Vertebral bone marrow fat changes after gastric bypass surgery

Last Updated: 2015-02-16

By Will Boggs MD

NEW YORK (Reuters Health) - Patients with diabetes, but not those without the condition, experience reductions in vertebral bone marrow fat following gastric bypass surgery, researchers report.

"Our findings surprised us," Dr. Anne L. Schafer from the University of California, San Francisco, told Reuters Health by email.

"Based on published observations that caloric restriction paradoxically increases bone marrow fat in mice and that women with anorexia nervosa have high marrow fat, we had hypothesized that marrow fat content would increase after Roux-en-Y gastric bypass (RYGB)," she said. "In fact, we expected that marrow fat content would increase markedly, given the consistent and dramatic declines in total body fat induced by RYGB. However, our findings did not support this hypothesis."

Gastric bypass surgery results in weight loss and, often, improvement in glycemic control. No studies have assessed changes in bone marrow fat in the setting of improving or declining glycemic control.

In a pilot study of 11 morbidly obese diabetic (n=6) and nondiabetic (n=5) women undergoing RYGB, Dr. Schafer's team examined the effects of RYGB on vertebral bone marrow fat. Their findings were published online January 17 in Bone.

Mean preoperative marrow fat content was 54.2% in diabetic and 43.1% in nondiabetic women and correlated with visceral fat levels. Marrow fat content was inversely correlated with spinal bone mineral density (BMD).

All participants lost weight and total body fat mass after RYGB, and HbA1c levels decreased to a greater extent among diabetic than nondiabetic participants. BMD also decreased during the six-month follow-up period, with no consistent differences between diabetic and nondiabetic women.

Marrow fat showed very little change among nondiabetic women (mean change, +0.9%; p=0.84), but decreased significantly among diabetic women (mean change, -7.5%; p=0.05), according to the report.

Changes in total body fat correlated inversely with changes in marrow fat content among nondiabetic women, whereas there were no such correlations between changes in total body fat or visceral fat and marrow fat content among diabetic women.

There was no significant correlation between the change in HbA1c and the change in marrow fat, but the direction of correlation was opposite for women with (direct) and without (inverse) diabetes, the researchers say.

"Our findings suggest that among those without diabetes, marrow fat content is maintained after RYGB despite dramatic declines in overall fat mass," Dr. Schafer said. "In contrast, among those with diabetes, RYGB may reduce marrow fat content, possibly by improving glycemic control. Future studies of marrow fat during weight loss should take into account participants' diabetes status, and studies should continue to explore the physiological significance and regulation of marrow fat."

Dr. Schafer concluded, "Bone marrow is a depot for adipose tissue, but the regulation of bone marrow fat is poorly understood. Our findings suggest that marrow fat has unique metabolic behavior compared to other fat depots."

Dr. Elaine W. Yu from Massachusetts General Hospital and Harvard Medical School in Boston, who was not involved in the work, agreed.

"Not all body fat is the same," she told Reuters Health by email. "Fat depots within different compartments of the body are regulated differently and may have varying effects on human metabolism. We have already had hints of this with the different associations of visceral and subcutaneous fat with cardiovascular disease (visceral fat = bad prognostic factor; subcutaneous fat = neutral). Another fat depot, brown fat, has thermoregulatory properties and is important for regulating energy expenditure. It will be fascinating to learn more about whether bone marrow fat also plays a systemic role in human metabolism."

"Importantly, there has been a theoretical concern that our standard bone density imaging techniques (e.g., Dual-energy x-ray absorptiometry, DXA; and quantitative computed tomography, QCT) may have been exaggerating the amount of bone loss seen after gastric bypass due to imaging artifacts from increasing marrow fat," Dr. Yu explained. "However, this pilot study shows either no change or possible decreases in marrow fat after gastric bypass. This means that the bone loss observed after gastric bypass is a real phenomenon that may have important clinical consequences."

SOURCE: http://bit.ly/1JlFH9v

Bone 2015.