The double-blind, placebo controlled study, by researchers at Maastricht University's VitaK and the Cardiovascular Research Institute (CARIM), followed 325 healthy women with no osteoporosis for three years and also found that vitamin K2 supplements boosted the women's bone mineral content (BMC), compared to placebo "These findings are quite exciting," said study co-author Leon Schurgers.
"Research has shown many indications that vitamin K contributes to the maintenance of bone strength.
Besides various population-based studies and a number of clinical intervention trials investigating the effects of increased vitamin K intake, it has also been reported that subjects receiving vitamin K antagonists (oral anticoagulant therapy) have lower bone mineral density and increased fracture risk.
"Vitamin K2 has significant potential to benefit bone health," he said.
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 (vitamins 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 (menatetrenone) 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.
The results of the new study, published in the journal Osteoporosis International, show that supplements of vitamin K2 (MK-4, Eisai Co, Tokyo) to give daily doses of 45 mg per day (capsules of 15 mg, three times per day), or placebo to 325 postmenopausal women (average age 66, average BMI 27.2 kg per sq. m).
At the end of the study, the researchers report that, while no effect was observed on bone mineral density (BMD), significant improvements were observed for compression strength (2.03 per cent), bending strength (3.83 per cent), impact strength (1.72 per cent), femoral neck width (1.34 per cent), and hip axis length (0.23 per cent), compared to placebo.
"In this paper we demonstrate that the main effect of vitamin K2 on bone in the hip is an increase of the femoral neck BMC and width, resulting in maintenance of the calculated bone strength even at decreasing BMD after the menopause," wrote the authors.
"This makes K2 an interesting compound for combination therapy with other food supplements (calcium, vitamin D)… with known effect on BMD," they wrote.
The researchers note that they used MK-4 supplements with a relatively high dose of 45 milligrams, which exceeds the present recommendations of 90 to 120 micrograms per day, but state that MK-4 only stays in the body for a very short time, and even on administration of three 15 mg doses, serum levels of the vitamin can fluctuate.
"A possible alternative to MK-4 might be using MK-7, the K2 vitamin most abundantly found in the Japanese food natto," they said.
"MK-7 has probably a comparable effect as MK-4, but it has a half-life in circulation of three days, resulting in more constant plasma levels and accumulation."
This, said the researchers, makes MK-7 the logical choice for low-dose food supplementation.
They also said that it would be desirable to evaluate the cost-benefits of supplementation to all postmenopausal women, but the optimal dose, and synergistic effects with calcium and vitamin D needs further research.
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.
Source: Osteoporosis International Published on-line, doi: 10.1007/s00198-007-0337-9 "Vitamin K2 supplementation improves hip bone geometry and bone strength indices in postmenopausal women"
Authors: M.H.J. Knapen, L.J. Schurgers, C. Vermeer