Introduction Roux-en-Y gastric bypass (RYGB) restricts food intake. Consequently, patients consume less calcium. In addition, food no longer passes through the duodenum, the main site of calcium absorption. Therefore, calcium absorption is significantly impaired. The goal of this study is to compare two common calcium supplements in gastric bypass patients. Method Nineteen patients were enrolled in a randomized, double-blinded, crossover study comparing the absorption of calcium from calcium carbonate and calcium citrate salts. Serum and urine calcium levels were assessed for peak values (Cmax) and cumulative calcium increment (area under the curve [AUC]). Serum PTH was assessed for minimum values (PTHmin) and cumulative PTH decrement (AUC). Statistical analysis was performed using a repeated analysis of variance model. Results Eighteen subjects completed the study. Calcium citrate resulted in a significantly higher serum Cmax (9.4+0.4 mg/dl vs. 9.2+0.3 mg/dl, p=0.02) and serum AUC (55+2 mg/dl vs. 54+2 mg/dl, p=0.02). Calcium citrate resulted in a significantly lower PTHmin (24+11 pg/ml vs. 30+13 pg/ml, p=0.01) and a higher AUC (−32+51 pg/ml vs. −3+56 pg/ml, p=0.04). There was a non-significant trend for higher urinary AUC in the calcium citrate group (76.13+36.39 mg/6 h vs. 66.04+40.82, p=0.17). Conclusion Calcium citrate has superior bioavailability than calcium carbonate in RYGB patients.

Purpose Roux-en-Y gastric bypass surgery is associated with an increased risk of nephrolithiasis but obesity itself is a known risk factor for kidney stones. To assess the mechanism(s) predisposing to nephrolithiasis after Roux-en-Y gastric bypass we compared urinary tract stone risk profiles in patients who underwent the procedure and normal obese individuals. Materials and Methods In this cross-sectional study urine and serum biochemistry was evaluated in 19 nonstone forming patients after Roux-en-Y gastric bypass and in 19 gender, age and body mass index matched obese controls without a history of nephrolithiasis. Results Compared with obese controls surgical patients had significantly higher mean ± SD urine oxalate (45 ± 21 vs 30 ± 11 mg daily, p = 0.01) and lower urine citrate (358 ± 357 vs 767 ± 307 mg daily, p <0.01). The prevalence of hyperoxaluria (47% vs 10.5%, p = 0.02) and hypocitraturia (63% vs 5%, p <0.01) was significantly higher in surgical patients, who also had significantly lower urine calcium than obese controls (115 ± 93 vs 196 ± 123 mg daily, p = 0.03). The calcium oxalate urine relative supersaturation ratio was not significantly different between the 2 groups. Conclusions Almost half of patients with Roux-en-Y gastric bypass without a history of nephrolithiasis showed hyperoxaluria or hypocitraturia. This prevalence was significantly higher than in body mass index matched controls. These risk factors were negated by lower urine calcium excretion in patients with Roux-en-Y gastric bypass.

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