Then, we evaluated the serum levels of the serological markers, and we found that serum levels of C3 were higher in patients with IBS classified as overweight (median [152.10 mg/dL]; range [85.10 mg/dL–200 mg/dL]) and obesity (median [164.30 mg/dL]; range [94.40 mg/dL–213.20 mg/dL]) than in IBS patients with normal weight (median [129.70 mg/dL]; range [83.40 mg/dL–165.40 mg/dL]; p = 0042 and p < 0.001). This evidence concerns the gene C3 and Obesity.