Gastric Bypass Patients More Likely to Develop Kidney Stones
By Nancy Fowler Larson
Medscape Medical News
March 12, 2010 — Patients who have Roux-en-Y gastric bypass (RYGB) surgery develop a greater risk for kidney stones as a result of altered urine composition, according to a study published in the March issue of the Journal of Urology.
"Obesity and RYGB surgery are separately associated with an increased risk of kidney stones," write Naim Maalouf, MD, assistant professor of internal medicine, University of Texas Southwestern Medical Center, Dallas, and colleagues. "We compared urinary tract stone risk profiles in patients after RYGB and obese controls to assess the mechanism(s) by which RYGB heightens the nephrolithiasis risk."
Researchers evaluated 38 patients. Half had undergone RYGB, a common weight-loss surgery involving stapling or banding the stomach, in the previous 1.8 to 3.5 years. These patients were compared with a control group of obese individuals, matched according to body mass index and age (mean age, 49 ± 11 years for the surgery group vs 47 ± 10 years for the control group [p = 0.58]). None of the 3 men and 16 women in each group had a history of nephrolithiasis.
In this cross-sectional study, participants completed a 24-hour urine collection while on their regular diet. Using these samples, the study authors measured levels of oxalate, citrate, and calcium. Researchers also collected fasting blood samples.
Gastric Bypass Patients Have Higher Risk in 2 Areas
The results showed that the urine of patients who had RYGB surgery contained higher levels of oxalate, an acid found in most kidney stones (45 ± 21 vs 30 ± 11 mg daily; P = .01). The urine of patients who had RYGB surgery also revealed lower levels of citrate (358 ± 357 vs 767 ± 307 mg daily; P < .01), which inhibits the formation of stones. Hyperoxaluria (excessive urine oxalate) was more prevalent in patients who had RYGB surgery (47% vs 10.5%; P = .02), as was the prevalence of hypocitraturia (low urine citrates) (63% vs 5%; P < .01).
One potentially mitigating factor was the lower level of calcium in the urine of gastric bypass patients (115 ± 93 vs 196 ± 123 mg daily; P ? .03), a condition that can offset the risk for nephrolithiasis. Still, the overall risk factors for kidney stone formation in this population were significant and may increase over time.
"Almost half of the patients who had undergone gastric bypass and did not have a history of kidney stones showed high urine oxalate and low urine citrate — factors that lead to kidney-stone formation," Dr. Maalouf said in a press release. "This complication may not be well-recognized in part because it tends to occur months to years after the bypass surgery."
The investigators stated the following limitations to their study:
No cause for hypocitraturia could be determined because the urine collection process prohibited the researchers from measuring net acid.
The results are based on a single 24-hour urine sample per subject; in a previous study, in which 2 samples were collected, researchers noted marked differences in each sample regarding the amount of oxalate in the urine of patients who had RYGB surgery.
The cross-sectional design of the study prevented the discovery of extensive information about the time course of changes in urine factors in patients who had RYGB surgery.
Insight into time course changes could be determined in a prospective follow-up study of risk factors in gastric bypass patients. More research is also needed to measure the actual occurrence of kidney stones and to manage the risk.
"The true incidence of nephrolithiasis and optimal treatment for lithogenic risk factors in this population remain to be established," the study authors write.
The National Institutes of Health supported the study. The study authors have disclosed no relevant financial relationships.
