Factors influencing body weight one year after bariatric surgery

**Background:** Obesity is a major health challenge, with prevalence in Saudi Arabia approximately 34.8% in men and 35.6% in women. Bariatric surgery (BS) is an effective long-term treatment for obesity, reducing mortality risk by 51% and improving obesity-related comorbidities. However, weight loss outcomes can be influenced by factors such as age, sex, initial body weight, and type of surgery. Rapid weight loss after BS is associated with an increased risk of cholelithiasis, with prevalence rates between 2% and 50% worldwide. Ursodeoxycholic acid (UDCA) has been suggested as prophylaxis against gallstone formation, but its effectiveness in Saudi Arabian patients was unclear. This study aimed to investigate factors influencing weight change during the first year after BS and to evaluate the efficacy of prophylactic UDCA in preventing gallstone formation.

**Methods:** This retrospective cohort study included 162 patients with morbid obesity who underwent BS at King Abdullah Medical City between May 2018 and May 2020 and completed 1 year of follow-up. Inclusion criteria were age 18-65 years. Exclusion criteria were BS outside the study timeframe, age >65 or <18 years, cholelithiasis before surgery, and cholecystectomy before BS. Data were extracted from electronic medical records at baseline (immediately after surgery) and at 2 weeks, 3, 6, 9, and 12 months postoperatively. Collected data included age, sex, type of surgery, weight, height, and BMI. Percentage of excess weight loss (%EWL) was calculated at 6, 9, and 12 months. Successful weight loss was defined as ≥50% EWL. Postoperative occurrence of cholecystectomy was recorded. Statistical analysis used paired t-tests, Pearson correlation, chi-square tests, and Cohen's d effect size, with significance set at P≤0.05.

**Key Results:** Baseline characteristics: 67.9% female, mean age 40.6±10.4 years, mean baseline BMI 49.06±7.39 kg/m², mean baseline weight 129.1±24.3 kg. Surgery types: laparoscopic sleeve gastrectomy (LSG) 85.8%, Roux-en-Y gastric bypass (RYGB) 5.6%, mini-gastric bypass (MGB) 8.6%. BMI decreased progressively from 49.1±7.4 preoperatively to 30.9±5.3 at 12 months. Successful weight loss (≥50% EWL) was achieved by 56.8% at 6 months, 83.3% at 9 months, and 92.6% at 12 months. Mean %EWL increased significantly over time: 52.8±14.1 at 6 months, 66±15.7 at 9 months, and 76.9±17.5 at 12 months (all pairwise comparisons P<0.001). Effect sizes were large for 6 vs 9 months (Cohen's d=0.89) and 6 vs 12 months (d=1.52), and medium for 9 vs 12 months (d=0.65). No significant association was found between %EWL and sex or type of surgery at any interval. Age showed a weak negative correlation with %EWL only at 6 months (r=-0.18, P=0.024). Initial weight had a weak negative correlation with %EWL at 9 months (r=-0.169, P=0.031) and 12 months (r=-0.199, P=0.011). Initial BMI showed a medium negative correlation with %EWL at all intervals: 6 months (r=-0.31, P<0.001), 9 months (r=-0.35, P<0.001), and 12 months (r=-0.33, P<0.001). Among 140 patients without prior cholecystectomy, only 8 (5.7%) underwent cholecystectomy after BS, attributed to prophylactic UDCA (300 mg twice daily from week 2 to 6 months postoperatively).

**Clinical Implications:** This study demonstrates that bariatric surgery leads to substantial and progressive weight loss over the first year, with 92.6% of patients achieving successful excess weight loss at 12 months. Higher initial BMI and weight are associated with lower %EWL, suggesting that patients with greater obesity may need more intensive postoperative support. Age had a minimal effect, while sex and surgery type did not significantly influence weight loss outcomes. The very low incidence of post-surgical cholecystectomy (5.7%) supports the use of prophylactic UDCA in bariatric patients, consistent with previous studies showing reduction in gallstone formation from 22% to 6.5% with UDCA. These findings are particularly relevant for clinical practice in Saudi Arabia, where obesity prevalence is high. Limitations include the predominance of female patients and LSG procedures, lack of data on diet and physical activity, and no recorded adherence to UDCA. Future research should include more balanced sex and surgery type distributions, and assess lifestyle factors and UDCA adherence.