Little is known about the impact of most currently offered bariatric surgeries on the risk for nephrolithiasis. Since obesity and insulin resistance have been implicated as risk factors for nephrolithiasis, especially uric acid stones, one might reasonably hypothesize that RYGB could ameliorate kidney stone risk [8
]. Further, the RYGB operation with a Roux limb < 150 cm in length has generally been believed not
to cause fat malabsorption, thought to be a critical factor in the development of enteric hyperoxaluria.
However, we recently noted a seemingly large number of patients with calcium oxalate stones and relatively marked hyperoxaluria after RYGB in our institution. Therefore, in 2005 we conducted a systematic review of all Mayo Clinic patient records to identify potential cases of enteric hyperoxaluria in patients who had received RYGB between 1984–2005 (n=1436) [31
]. In addition, a survey was sent to the subgroup that had undergone the potentially more malabsorptive distal RYGB (Roux limb ~300 cm and common channel length ~125 cm; n= 258), since we hypothesized those patients might be more susceptible to this complication. A total of 23 cases of enteric hyperoxaluria were identified by the initial record review, 14 after proximal RYGB and 9 after distal RYGB. Most concerning were two patients that presented with renal failure and biopsy-proven oxalate nephropathy. Neither had a prior history of renal disease or nephrolithiasis. Amongst the distal RYGB group, 188/258 patients returned the supplemental survey. Of these 27 (16%) had experienced nephrolithiasis after the procedure; only 8 had had a stone preoperatively. We cannot reliably estimate the prevalence of stones in the proximal RYGB since they were not surveyed, many of these patients do not receive regular medical care at Mayo, and renal stones are not always recorded in the medical record. However, this study did suggest that nephrolithiasis was common after RYGB, and perhaps more so in the distal RYGB group.
In early 2006, we updated the Mayo Clinic series to include an increasing number of patients referred to our Stone Clinic after RYGB, and for whom we had detailed metabolic data [42
]. A total of 60 patients were identified, including 31 who had been seen in the Stone Clinic. A large percentage (17/31) had been seen for the first time in stone clinic over the prior two years. The mean BMI of patients pre-operatively was 57 kg/m2
, with a decrease of 20 kg/m2
at the time of the first stone event which averaged 2.9 years after RYGB. Although the distal RYGB only accounts for only ~18% of the total RYGB procedures performed at Mayo, there were 36 distal RYGB and 24 proximal RYGB patients identified. Therefore, patients may be at greater risk for stones after the distal procedure. Amongst those analyzed, stones were 100% calcium oxalate in 19 patients, and mixed uric acid/calcium oxalate in two other instances.
Amongst the subset seen in stone clinic, hyperoxaluria was commonly observed (present in 17/31), with a mean urine oxalate of 0.66 mM/24 hours (). Urinary citrate and calcium were modestly reduced, and overall urinary supersaturation for calcium oxalate was quite high. When divided into proximal and distal patients, there was no significant difference in urinary parameters (). However, there did seem to be differences in the urine composition depending on time of presentation after RYGB. Those who presented less than 6 months postoperatively rarely had elevated urinary oxalate (mean 0.44 mM/day), whereas those longer than 6 months out often did (mean 0.77 mM/day; ). Urinary supersaturation was equally high in both groups however (), due in large part to low urine volumes in the less than 6 month group. These differences may reflect changes in gastrointestinal function and diet that developed over the first year after the procedure.
Urinary Chemistries in patients with and without gastric bypass surgery
Urine oxalate excretion and calcium oxalate supersaturation amongst those stone formers who present early (<6 months) or late (> 6 months) after RYGB
In order to get a better sense regarding how common hyperoxaluria might be in the total group of patients that undergo RYGB, we next completed a small pilot study of patients randomly selected before (n=20), 6 months (n=8), and 12 months (n=13) after proximal RYGB. At baseline hyperoxaluria was rare (mean oxalate 0.35 mM/day) and urinary calcium oxalate supersaturation was not increased above the reference mean (). Urinary composition was not significantly changed in the 6 months post operative group, but by 12 months mean urinary oxalate (0.74 mM/24 hrs) and calcium oxalate supersaturation were both elevated in this group of non-stone forming patients. Other urinary changes included a modest fall in urinary citrate and calcium ( and ). These data suggest that many patients may have sub clinical enteric hyperoxaluria and be at risk for stones after standard RYGB, since over half (7/13) were hyperoxaluric and nearly all (12/13) had elevated calcium oxalate supersaturation at the 12 months time point.
Urine chemistries amongst a random sampling of patients before (n=20), 6 months after (n=8) and 12 months after (n=13) RYGB
Other data is emerging that links RYGB to kidney stones. A very recent report lists urinary tract calculus as a common cause for emergency room visits (3.6%) and readmission to hospital (3.0%) within the first 180 days after bariatric surgery [43
]. The University of Pittsburgh also recently examined their longer term experience with a specific focus on stone prevalence [44
]. The medical records of a total of 972 persons that underwent RYGB between the years of 1997–2004 at their bariatric surgery center were examined for stone events, including review of radiology reports. In their group, 85 patients (8.8%) had a preoperative stone history. Of these, 26 (31.4%) had recurrent stones postoperatively (mean time 1.9 years), whereas an additional 31 developed stones de novo
at a mean time of 2.8 years (3.5%). These data may underestimate the scope of the problem since stone history was obtained from record review alone and no information is provided regarding time or extent of follow-up in the cohort. Nevertheless, the data did suggest that stone prevalence was enriched by at least 70% in this population, compared to expected rates derived from Nutrition Examination Survey (NHANES) III data [44
Recently, a large referral lab reported urinary chemistry values for 132 patients that were identified as having undergone “modern” bariatric surgery for obesity [45
]. Only abbreviated patient history was available; for example the sub-type of surgery (e.g., banding versus gastric bypass) was not known. Nevertheless, the urinary data are remarkably similar to those observed in our patient group at Mayo Clinic. Mean urine oxalate was elevated (83 mg/day), with a corresponding increase in urinary calcium oxalate supersaturation (). Urine calcium excretion was slightly reduced, while citrate excretion and total volume were both fairly normal. Urine oxalate excretion was not as high as in an older group with JI bypass; nevertheless the calcium oxalate supersaturation was actually marginally higher in the modern bariatric group. Importantly, 23% of this referral lab cohort had a daily urinary oxalate excretion of greater than 100 mg, a level at which renal damage has been well described. Time to first stone was also comparable to the Mayo cohort (3.6 vs. 2.9 years) although relatively fewer had pre-existing stones (1/132 versus 11/31).
To our knowledge, no information is currently available to assess the relative potential risk for nephrolithiasis and/or hyperoxaluria after the various forms of bariatric surgery. However, very limited data are available regarding the degree of fat malabsorption in patients after selected procedures [34
]. After JI bypass overall fat absorption was reported to be only 15%, whereas it was 97% after VBG or LAGB. Fat absorption was also severely compromised after BPD, with or without duodenal switch (19%) and intermediate after RYGB (67%). Based upon this data, one might hypothesize that the risk for enteric hyperoxaluria would be greatest after BPD, lowest for VBG or LAGB, and intermediate for RYGB. However, even these inferences are tentative since the fat absorption numbers were based on measurements in only 9 patients in the RYGB group. We do note, however, that even though patients in the RYGB group had an elevated average of 44 grams of fecal fat (versus 139 grams in the BPD groups) the patients did not report prominent symptoms of diarrhea (average 1.5 bowel movements per day versus 3.6 in the BPD groups). This observation correlates with our personal clinical experience that the RYGB patients rarely report clinical diarrhea.