Thin bones: Vitamin D and calcium handling after bariatric surgery

Abstract

Bariatric surgery has proven to be a valuable treatment option for morbid obesity. However, these procedures can lead to impaired intestinal absorption of calcium and vitamin D, thereby challenging calcium homeostasis and possibly contributing to bone loss leading to an increased fracture risk. Besides calcium and vitamin D malabsorption, hormonal changes occurring after surgery can also be the source of observed bone loss. In this review, first, a case report will be discussed, highlighting the relevance of this topic. Afterwards, changes in bone density and fracture risk, after the two most performed types of bariatric surgery, Sleeve Gastrectomy (SG) and Roux-en-Y Gastric Bypass (RYGB) will be discussed. In addition, we discuss the putative underlying mechanisms leading to bone changes based on both preclinical and clinical observations. Nonetheless, it is clear further research is needed to further elucidate the exact mechanisms of bone loss following bariatric surgery and subsequently identify potential treatment options for bone preservation.

Keywords: Bariatric surgery; Bone mineral density (BMD); Calcium; Fractures; RYGB, Roux-en-Y Gastric Bypass; Roux-en-Y Gastric Bypass; SG, Sleeve Gastrectomy; Sleeve Gastrectomy; Vitamin D.

Fig. 1

Sleeve Gastrectomy (left) and Roux-en-Y Gastric Bypass (right).

Fig. 2

Intestinal (left) and renal (right) calcium (re)absorption. Calcium is taken up by the cell through TRPV6 or TRPV5, transported in the cytoplasm bound to calbindinD_9k and transported across the basolateral membrane into the blood by PMCA1b or NCX1.

Fig. 3

Calcium and vitamin D homeostasis. PTH and 1,25(OH)₂D tightly regulate serum calcium levels. When calcium levels drop, PTH is secreted by the parathyroid glands. PTH will have its effects on kidneys and bone to increase calcium levels in the serum. Red lines indicate inhibitory effects, green lines indicate stimulatory effects.

Fig. 4

The relationship between adipose tissue and bone homeostasis. Lean white adipose tissue produces more adiponectin. With obesity, adipose tissue expands and more leptin will be produced. These hormones are influenced by factors from the bone homeostasis and vice versa, as indicated by the lines. Red lines indicate inhibitory effects, green lines indicate stimulatory effects. The black dashed line indicates the relationship between bone and leptin, which is not entirely clear yet.