Intestinal Calcium Absorption Decreases Dramatically After Gastric Bypass Surgery Despite Optimization of Vitamin D Status

Abstract

Roux-en-Y gastric bypass (RYGB) surgery has negative effects on bone, mediated in part by effects on nutrient absorption. Not only can RYGB result in vitamin D malabsorption, but the bypassed duodenum and proximal jejunum are also the predominant sites of active, transcellular, 1,25(OH)2D-mediated calcium (Ca) uptake. However, Ca absorption occurs throughout the intestine, and those who undergo RYGB might maintain sufficient Ca absorption, particularly if vitamin D status and Ca intake are robust. We determined the effects of RYGB on intestinal fractional Ca absorption (FCA) while maintaining ample 25OHD levels (goal ≥30 ng/mL) and Ca intake (1200 mg daily) in a prospective cohort of 33 obese adults (BMI 44.7 ± 7.4 kg/m2). FCA was measured preoperatively and 6 months postoperatively with a dual stable isotope method. Other measures included calciotropic hormones, bone turnover markers, and BMD by DXA and QCT. Mean 6-month weight loss was 32.5 ± 8.4 kg (25.8% ± 5.2% of preoperative weight). FCA decreased from 32.7% ± 14.0% preoperatively to 6.9% ± 3.8% postoperatively (p < 0.0001), despite median (interquartile range) 25OHD levels of 41.0 (33.1 to 48.5) and 36.5 (28.8 to 40.4) ng/mL, respectively. Consistent with the FCA decline, 24-hour urinary Ca decreased, PTH increased, and 1,25(OH)2D increased (p ≤ 0.02). Bone turnover markers increased markedly, areal BMD decreased at the proximal femur, and volumetric BMD decreased at the spine (p < 0.001). Those with lower postoperative FCA had greater increases in serum CTx (ρ = −0.43, p = 0.01). Declines in FCA and BMD were not correlated over the 6 months. In conclusion, FCA decreased dramatically after RYGB, even with most 25OHD levels ≥30 ng/mL and with recommended Ca intake. RYGB patients may need high Ca intake to prevent perturbations in Ca homeostasis, although the approach to Ca supplementation needs further study. Decline in FCA could contribute to the decline in BMD after RYGB, and strategies to avoid long-term skeletal consequences should be investigated.

Outcomes of Laparoscopic Sleeve Gastrectomy and Roux-en-Y Gastric Bypass in Patients Older than 60

“…More studies with bigger sample sizes of older patients may better elucidate whether older patients truly have more complications. Other factors to consider include the reported decrease in Vitamin D and calcium levels in patients after malabsorptive bariatric surgery and the subsequent effect on physiological parameters such as bone density [15,16]. However, these studies were not done in the older population; it remains to be examined if these trends hold true in older patients and how these potential drawbacks weigh in against the benefits of bariatric surgery.…”

Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects

“…Extensive clinical experience and some randomized control trials indicate that daily intake of 400 IU vitamin D 3 is sufficient to prevent this type of rickets in children (55,264). In adults, clinical vitamin D-related osteomalacia is usually found in individuals with low sun exposure or in patients with impaired intestinal vitamin D absorption as part of intestinal fat malabsorption, like after bariatric surgery or with inflammatory bowel disease (395).…”

Short-Term UVB Treatment or Intramuscular Cholecalciferol to Prevent Hypovitaminosis D After Gastric Bypass—a Randomized Clinical Trial

“…Although larger studies are needed to evaluate the importance of extra vitamin D supplementation in healthy individuals, gastric bypass patients are in need to raise their low levels. The result in the present study has, together with the recent literature [18,19], triggered us to measure vitamin D levels more often.…”