is less prevalent in the diet. Although tissues can make vitamin K2 from vitamin K1, the liver preferentially hoards vitamin K1, and the menaquinone-4 form of K2, so that little reaches other parts of the body.
The liver mainly distributes vitamin K in the form of long-chain menaquinones (especially MK-7) and intakes of these are limited (natto, aged cheese, fermented foods and supplements are the main sources). While this preserves the liver’s ability to make blood clotting proteins (which is obviously important), it does mean that other tissues, such as the bones, arteries and the pancreas, often do not obtain sufficient vitamin K2 for optimum regulation of calcium and to prevent its unwanted deposition in tissues other than the bones.
Vitamin K dose
These recommended intakes are based on blood clotting requirements rather than the effects of vitamin K2 on Gla proteins involved in bone and soft tissue health, however, and it is widely accepted that they need revision.
Take supplements providing vitamin K2 rather than vitamin K1 to obtain benefits for bone, arteries and other tissues.
Vitamin K2 supplements are usually taken at doses of 75mcg to 200mcg. Higher doses may be suggested for treating osteoporosis. As it is fat-soluble, vitamin K supplements are best taken with meals for optimum absorption.
Vitamin K safety
The upper safe level for long-term use from supplements is suggested as 1000 mcg (1mg) vitamin K.
Don’t take supplements containing vitamin K if you are on warfarin treatment unless your doctor tells you to.
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