Vitamin K Supplementation Reduces Insulin Resistance in Men

Vitamin K supplementation for 36 months at doses that could be achieved through dietary intake may reduce progression of insulin resistance in older men, according to the results of a new study.

"Vitamin K has a potentially beneficial role on insulin resistance, but evidence is limited in humans," write Makiko Yoshida, PhD, from the Jean Mayer US Department of Agriculture Human Nutrition Research Center at Tufts University in Boston, Massachusetts, and colleagues. "We tested the hypothesis that vitamin K supplementation for 36 months will improve insulin resistance in older men and women."

This was an ancillary study of a 36-month, randomized, double-blind, controlled clinical trial in which the primary objective was to determine the effect of supplementation with 500 μg/day of phylloquinone (vitamin K) on bone loss. The study sample consisted of 355 nondiabetic adults aged 60 to 80 years; 60% were women. The main endpoint was insulin resistance, measured by the homeostasis model (HOMA-IR) evaluated at 36 months, and secondary endpoints were fasting plasma insulin and glucose.

The affect on the HOMA-IR of 36-month vitamin K supplementation was different in men and in women (sex-by-treatment interaction: P = .02). Compared with men in the control group, men in the supplement group had statistically significantly lower HOMA-IR at 36 months (P = .01), after adjustment for baseline HOMA-IR, body mass index (BMI), and body weight change. Women in the 2 intervention groups did not differ significantly in HOMA-IR or in other outcome measures.
"Vitamin K supplementation for 36 months at doses attainable in the diet may reduce progression of insulin resistance in older men," the study authors write. "There was an inverse association between plasma phylloquinone concentrations and BMI in the women suggestive of an impaired response to vitamin K supplementation."

Limitations of this study include analyses of data obtained from a study designed to determine the effect of vitamin K supplementation on changes in bone mineral density and vascular calcification in older men and women; possible lack of generalizability to the overall population; measures presented not necessarily obtained by optimal techniques; limited statistical power to detect differences in HOMA-IR in response to vitamin K supplementation; and predominantly Caucasian sample.
"In summary, 36 months of vitamin K supplementation had a beneficial effect on insulin resistance in older men, but not older women," the study authors conclude. "As the parent study was not designed to test this hypothesis, these findings need to be replicated in a study designed specifically to test the hypothesis that vitamin K plays a protective role in insulin resistance in older adults."

Practice Pearls:

• In older men, use of vitamin K supplements (500 μg/day of phylloquinone) for 36 months is associated with reduced insulin resistance and insulin levels and no change in fasting glucose.
• In older women, use of vitamin K supplements (500 μg/day of phylloquinone) for 36 months is not associated with significant reduction in insulin resistance, insulin levels, or fasting glucose.

Hermes Arzenemittel GMBH, Munich, Germany, donated the supplements used for this study. The US Department of Agriculture and American Heart Association supported this study. The Ministry of Education, Culture, Sports, and Technology in Japan supported one author and the American Diabetes Association supported another author.
Diabetes Care. Published online August 12, 2008.

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