The relationship between vitamin D and insulin resistance is still controversial. Many factors could influence this relationship. In addition, this relationship in different genders was still unclear. Methods. A total of 6597 subjects, including 2813 males and 3784 females, were analyzed. The serum levels of 25(OH)D, fasting blood glucose (FBG), fasting insulin, HbA1c, and other metabolic parameters were tested. The waist circumference (WC), weight, and height were also measured. Questionnaires regarding smoking and drinking were collected from these subjects. Results. Serum 25(OH)D was categorized into quartiles. Increasing 25(OH)D levels were associated with reduced trend of homeostasis model assessment of insulin resistance (HOMA-IR) in both males and females. Pearson’s correlation indicated 25(OH)D level was inversely associated with the HOMA-IR for male subjects (r = −۰ ۰۵۵, P = 0 028) but not for female subjects (r = −۰ ۰۰۵, P = 0 798). Age, triglyceride (TG), and low-density lipoprotein (LDL) were associated with the vitamin D levels in males and females. In regression analysis, serum 25(OH)D concentration was significantly associated with HOMA-IR only in overweight males. Conclusion. We found an inverse association between 25(OH)D and HOMA-IR in Chinese overweight males. Vitamin D supplementation might be beneficial in this population. However, further clinical trials are needed to confirm this association.

Introduction

As a major noncommunicable disease, diabetes is becoming more and more prevalent in China. According to the 2010 American Diabetes Association (ADA) criteria for diabetes, the estimated prevalence is 11.6% for diabetes and 50.1% for prediabetes in Chinese adults [1]. Type 2 diabetes is the predominant form of diabetes in adults. Insulin resistance is frequently caused by overweight or obesity, which have been recognized as important risk factors for type 2 diabetes [2]. In the human body, vitamin D is mainly generated by cutaneous synthesis in response to UV radiation [3] and is then converted to 25-hydroxyvitamin D in the liver through hydroxylation; this form is the biochemical marker representing the vitamin D status in the human body [4]. Finally, the active form of vitamin D (1,25(OH)2D) is formed by 1-α-hydroxylase in the kidneys. Previously, vitamin D was considered to play a vital role in calcium homeostasis and bone metabolism. Recent studies indicated an association between low serum levels of 25(OH)D and cardiovascular diseases, diabetes, obesity, hypertension, and dyslipidemia [5]. However, the relationship between 25(OH)D and insulin resistance was debated [6–۱۰]. Some studies also showed lower serum 25(OH)D concentrations in overweight and obese patients [11–۱۳]. This inconsistency might be related to the body mass index (BMI). Furthermore, the relationship is still unknown in the Chinese population with different BMI. In this study, we carried out a large clinical investigation (SPECT-China) to analyze the sex-specific differences of the relationship between the serum vitamin D concentrations and insulin resistance in the Chinese population.