NEW YORK (Reuters Health) – Patients lose bone density after weight loss surgery, and the more weight they lose, the greater the drop, new research shows.

While the effect of bone density loss on future fracture risk is not yet clear, “for the present, a high degree of vigilance for nutritional deficiencies and bone loss in patients both before and after bariatric surgery is critical,” the researchers write.

Dr. Shonni J. Silverberg and colleagues from Columbia University College of Physicians and Surgeons in New York City explain that after people have obesity surgery, they have a smaller gut area available to absorb nutrients from the food they eat. Evidence is mounting that weight-loss surgery may impair bone and mineral metabolism, the team notes in the Journal of Clinical Endocrinology and Metabolism.

To investigate, they followed 23 men and women who had undergone Roux-en-Y gastric bypass surgery, one of the most popular types of weight-loss surgery. This operation involves bypassing the duodenum, the portion of the small intestine that joins to the stomach, which is the chief site of calcium absorption by the body.

Before the surgery, the patients had less-than-optimal blood levels of vitamin D, even though their intake of the nutrient was relatively high.

After the surgery, they doubled their calcium intake and were consuming 2.6 times more vitamin D, but their blood levels of vitamin D remained low, and their blood levels of calcium actually fell.

Signs of poor calcium absorption appeared within three months of the surgery. Concentrations of parathyroid hormone, which fosters release of calcium from the bones, had been in the high normal range before the operation, and increased after the surgery.

Within a year of the surgery, patients had lost an average of 45 kilograms in weight. At that point, the density of their upper thigh bones had dropped by 9.2% and their hip bone density fell 8%, but most patients didn’t show loss of bone density in the lower back or the forearm.

The degree of bone loss was closely linked to how much weight patients lost, Silverberg and colleagues found. This may have occurred, the researchers suggest, because bones no longer had to bear as much weight, and weight-bearing is a key component of maintaining and building bone strength. Increased secretion of parathyroid hormone could also have been a factor, they add.

“The small sample size of this study argues for larger, long-term studies to answer the critical question of how these losses relate to bone quality and fracture risk,” the researchers state.

SOURCE: Journal of Clinical Endocrinology and Metabolism, published online July 22, 2008.