BERKELEY, CA (UroToday.com) - As is widely reported in the popular press, obesity is considered a national health crisis in the United States, and it increases the risk of chronic diseases such as diabetes, hypertension, and obstructive sleep apnea. To address this crisis, an increasing number of obese people have undergone bariatric surgical procedures. Of these, Roux-en-Y gastric bypass (RYGB) has been most commonly employed, with 100 000 – 150 000 procedures performed per year in the United States over the last 2 decades. Although randomized trials and large cohort studies confirm the effectiveness of this approach for allowing sustained weight loss and improved mortality, this benefit does not come without a price. We and others have documented enteric hyperoxaluria, chronic kidney disease, and calcium oxalate kidney stones are also a common and inadvertent side effect. However, the magnitude of this risk has been unclear. In the current population-based study we were able to provide a better estimate of the risk. After a standard RYGB, the odds of a new stone are about 2% per year, or roughly double those of obese individuals who do not undergo surgery. Patients with more extensive weight loss procedures (e.g., biliopancreatic diversion) appear to have even greater stone risk, while restrictive procedures such as gastric lap bands do not seem to be associated with more kidney stones. One way to detect those at higher risk of kidney stones is to measure 24 hr urinary oxalate excretion at 6-12 months after the procedure.
The other serious renal complication of RYGB appears to be oxalate nephropathy. A small but consistent proportion of such cases have been identified in our renal clinics, many requiring dialysis and/or kidney transplant. In the current study, the overall risk of chronic kidney disease after RYGB did not appear to be statistically significant compared to non-operated obese controls. This observation suggests that the percent at risk for this dreaded complication is relatively low. However, even if the percentage at risk is low, given the number of patients operated per year (> 100 000), the absolute number of individuals with kidney damage could still be substantial.
Now that we have identified the potential magnitude of the problem, the next step is to identify better treatment methods. Given the common nature of stones after RYGB, and the known morbidity associated with stone events, it might be reasonable to screen all RYGB patients at one year post-procedure for evidence of enteric hyperoxaluria. Certainly, all patients who do develop stones should be screened. Once identified, patients should be placed on an appropriate diet (low fat, low oxalate), with plenty of supplemental calcium to act as an oxalate binder. The optimum amount of calcium varies by patient and should be determined based upon serial urine measurements. Many research questions remain. Why does only a subset of patients develop this complication? What puts patients at risk for oxalate nephropathy? Are other treatment strategies possible, such as probiotics or oxalate degrading enzymes? What is the role of the intestinal microbiome? Can we devise strategies to better protect the kidneys of those at risk? Prospective studies of cohorts of patients with enteric hyperoxaluria following bariatric surgery, such as that being assembled by the Rare Kidney Stone Consortium, can help to answer these questions. Even if everyone stopped doing RYGB procedures today, there are literally millions of patients who have already been treated, and will require our vigilance for decades to come.
Written by:
John C. Lieske, MD,1,2,3 David S. Goldfarb, MD,4 and Dawn S. Milliner, MD1,2 as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
1Rare Kidney Stone Consortium
2Division of Nephrology and Hypertension, Department of Internal Medicine, Mayo Clinic, Rochester, MN USA
3Renal Laboratory, Department of Laboratory Medicine, Mayo Clinic, Rochester, MN USA
4Nephrology Section, NY Harbor VA Medical Center, and Division of Nephrology, NYU School of Medicine, New York, NY, USA