SummaryThis study was to determine serum 25-hydroxyvitamin D (25(OH)D), the complement 3 (C3), and C-reactive protein (CRP) levels, and their association with the risk of insulin resistance (IR). A case-control study was carried out among 134 participants with body mass index (BMI) $30 kg/m 2 and BMI518.5-24.99 kg/m 2 . Anthropometric and body composition indicators were measured. Serum levels of C3, CRP, 25(OH)D, insulin, and glucose were also measured. IR was assessed by the homeostasis model assessment (HOMA-IR). C3, CRP, insulin, and HOMA-IR levels were higher in participants with obesity than that of controls (p,0.001). After adjustment for the potential confounders, anthropometric and body composition indicators were correlated positively with C3 (p,0.001), and negatively with 25(OH)D (p,0.05). C3, and 25(OH)D were correlated with r520.212; p,0.05). In logistic regression analyses, C3 and CRP were significantly related to increased odds of IR among participants with obesity as compared to controls after progressively adjusting for the potential confounders (p,0.001), whereas 25(OH)D was negatively, but insignificantly, related to decreased odds of IR among participants with obesity (p.0.05). C3 was associated positively with 25(OH)D insufficiency/deficiency independent of HOMA-IR and/or BMI (b50.183, p,0.05). Obesity is associated with elevated levels of proinflammatory biomarkers and IR. 25(OH)D insufficiency/deficiency was associated with C3 regardless of HOMA-IR or BMI, which could in turn, have a role in the augmentation of IR during obesity. Key Words adipose tissue, ASP, BIA, 25-hydroxyvitamin D deficiency, inflammation Adipose tissue is a vital endocrine organ that can affect the homeostasis of the body; it expresses several proinflammatory cytokines such as, tumor necrosis factor-alpha (TNF-a) and C-reactive protein (CRP), and anti-inflammatory factors such as adiponectin. Obesityinduced inflammation has been implicated in different cardiometabolic disorders including insulin resistance (IR) (1). The complement system is activated during the state of subclinical inflammation associated with obesity; of which the complement 3 (C3) plays the central role in all pathways; namely, the lectin, classical, and alternative pathways (2). The role of the complement system in the human immune integration is well known, yet the effect of improbable management of the complement system on health requires further investigation (2, 3). Vitamin D is a steroid hormone and its role in different segments of the body is well-established. Its deficiency has been associated with obesity as well as immune system defects and IR (4). The serum 25-hydroxyvitamin D (25(OH)D) blood level is known as a valid indicator to assess vitamin D status; whether it is deficient, sufficient or intoxicated. It has a half-life of 2-3 wk that reflects vitamin D stores over a long period and it represents the two main sources of vitamin D; food intake and sun exposure (5).To date, only one report by De Pergola et al. (4), University of Bar...
