Purpose Obesity contributes to the technical difficulty of rectal surgery and is considered to be a risk factor for postoperative complications. The impact of obesity on the long-term outcomes of rectal cancer surgery remains unclear. Methods A total of 263 consecutive rectal cancer patients who underwent surgery were categorized into two groups according to the body mass index (BMI) based on the Asian BMI classification: non-obese (BMI <25 kg/m2) and obese (BMI ≥25 kg/m2). The postoperative survival and recurrence rates and oncological surgical quality indicators were compared between groups using the univariate and multivariate analyses. The differences in recurrence patterns were assessed by a competing risk regression analysis. Results 64 (24 %) patients were included in the obese group. The number of retrieved lymph nodes was significantly greater in the non-obese group than in the obese group (22.4 vs. 16.0, P < 0.01). The 5-year disease-free survival (DFS) rates were 86.5 and 68.8 % in the obese and non-obese groups, respectively (P = 0.01). The multivariate analysis demonstrated that obesity significantly decreased the postoperative recurrence rate (P = 0.04). Moreover, the BMI was significantly associated with distant metastasis (P = 0.04). Conclusions Obese rectal cancer patients have high DFS rates and a decreased incidence of distant metastases compared to non-obese patients. The BMI may be a key factor for predicting the postoperative prognosis and determination of an appropriate strategy for the treatment of rectal cancer patients.
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