Gastric bypass surgery (GBS) is an operation where the stomach is made smaller and food is allowed to bypass part of the small intestine, thus reducing the amount of food and calories a person eats because they feel full quicker. For many people struggling with obesity and weight issues, GBS has been successful in helping them lower their body weight, thus improving their health and quality of life.

Now, however, doctors are discovering that some patients who have had GBS experience changes in their urinary composition that places them at a higher risk for developing kidney stones than obese individual who do not undergo GBS. The findings, published in the Journal of Urology, suggest that there may be additional adverse health consequences to the surgery.

In the study in question, researchers gathered 38 individuals who fell into one of two categories: those that had Roux-en-Y gastric bypass (RYGB) performed on them, and those that had not. RYGB is in fact the most common form of GBS used today for weigh loss.

What they found was that compared to subjects who had no surgery, the urine of the RYGB subjects was higher in a compound known as oxalate (47% versus 10.5%). RYGB subjects also had lower concentrations of citrate (32% versus 5%) in their urine. The significance of this is rooted in the fact that oxalate is found in a majority of kidney stones, and citrate acts to inhibit kidney stone formation.

Furthermore, nearly half of the RYGB subjects had no history of kidney stones, thus supporting the role that the surgery may played in the development of the stones. How or why the procedure might contribute to kidney stone formation is not clear, but findings nonetheless highlight the need for doctors and patients to be aware of the increased risk, a fact which is complicated by the long interval of time between the surgery and the appearance of kidney stones, which can sometimes take years.

GBS has become more common in recent years as a way to combat obesity, and while different hospitals may have different rule and regulations regarding the procedure, there are certain general guidelines. GBS is usually an option for people who suffer from life threatening obesity or have a body mass index (BMI) of 40 or higher (this corresponds to a person who is 5’6” and weighs about 250 pounds). Because it is an invasive procedure, GBS is generally considered only after most other weight loss options have been exhausted, and in many instance, certain criteria must be met.

And while GBS has been successful in helping obese people to lose weight, the procedure itself, like all surgeries, comes with its own inherent risks.

If you are considering GBS, speak with your physician about your options. For more information about the procedure, visit website for the National Institutes of Health (NIH). To learn more about kidney stones, check out the website for the National Kidney and Urologic Diseases Clearinghouse (NIDDK), a division of NIH.