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Review
. 2017 Oct:173:202-210.
doi: 10.1016/j.jsbmb.2016.12.012. Epub 2016 Dec 24.

Vitamin D and intestinal calcium transport after bariatric surgery

Anne L Schafer  1
Affiliations
Review

Vitamin D and intestinal calcium transport after bariatric surgery

Anne L Schafer. J Steroid Biochem Mol Biol. 2017 Oct.

Abstract

Bariatric surgery is a highly effective treatment for obesity, but it may have detrimental effects on the skeleton. Skeletal effects are multifactorial but mediated in part by nutrient malabsorption. While there is increasing interest in non-nutritional mechanisms such as changes in fat-derived and gut-derived hormones, nutritional factors are modifiable and thus represent potential opportunities to prevent and treat skeletal complications. This review begins with a discussion of normal intestinal calcium transport, including recent advances in our understanding of its regulation by vitamin D, and areas of continued uncertainty. Human and animal studies of vitamin D and intestinal calcium transport after bariatric surgery are then summarized. In humans, even with optimized 25-hydroxyvitamin D levels and recommended calcium intake, fractional calcium absorption decreased dramatically after Roux-en-Y gastric bypass (RYGB). In rats, intestinal calcium absorption was lower after RYGB than after sham surgery, despite elevated 1,25-dihyroxyvitamin D levels and intestinal gene expression evidence of vitamin D responsiveness. Such studies have the potential to shed new light on the physiology of vitamin D and intestinal calcium transport. Moreover, understanding the effects of bariatric surgery on these processes may improve the clinical care of bariatric surgery patients.

Keywords: Bariatric surgery; Calcium; Gastric bypass; Malabsorption; Sleeve gastrectomy; Vitamin D.

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Figures

Fig. 1
Fig. 1
Intestinal fractional calcium absorption before and 6 months after Roux-en-Y gastric bypass surgery. Values are mean ± SD. Reproduced from Schafer et al. [15], by permission of John Wiley & Sons.
Fig. 2
Fig. 2
Correlation between fractional calcium absorption 6 months after Roux-en-Y gastric bypass surgery and change in serum CTx level. Reproduced from Schafer et al. [15], by permission of John Wiley & Sons.
Fig. 3
Fig. 3
Effects of Roux-en-Y gastric bypass surgery on calcium homeostasis. After RYGB, impaired intestinal calcium absorption threatens serum calcium concentration, and PTH secretion increases. PTH enhances renal calcium reabsorption, stimulates conversion of 25(OH)D to 1,25(OH)2D, and increases bone resorption in order to maintain serum calcium concentration. There may be direct skeletal effects of 1,25(OH)2D as well. Concurrent non-PTH-mediated stimulation of bone resorption likely occurs as well, due to factors such as mechanical unloading and changes in adipokine, sex steroid, and gut-secreted hormone levels, and the enhanced mobilization of calcium from the skeleton may dampen the impetus for greater PTH secretion.
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