In a recent three-year trial, 500 micrograms a day of vitamin K slowed the onset of insulin resistance in elderly men, but not in women. But there may be a good explanation for that.

Insulin is a hormone which transports sugar into cells, where the sugar is converted into energy. In people with type 2 diabetes, insulin performs this function less effectively. The insulin is still there, it just doesn't work as well. This causes an increase in blood sugar and is termed "insulin resistance." As people age, diabetic or not, their insulin resistance tends to rise.

Foods richest in vitamin K are dark-green leafy vegetables (kale, spinach and collard greens) and cruciferous vegetables (brussels sprouts and broccoli).

In a study of 355 non-diabetic men and women, aged 60 to 80, vitamin K significantly slowed this rise in men, but not in women. At the study's end, in addition to improved insulin resistance, the men who took vitamin K also had lower blood insulin levels than men who took no vitamin K. A rise in blood insulin is often a sign of increased insulin resistance, as the body tries to compensate for the reduced effectiveness of the insulin by producing more of it. A high enough insulin resistance can cause blood sugar to rise to a diabetic level.

There has been little previous research into the relationship between vitamin K and insulin; this particular study was originally designed to test the effect of vitamin K on bone mineral density.

The currently recommended intake of vitamin K is 130 micrograms daily for men aged 19-70 and 90 micrograms for women aged 19-70. The study participants who received vitamin K received it as a 500 microgram dietary supplement. The study authors point out that this amount can also be obtained from eating a diet rich in vitamin K. Foods richest in vitamin K are dark-green leafy vegetables (kale, spinach and collard greens) and cruciferous vegetables (brussels sprouts and broccoli).

The authors also point out that the results were from a sample group of elderly adults, most of whom were Caucasian, and thus may not apply to the general populace. A possible explanation for the lack of effect of vitamin K seen on women has to do with the fact that any of the women in the study were obese or overweight. Vitamin K is a fat-soluble vitamin, and in the presence of excess fat there may be less vitamin K circulating in the bloodstream.

Vitamin K is instrumental in the blood clotting process and strongly interferes with the action of blood thinning medications such as warfarin (Coumadin®). Anyone taking this type of medication should not attempt to alter their vitamin K consumption without first discussing it with their doctor.

The study participants were advised to maintain their usual diets and to avoid taking dietary supplements other than those provided to them by the study team. All participants received a multivitamin supplement, to be taken daily. The control group's supplement contained no vitamin K, while the experimental group's supplement contained 500 micrograms of vitamin K. All participants also received a second daily supplement containing calcium and vitamin D. Blood samples were taken and analyzed six and 36 months into the study.

The research was conducted at the Jean Mayer Human Nutrition Research Center on Aging at Tufts University. The study results were published in the November 2008 issue of Diabetes Care.