Insulin resistance, whereby insufficient insulin is released to produce a normal glucose response from fat, muscle and liver cells, was significantly lower in men following a daily vitamin K1 supplement, according to results of a 36-month, randomised, double-blind, controlled trial.
No effects were observed in women, report the researchers, led by Sarah Booth from the Jean Mayer USDA Human Nutrition Research Center at Tufts University, in this month’s issue of Diabetes Care.
The authors speculated that weight might be influencing the effects of vitamin K in men and women. "In our study, there was a higher prevalence of obese or overweight women in the vitamin K supplementation group compared to the male supplementation group," said Booth. "Vitamin K is stored in fat tissue. If there is excess fat, vitamin K may not be readily available to cells that require it to process glucose."
There are two main forms of vitamin K: phylloquinone, also known as phytonadione, (vitamin K1) and menaquinones (vitamins K2). K1 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; while K2, which makes up about 10 per cent of Western vitamin K consumption and can be synthesised in the gut by microflora.
The researchers recruited 355 non-diabetic men and women between the ages of 60 and 80. Sixty per cent of the participants were women. The participants were randomly assigned to receive a daily vitamin K1 supplement (500 micrograms per day of phylloquinone) or placebo for 36 months. The vitamin K doses is approximately five times the adequate intake, said the researchers. All of the participants also received a calcium and vitamin D supplement.
Booth and her co-workers report that insulin resistance, assessed using the homeostasis model (HOMA-IR), improved in men consuming the vitamin K supplements. On the other hand, progression of insulin resistance continued in all women, and in the men in the placebo group.
Previously, researchers from America, Canada and Britain reported in the journal Cell that vitamin K may have an effect on diabetes development via the vitamin K-dependant protein osteocalcin. The study with mice looked at genes that operate primarily in the bone cells that are linked to glucose metabolism.
By "knocking out" these genes in mice so that they could not function, the animals lacking a functional osteocalcin gene gained fat, showing that osteocalcin helps regulate the cells that produce insulin in the pancreas and release it into the bloodstream.
Booth and co-workers dismiss this as the mechanism, however, noting that men in vitamin K group actually had less of the functional osteocalcin than men in the placebo group.
“It is plausible,” they stated, “That vitamin K may improve insulin sensitivity through suppression of inflammation. In vivo and in vitro studies have shown that vitamin K reduced lipopolysaccharide-induced inflammation.
“More recently, it was reported that biochemical and dietary measures of vitamin K status were inversely associated with inflammatory markers in an observational study,” they said.
Booth and her co-workers note that the study was limited to Caucasian adults and that generalization of the results may not be possible. Additional studies are recommended.
The study was funded by the United States Department of Agriculture Agricultural Research Service, the National Institutes of Health, the American Heart Association, the Ministry of Education, Culture Sports and Technology in Japan and the American Diabetes Association.
An estimated 19 million people are affected by diabetes in the EU 25, equal to four per cent of the total population. This figure is projected to increase to 26 million by 2030.
In the US, there are almost 24 million people with diabetes, equal to seven per cent of the population. The total costs are thought to be as much as $174 billion, with $116 billion being direct costs from medical expenditures, according to 2007 American Diabetes Association figures.
Source: Diabetes Care November 2008, Volume 31, Pages 2092-2096, doi: 10.2337/dc08-1204 "Effect of Vitamin K Supplementation on Insulin Resistance in Older Men and Women”Authors: M. Yoshida, P.F. Jacques, J.B. Meigs, E. Saltzman, M.K. Shea, C. Gundberg, B. Dawson-Hughes, G. Dallal, S.L. Booth