But it may help them avoid fractures or cancers.
The findings relating to fracture and cancer avoidance were "unexpected," says Angela Cheung, MD, of Toronto's University Health Network and lead author of the study. "It is intriguing and gives us reason for additional research."
The study, published in the journal PLoS Medicine, involved 440 postmenopausal women diagnosed with osteopenia, a "warning" condition of bone loss that can be a precursor to osteoporosis. The disease causes bones to become more fragile and likely to break.
Cheung says the women were either given a vitamin K supplement or a placebo for two years, with 261 continuing for two more years.
"Bone density scans after two years and after four years revealed there were no differences in bone density between the two groups, and that bone density had decreased by similar amounts," Cheung says. "But fewer women over the four-year period had fractures, and fewer had cancer."
Vitamin K and Osteoporosis
Low-dose vitamin K for osteoporosis has been "promoted by the lay media for increasing bone mass" and is widely available in health food stores and over the Internet, according to study researchers.
In Japan, Cheung tells WebMD, vitamin K is an approved treatment for osteoporosis.
But the study researchers say the value of vitamin K to lower risk of bone loss or fractures "remains controversial." The study, whose participants were chosen because they had been diagnosed with osteopenia, concludes that 5 milligrams of vitamin K supplementation daily did not protect them against age-related declines in bone mineral density "at the lumbar spine, total hip, femoral neck, or ultradistal radius" even if they had plenty of vitamin D.
Roberto Pacifici, MD, director of the division of endocrinology at Emory University in Atlanta, says he was unimpressed with the study because it's "been known for some time that there really is no connection between osteoporosis and vitamin K. If you have enough vitamin K deficiency to cause bone weakness, you bleed to death."
He says vitamin D is "very important" for bone strength, but that "people should not go out and buy vitamin K because of this study."
Although Cheung says it would be premature to recommend vitamin K for osteoporosis, she believes it deserves further study in larger groups of women.
About 10 million Americans, 80% of them women, have been diagnosed with osteoporosis; another 34 million people in the U.S. are at risk, according to the National Osteoporosis Foundation.
Previous research suggesting vitamin K supplements might strengthen bones may have had "design and methodological" flaws, Cheung says.
Women in the study did develop fewer fractures, a finding that Cheung says she could not explain and needed further research. Over the four years, nine women had fractures among those taking vitamin K supplements, vs. 20 in the placebo group.
Vitamin K and Cancer
There were also similar surprise findings on vitamin K and cancer.
In the study, only three women in the high supplement group developed cancer over four years, compared to 12 taking placebo.
The finding suggesting vitamin K might reduce cancer risk was "consistent with other research," but the numbers were so small that "a chance association" was possible, Cheung writes.
The mere possibility that vitamin K might reduce cancer risk is reason enough to study the matter further, Cheung notes.
Should you take a vitamin K supplement? Cheung says there is not enough evidence in this study to draw any conclusions.
She says people with osteopenia and those who've been diagnosed with osteoporosis should definitely make sure they get enough vitamin D by taking supplements or eating foods such as egg yolks, butter and margarine, cheese, fish liver oils, fortified cereals, bread, milk, herring, mackerel, oysters, and salmon.
Cheung also says she also recommends foods rich in vitamin K such as broccoli, Brussels sprouts, cauliflower, chick peas, dairy products, kale, and seeds.