Associations of Plasma 25-Hydroxy Vitamin D and Dietary Vitamin D Intake with Insulin Resistance in Healthy Japanese Women.

We investigated the associations of plasma 25-hydroxy vitamin D (25(OH)D) concentration and the dietary intake of vitamin D with insulin resistance in Japanese women. Study participants were 406 Japanese women attended a health examination. They were not taking hormones or medications for diabetes and had no history of cancer, ischemic heart disease, or stroke. Information regarding medical history and lifestyle factors was obtained by a self-administered questionnaire, while hours of sun exposure were determined through interviews. Dietary intake of vitamin D was evaluated using a validated food frequency questionnaire. Fasting plasma glucose and insulin concentrations were measured, and insulin resistance (HOMA-IR) scores were calculated based on homeostasis model assessment. Women with vitamin D deficiency (25(OH)D<20 ng/mL) had significantly higher fasting plasma insulin concentration and HOMA-IR than did the other women. Plasma 25(OH)D concentration was significantly and inversely associated with fasting plasma insulin level and HOMA-IR after controlling for age, season, menopausal status, BMI, smoking status, alcohol intake, physical exercise, and intakes of fat and calcium. Dietary vitamin D intake was not associated with HOMA-IR concentration after adjusting for these covariates and hours of sun exposure. Although significant inverse association between plasma 25(OH)D concentration and HOMA-IR was observed in women with a low BMI, low fat intake, or a high calcium intake, the interaction terms were not statistically significant. Data suggest that plasma vitamin D, but not dietary vitamin D, is inversely associated with fasting plasma insulin concentration and HOMA-IR in non-diabetic Japanese women.