Objective: This study aimed to investigate the relationship between body mass index (BMI) and risk of death in patients with acute pancreatitis (AP) using a Japanese national administrative database. Methods: We analyzed a total of 6002 patients with AP. We collected patient information, including sex, age, BMI, severity of AP based on the Japan Pancreas Society scoring system, and prognosis. We classified BMI into 5 categories (underweight [BMI, <18.5], normal range [18.5-24.9], preobese [25-29.9], obese class I [30-34.9], and obese class II/III [>35]) and investigated the relationship between each category and risk of death in AP. Results: There was a good correlation between the Japanese AP severity score and in-hospital mortality. Overall mortality of severe pancreatitis was 7.0% (n = 2245). Mortality in each BMI category was as follows: underweight, 6.4%; normal range, 3.6%; preobese, 2.4%; obese class I, 3.2%; and obese class II/III, 5.7%. Underweight and obese class II/III patients had significantly higher relative risk (RR) of death in AP compared with preobese patients after adjusting for sex, age, and severity of AP (RR, 2.7; 95% confidence interval, 1.6-4.5; and RR, 6.4; 95% confidence interval, 1.9-20.9, respectively). Conclusions: Underweight or overweight was the independent risk factor for mortality in AP.
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