Context: Bariatric surgery is common and may be associated with deleterious effects on the skeleton.

Objective: Assess bone metabolism and density (BMD) following Roux-en-Y gastric bypass (RYGB).

Design: One year prospective longitudinal study.

Setting: University hospital bariatric surgery practice and metabolic bone disease unit.

Participants: 23 obese (mean BMI 47 kg/m²) men and women, aged 20 to 64.

Intervention: None.

Main Outcome Measures: Serum PTH, 25-hydroxyvitamin D (25OHD), osteocalcin urinary N-telopeptide (NTx), and BMD.

Results: Patients lost 45 ± 2 kg one year postoperatively (p<0.01). PTH increased early (3 months: 43 to 50 pg/ml; p<0.001) and urinary calcium dropped (161 to 92 mg/24 hour; p<0.01), despite doubling of calcium intake (1318 to 2488 mg/d; p<0.001). Serum 25OHD concentrations were unchanged (23 to 26 ng/ml), although vitamin D intake increased by 260% (658 IU/d at baseline to 1698 IU/d at 12 months; p<0.05). Markers of bone remodeling rose (p<0.01 for both NTx and osteocalcin), while BMD decreased at the femoral neck (FN: 9.2%, p<0.005) and at the total hip (TH: 8.0%, p<0.005). These declines were strongly associated with the extent of weight loss (FN: r=0.90, p<0.0001; and TH: r=0.65, p=0.02). Lumbar spine and distal radius sites did not change.

Conclusions: Following RYGB there was evidence of calcium and vitamin D malabsorption. Bone turnover increased and hip bone density rapidly declined. The decline in hip BMD was strongly associated with weight loss itself. Vigilance for nutritional deficiencies and bone loss in patients both before and after bariatric surgery is crucial.