Vitamin K supplementation for 36 months at doses attainable in the diet may reduce progression of insulin resistance in older men, according to the results of a randomized controlled trial.
"Vitamin K has a potentially beneficial role in insulin resistance, but evidence is limited in humans," write Makiko Yoshida, PhD, from the Jean Mayer US Department of Agriculture Human Nutrition Research Center, 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."
Vitamin K may be inversely associated with insulin resistance, according to limited data from human and animal studies. The physiologic mechanisms underlying the relationship between vitamin K and insulin and glucose metabolism are unclear. An observational study in a community-based cohort showed that higher dietary and supplemental vitamin K intakes were associated with greater insulin sensitivity and better glycemic status in both men and women. In a small metabolic study of young men, insulin response after an oral glucose challenge was improved by short-term vitamin K supplementation.
This was an ancillary study of a 36-month, randomized, double-blind, controlled trial evaluating the effect on bone loss of supplementation with 500 µg/day of phylloquinone in 355 older nondiabetic men and women. Age range was 60 to 80 years, and 60% were women. The main endpoint of this ancillary study was insulin resistance at 36 months, measured by homeostasis model assessment of insulin resistance (HOMA-IR). Secondary endpoints were fasting plasma insulin and glucose levels.
There was a significant interaction between sex and treatment (P = .02), with HOMA-IR significantly lower at the 36-month visit among men in the supplement group vs men in the control group (P = .01), after adjustment for baseline HOMA-IR, body mass index (BMI), and change in body weight. For women, outcome measures did not differ significantly between intervention groups.
Limitations of this study include failure to test the effect of this intervention on post glucose challenge measures of insulin resistance, overweight or obesity more prevalent in women who received vitamin K supplementation vs those who did not, and possibly insufficient statistical power to detect statistically significant differences in HOMA-IR in response to vitamin K supplementation in women.
Other limitations 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; findings possibly not representative of the general population; use of HOMA-IR, which only provides an indirect estimate of insulin resistance vs the hyperinsulinemic euglycemic clamp; body composition evaluation limited to BMI and percent body fat; and predominantly Caucasian sample.
"Vitamin K supplementation for 36 months at doses attainable in the diet may reduce progression of insulin resistance in older men…but not in older women," the study authors write. "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."
- A significant interaction between sex and treatment effect of vitamin K supplementation on progression of insulin resistance existed. At 36 months, HOMA-IR and plasma insulin levels were significantly lower for men in the supplement group vs those in the control group, after adjustment for baseline HOMA-IR, BMI, and weight change.
- HOMA-IR, fasting plasma insulin levels, and glucose levels did not differ significantly between treatment groups, even after adjustment for BMI and other factors in women.
Diabetes Care. 2008;31:2092-2096.
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