Vitamin D Doesn't Prevent Calcium Drop After Gastric Bypass

Nancy A. Melville

September 14, 2014

HOUSTON – The significant reduction in calcium absorption known to result from gastric-bypass surgery occurs even when healthy levels of vitamin D are maintained postoperatively and patients receive recommended calcium supplementation, according to a small study presented here at the American Society for Bone and Mineral Research (ASBMR) 2014 annual meeting.

"We found that fractional calcium absorption decreased dramatically after roux-en-Y-gastric-bypass [RYGB] surgery, even with 25(OH)D levels that were normal — at or above 30 ng/mL," said lead author Anne Schafer, MD, of the San Francisco VA Medical Center and assistant professor at the University of California, San Francisco.

"These decreases are more severe than previously reported," she observed.

The effects of RYGB on vitamin-D absorption — and subsequent adverse effects on the skeleton — are known unwanted effects that result because the surgery bypasses the duodenum and proximal jejunum, both important sites of vitamin-D and calcium absorption.

Some have theorized, however, that the loss in calcium is somewhat compensated by absorption elsewhere in the intestine and that postoperative declines can be mitigated by maintaining healthy vitamin-D levels and recommended calcium intake.

Thus, this study is notable in that it is among the first to examine the important question of whether maintaining an optimal vitamin-D status protects against the decrease in calcium absorption after gastric bypass, said session moderator Catherine M. Gordon, MD, director of the division of adolescent medicine, Hasbro Children's Hospital, and professor and vice chair of clinical research, department of pediatrics, at the Alpert Medical School of Brown University, Providence, Rhode Island.

"The authors are to be commended on an excellent study hypothesis, which if proven true, could have represented a simple but important public-health intervention, as vitamin-D supplementation is inexpensive and easy to administer as an oral preparation," said Dr. Gordon. Importantly, the results suggest the intervention could instead have no effect on calcium levels, she told Medscape Medical News.

"Unfortunately, the investigators showed that calcium absorption decreased despite optimization of vitamin-D status," she noted. "Thus, provision of calcium to these patients is highlighted as a critically important intervention for these patients."

Study Results

Dr. Schafer and colleagues evaluated 33 obese patients (76% women) with depleted serum 25-hydroxyvitamin D (25(OH)D) levels after RYGB. Patients were provided with individually tailored vitamin-D supplementation to restore levels and maintain them at 30 ng/mL or higher. Patients also maintained a total daily calcium intake of 1200 mg with calcium-citrate supplements.

At the 6-month postop follow-up, patients had a mean weight loss of 32 kg (26% of preop body weight) and showed steep decreases in fractional calcium absorption, dropping from preop levels of 0.33 to just 0.07 (P < .001), despite serum 25(OH)D levels of 42 ng/mL (preop) and 36 ng/mL (postop) and a calcium intake of 1200 mg/day.

In terms of calciotropic hormones, urinary calcium decreased (P < .01) and parathyroid levels increased (P = .02) at 6 months.

Markers of bone turnover rose dramatically: postoperative levels of CTX increased by a mean of 276%, and P1NP increased by 104% (both P < .01).

"We found that those with lower postop fractional calcium absorption were those with greater increases in CTX, and multivariable analysis showed that this was independent of the amount of weight loss," Dr. Schafer noted.

Meanwhile, bone-mineral density (BMD) declined at the femoral neck and total hip by 4.8% over the 6 months (P < .01); however, there was no significant association between the amount of fractional calcium absorption and BMD change.

Dramatic Correlation Between Weight Loss and Drop in Calcium Absorption

But a striking correlation was seen between the amount of weight loss and extent of reduction in calcium absorption. Higher percentages of weight loss and decreases in insulinlike growth factor 1 (IGF-1) and IGF-binding protein 3 levels correlated with greater decreases in fractional calcium absorption (all P ≤ .04) independent of patients' age, sex, and race.

Dr. Schafer speculated that evolving gastric-bypass techniques could be a reason for the greater reductions in fractional calcium absorption that were observed compared with previous studies.

"A possible explanation for greater severity could include differences in surgical approach — mainly differences in the length of the jejunum that gets bypassed."

Interventional trials are needed, however, to better evaluate changes in weight and calcium absorption, said Dr. Schafer. She added that a 6-month follow-up may be too short to observe the full effect of fractional calcium absorption on changes in BMD.

The findings nevertheless underscore the need to closely monitor calcium intake after gastric bypass surgery, she said.

"RYGB patients may need high calcium intakes to prevent perturbations in calcium homeostatic response to the lower fractional calcium absorption," she said.

"Decline in calcium absorption could be one of the determinants of the decline in bone-mineral density observed after gastric bypass, and strategies to avoid long-term skeletal consequences should be investigated."

Dr. Gordon said an important study limitation is the small size, but noted its value nevertheless in contributing to evidence on the issue.

"[Because of the size], the findings cannot be viewed as definitive. However, the study adds to growing knowledge about short- and long-term skeletal consequences of gastric-bypass surgery as well as the importance of calcium supplementation for these patients," she concluded.

Drs. Schafer and Gordon have reported no relevant financial relationships.

American Society for Bone and Mineral Research 2014 Annual Meeting. September 13, 2014; Houston, Texas. Abstract 1077.

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