"We aren't exactly sure what is causing the increase in stone formation or creating the environment for the stones to form, but we believe it is part of the process of malabsorption of nutrients that results from the most common surgical procedure, the Roux-en-Y gastric bypass," explained researcher John Lieske, MD, professor of medicine, Mayo Clinic College of Medicine, Rochester, Minnesota.

In his presentation November 17^th, Dr. Lieske said there was no argument that the treatment works for most people undergoing the surgery. He cited a Swedish study that indicated patients lost an average of 60 pounds (30 kg) and reduced hypertension, diabetes and cholesterol risk from 62% to 90% when compared with similar obese individuals who were treated conventionally.

However, the cost of surgery may not be felt until 2 years after surgery, when kidney stone development appears to occur, he said.

Dr. Lieske and colleagues evaluated 31 patients who underwent standard gastric bypass surgery at Mayo Clinic and found that those patients had decreased body mass index from an average of 57 to 40 after 2 years. However, 9 of the 31 patients also developed oxalate stones -- the first stones occurring about 2.2 years after the surgery.

Those 9 patients produced 27 stones, usually of the calcium oxalate formation, an indication that mal-absorption of food was the cause, Dr. Lieske said.

"This random sample of Roux-en-Y gastric bypass surgery patients developed a marked increase in urinary oxalate excretion, a significant reduction in urine citrate excretion and a resulting increase in calcium oxalate supersaturation within 12 months postoperatively," he said.

"Hence, the risk of calcium oxalate nephrolithiasis may be quite high in Roux-en-Y gastric bypass patients as a group. The risk for other complications of hyperoxaluria, such as chronic renal damage, could also be high," he speculated.

Dr. Lieske said the risk of this complication is uncertain, but doctors need to be vigilant for its occurrence since more than 100,000 of the procedures are being performed each year.


[Presentation title: Increased Lithogenic Risk Factors Following Roux-en-Y Gastric Bypass Surgery for Morbid Obesity. Abstract TH-F-DS871]

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