There were a few previous discussion threads about Vitamin K2 and natto; the most recent thread is here [url]http://www.healthboards.com/boards/showthread.php?t=427975[/url]
A new research study is "Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7". The abstract is at PubMed, PMID: 17158229. Some media reports have additional details from the study, and (of particular interest to us osteo-challenged):
Blood levels of the K1 group and K2 (MK-7) group both peaked after four hours. However, the levels of vitamin K1 were 86 per cent lower after only eight hours, while K2 (MK-7) levels were only 50 per cent lower after 68 hours, indicating slower excretion from the body. (This would seem to indicate that my present consumption of 100g of natto every other day should be keeping my K2 levels fairly high.
Bone turnover was also measured (as a ratio of circulating carboxylated and uncarboxylated osteocalcin), and the researchers reported that both forms of vitamin K showed activity after three days. However, vitamin K1's effect did not increase further while MK-7's effect increased over the six-week study period.
But people who take blood thinning medication (and are told to avoid Vitamin K), need to particularly avoid K2.
Anyway, I wouldn't be surprised if within a decade K2 is routinely recommended (just like calcium and D) for patients with osteoporosis.