Vitamin K linked to insulin resistance in older men

Vitamin K slowed the development of insulin resistance in elderly men in a study of 355 non-diabetic men and women aged 60 to 80 who completed a three-year clinical trial at the Jean Mayer Human Nutrition Research Center on Aging at Tufts University (USDA HNRCA).

"Men who received vitamin K supplementation had less progression in their insulin resistance by the end of the clinical trial," said Sarah Booth, senior author and director of the Vitamin K Laboratory at the USDA HNRCA.

"Conversely, we saw progression in insulin resistance in women who received vitamin K supplementation, and in the men or women who were not given vitamin K supplements."

Among those given vitamin K, both men and women took daily multivitamins containing 500 micrograms of vitamin K, five times the Adequate Intake (AI) recommended by the Institute of Medicine’s Food and Nutrition Board, with instructions to maintain normal diets without any additional supplementation. They also received a calcium and vitamin D supplement. Men and women in the control group received no vitamin K supplementation but did receive the multivitamin and the calcium and vitamin D supplement.

For the present study, insulin resistance was assessed by the homeostasis model (HOMA-IR). Additionally, participants’ blood glucose and blood insulin levels were measured following a minimum 10-hour fast. In addition to improved insulin resistance, the supplemented men had lower blood insulin levels compared to the unsupplemented men at the conclusion of the study.

Insulin is a hormone which plays a role in transporting sugar into cells so it can be converted into energy. A pre-cursor to diabetes, insulin resistance occurs when the body cannot use insulin properly, causing glucose to build up in the blood. People who are obese or overweight are prone to insulin resistance because excess fat can interfere with insulin function.

Writing in the November issue of Diabetes Care, the authors speculate that weight might explain why only the vitamin K supplemented men improved their insulin resistance.

"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," Booth said.

"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."

Because there are few studies of vitamin K and insulin resistance, the authors encourage further investigation of their findings and alternative study designs.

"The original purpose of the present study was to assess the effect of vitamin K1, or phylloquinone, supplementation on changes in bone mineral density and vascular calcification," Booth said.

"For instance, there is a way to achieve a more direct measure of insulin secretion than HOMA-IR. Also, our study is limited to caucasian adults. We acknowledge our findings may not apply to the general population."

Although vitamin K supplements were used for the study, the authors say the study dosage is attainable by consuming a healthy diet. Foods considered good sources of vitamin K include brussels sprouts, broccoli, and dark, leafy greens, such as spinach and collards.

(Source: Tufts University: Diabetes Care. S Booth, et al.: November 2008)