Weight loss surgery patients might be losing more than pounds and inches – they might be risking bone loss, as well. A recent study warns that patients run an increased risk of losing bone density – the greater the weight lost, the greater the damage.

In the study, which was published in The Journal of Clinical Endocrinology and Metabolism, doctors monitored the calcium and vitamin D levels of a group of pre-operative gastric bypass surgery patients. As with many obese people, these levels were low, although their nutrient intake was considered adequate.

After their surgeries, each patient doubled his or her calcium intake and consumed 2.6 times more vitamin D than before – yet, their vitamin D and calcium levels decreased, along with their ability to absorb calcium.

One year after surgery, the study subjects had lost an average of 99 pounds – and had also lost more than 9% of the density of their upper thigh bones, and 8% of their hip bones.

Some doctors think such decreases could lead to more fractures and osteoporosis among weight loss surgery patients. Researchers studying weight loss surgery recommend that physicians monitor their patients’ diets both before and after surgery to head off any possibility of mineral deficiencies or bone loss.