However several ingredient suppliers have started offering vitamin K in recent months, suggesting that they see a market for stepping up its use in dietary supplements - particularly as the baby boom generation enters the age bracket where it starts to become more concerned with age-related health problems, including osteoporosis and health disease.
Their timing may be good; just last week a meta-analysis of studies of vitamin K and bone health published in peer reviewed journals found that supplementation is associated with increased bone mineral density (BMD) and reduced fracture incidence. The reduction in fracture incidence was also striking, with an approximate 80 per cent reduction in hip fractures.
Despite this, the researchers said there was a need for large, pragmatic randomized clinical trials to confirm the results before supplementation could be recommended.
But recommended or not, is the public primed and ready for more vitamin K?
In a small email survey conducted by NutraIngredients.com last week, 56 per cent of respondents said they had heard of vitamin K, but less than one per cent said they take vitamin K supplements. It is unlikely that the remainder are making a conscious effort to include vitamin K in their diet (although they may be consuming it incidentally), since only 2 per cent correctly identified any dietary sources.
As to vitamin K's role in health, one per cent of respondents were aware of its role in bone health and one per cent of its role in preventing blood clots.
The survey involved 39 adults aged between 20 and 70 years, mostly European residents.
There has also been some confusion in the industry over the different forms of vitamin K, with suppliers offering NutraIngredients.com differing accounts.
In summary, there are two main forms of vitamin K: phylloquinone, also known as phytonadione, (vitamin K1) which is found in green leafy vegetables such as lettuce, broccoli and spinach, and makes up about 90 per cent of the vitamin K in a typical Western diet; and menaquinones (vitamin K2), which make up about 10 per cent of Western vitamin K consumption and can be synthesised in the gut by microflora.
Menaquinones (MK-n: with the n determined by the number of prenyl side chains) can also be found in the diet; MK-4 can be found in animal meat, MK-7, MK-8, and MK-9 are found in fermented food products like cheese, and natto is a rich source of MK-7.
Both K1 and K2 have been shown to play a role in bone health, influencing the secondary modification of osteocalcin, a protein needed to bind calcium to the bone matrix.
Some large human studies have tested the bone health benefits of calcium alone, calcium plus vitamin D and calcium plus vitamin D plus vitamin K. The latter has shown the best effect on osteoporosis; its mechanism is different from, but synergistic with, the other two. (Bolton Smith et al, Ann Nutr Metab 2001; 45, p 246; Braam et al, 2003, Calcif Tissue Int 72).
However according to US supplier PL Thomas, only K2 has so far been seen to have additional cardiovascular by promoting promotes matrix Gla-protein, which inhibits vascular calcification. There is also evidence to suggest that K2 remains in the body for considerably longer than K1.
Osteoporosis is estimated to affect about 75m people in Europe, the USA and Japan. According to the International Osteoporosis Foundation, the total direct cost of osteoporotic fractures is €31.7bn in Europe, and 17.5bn in the US (2002 figure). The total annual cost of osteoporosis in the UK alone is over £1.7bn (€2.5bn), equivalent to £5m (€7.3m) each day.
A spokesperson for the UK's National Osteoporosis Society told NutraIngredients.com that people with osteoporosis have been found to have lower levels of vitamin K, "indicating that it plays an important role in preventing this disease".
She said that people who eat a healthy balanced diet will be doing their best by their bones; a survey conducted by the society earlier this year to assess changing dietary habits over the last 20 years found that there was a trend towards cutting out dairy, without identifying alternative sources of calcium.
Moreover, many people said they did not think about nutritional value when preparing a meal at home.