Vitamin K1 doesn’t prevent thinning of the bone in this study but it does prevent bone fracture and cancer
Researchers from the University of Toronto performed a 2-year long randomized, placebo-controlled, double-blind study, extended for earlier participants for up to an additional 2 years because of interest in long-term safety and fractures. A total of 440 postmenopausal women with osteopenia were randomized to take either 5 mg of vitamin K1 or placebo daily.
Over 2 years bone mineral density decreased by −1.28% and −1.22% at the lumbar spine and −0.69% and −0.88% at the total hip in the vitamin K and placebo groups, respectively. So there was no significant difference in bone loss with Vitamin K1 however, the number of women with fractures decreased significantly and the number of women with cancer dropped significantly in the Vitamin K1 group. The number of women dying from cancer although low in general was certainly lower in the Vitamin K1 group.
The researchers conclude that in this study 5 mg of vitamin K1 supplementation taken daily for 2 to 4 years does not protect against age-related decline in bone mineral density, but may protect against fractures and cancers in postmenopausal women with osteopenia. The apparent contradiction between the effects of vitamin K1 on not improving bone density but still decreasing the number of women with fractures could mean that vitamin K1 supplements strengthen bone by changing factors other than bone density, e.g., by changing its fine structure rather than making it denser; in other words improving the microarchitecture (filling in the Swiss cheese holes) that make bone more fragile. The study results are published in the recent edition of PLoS Medicine.