obesity is an increasingly prevalent problem in the united states . seventeen percent of children and 32% of adults in the us meet the clinical definition of obesity . in adult men and in adolescents of both sexes ,
these numbers have been climbing over the past 6  years , and in women the rate of obesity seems to be leveling off . the obesity epidemic has been related to sedentary lifestyle , increased use of processed or high caloric foods , and other trends .
a prospective study of three large cohorts found that compared to matched controls , obese patients were 1.44 ( men ) and 1.92 ( women ) times as likely to form stones .
other studies have shown that obese patients have an increased excretion of stone - promoting substances .
the newest trend in treatment of morbid obesity is roux - en - y gastric bypass or gastric banding .
the rate at which these procedures are being performed has increased 600% from 1996 to 2001 . in the 1970s ,
jejunoileal bypass led to significant complications , including nephrolithiasis , ranging from 11% at 1 to 5  years   to as high as 39% after 15  years .
in contrast , modern bariatric surgery procedures , which are touted to cause less malabsorption than jejunoileal bypass , have been postulated to have less impact on urinary stone risk .
yet kidney stones were recently found to be one of the top 10 postoperative diagnoses in modern bariatric patients .
a separate retrospective study found de novo stone formation in 3.2% of patients ; however , for patients with a history of nephrolithiasis , 31.4% were found to have recurrent stones after bypass surgery .
in addition , hyperoxaluria was recently confirmed to be the most significant abnormality in urinary stone risk profile after modern bariatric surgery .
furthermore , hyperoxaluria has been shown to increase as time from surgery increases . in both of these studies ,
the patients had low urinary volumes , and in neither study did the authors comment on the use of oral medications to modify stone risk .
given what has been described as a pending epidemic in nephrolithiasis in patients undergoing modern bariatric surgery ,   we sought to evaluate the effects of medical management on urinary biochemical data between hyperoxaluric patients with and without a history of bariatric surgery .
patients selected for the study had been referred to a urinary stone practice at a tertiary medical center for at least one episode of urolithiasis ; many patients had previously undergone 24-h urine collection and/or been placed on some form of medical therapy by the referring urologist .
included patients had submitted at least two outpatient 24-h urine collections ( within ~1  week s time ) and a questionnaire to a commercial laboratory ( litholink , chicago , ill ) . as in many practices
, we recommend to all of our stone patients to decrease sodium , limit animal protein , and drink sufficient fluids so as to maintain a urine volume around 2  l / day .
we also use the 24-h urine collection to make specific recommendations for dietary change and medical therapy .
however , given the known tendency to chronic acidemia , hypocitraturia , and enteric hyperoxaluria in patients who have undergone bariatric surgery , we place all of these patients on 1,200  mg oral calcium divided up with meals as well as 20  meq potassium citrate three times daily .
those patients with severe ( > 75  mg / day ) hyperoxaluria were identified .
retrospective chart review was performed to determine their medical and surgical histories , especially regarding intestinal surgery and use of medications .
an average value ( over the two samples submitted by each patient ) was determined for each urinary stone risk factor .
calcium , magnesium , uric acid , creatinine , phosphorous , sodium , potassium , and ammonium were measured using a beckman autoanalyzer ( beckman instruments , brea , ca ) .
urine supersaturations of calcium oxalate , calcium phosphate , and uric acid were calculated by the iterative computer program , equil ii .
in addition , the patient s self - reported information on 19 clinical parameters was included  including family history , personal stone history , and use of stone risk modifying medications .
comparison between groups was made using the student s t - test utilizing commercially available statistical analysis software ( spss , chicago , il ) .
we identified 39 patients who had severe hyperoxaluria ( oxalate > 75  mg / day ) , 26 of whom had clinic charts available for review of medical and surgical histories .
five of these 26 patients were found to have a history of roux - en - y gastric bypass surgery .
in addition , none of the operations had taken place within the year prior to 24-h urine collection .
all included non - bariatric patients had presumed dietary or idiopathic hyperoxaluria . the mean age , weight , and medication use
seventy - six percent of the non - bariatric patients were men , although 80% of the bariatric patients were female .
bariatric patients were heavier than non - bariatric patients ( 97 vs. 78  kg ) .
non - bariatric and bariatric patients had a similar number of lifetime stone episodes ( 4.1 vs. 3.6 ) .
oral calcium supplement ( 80 vs. 28% ) and potassium citrate ( 100 vs. 47% ) use were more common in the bariatric than non - bariatric patients.table  1characteristics of hyperoxaluric patientsnon - bariatric ( n  =  21)bariatric ( n  =  5)mean age ( years)5454males ( % ) 7620weight ( kg)7897hctz ( % ) 1920calcium supp .
( % ) 4860allopurinol ( % ) 100 characteristics of hyperoxaluric patients tables  2 , 3 , and 4 provide a summary of the urinary biochemical data .
those patients with a history of bariatric surgery had a significantly higher chloride ( 350 vs. 190 p  <  0.01 ) , a lower citrate ( 390 vs. 800 p  =  0.04 ) , higher oxalate ( 130 vs. 91 p  <  0.01 ) , and higher sodium ( 350 vs. 210 p  <  0.01 ) .
urinary volume was 500  ml greater in the bariatric than non - bariatric group ( 2.9 vs. 2.4  l p  =  0.36).table  2mean 24-h urine parameters by history of bariatric surgerynon - bariatricbariatricp - valueammonium41580.17calcium2401600.25chloride1903500.01citrate8003900.04creatinine1,9001,6000.36magnesium1301300.88oxalate911300.01ph6.36.10.58phosphorous1.21.20.75potassium931000.71sodium2103500.01sulfate51580.60urea nitrogen14130.77uric acid0.890.760.34volume2.52.90.39table  3mean 24-h urine concentrations by history of bariatric surgerynon - bariatricbariatricp - valueammonium19210.69calcium99540.13chloride851200.04citrate3901300.05creatinine8605900.11magnesium57490.48oxalate41470.48phosphorous0.540.420.38potassium42340.48sodium911200.08sulfate23190.57urea nitrogen6.34.80.32uric acid0.410.270.10table  4mean 24-h urine supersaturations by history of bariatric surgerynon - bariatricbariatricp - valuess ua0.820.670.76ss caox116.70.13ss caphos1.40.640.25 mean 24-h urine parameters by history of bariatric surgery mean 24-h urine concentrations by history of bariatric surgery mean 24-h urine supersaturations by history of bariatric surgery the concentration of each urinary solute also was calculated and compared between the two groups .
the supersaturation of different stone constituents is determined more by solute concentration , rather than net amount excreted per day .
these results can be found in table  3 . only chloride ( 120 vs. 85 p  =  0.04 ) and citrate ( 130 vs. 390 p  =  0.05 ) showed a statistically significant difference .
calcium concentration was almost 50% less in bariatric patients versus non - bariatric patients , although this did not reach statistical significance ( 54 vs. 99 p  =  0.13 ) .
the supersaturation of calcium oxalate was 40% less in the bariatric patients versus the non - bariatric patients with hyperoxaluria ( 6.7 vs. 11 p  =  0.13 ) .
oxalate excretion has been identified as a major risk factor for stone formation for decades .
hyperoxaluria following bariatric surgery was definitively shown to be from increased intestinal oxalate absorption in the early 1980s . in this study , six patients were placed on an oxalate diet , a low - oxalate diet , and total parenteral nutrition .
urinary oxalate on a low - oxalate diet decreased by half , while on total parenteral nutrition , a fourfold decrease in urinary oxalate was found .
a complementary study revealed the importance of high oral calcium intake in binding luminal oxalate .
the average urinary oxalate doubled when dietary calcium was reduced from 3,000 to 250  mg / day .
these studies clearly show that dietary or medical intervention can mitigate some of the effects of bariatric surgery . in an attempt to further define the effects of aggressive medical management on urinary stone risk profile in patients undergoing roux - en - y gastric bypass , we retrospectively identified and compared hyperoxalurics with and without a history of bariatric surgery .
the most notable finding of this study was that following bariatric surgery , patients had a surprisingly low supersaturation of calcium oxalate of 6.7 .
in fact , the supersaturation of calcium oxalate in our bariatric patients is similar to that found in a previous large cohort study of normal , overweight , and obese stone formers .
the obese stone formers had a supersaturation of calcium oxalate ( by the equil equation ) of 6.14 ( men ) and 5.97 ( women ) .
therefore , by comparison , our obese patients after bariatric surgery had a supersaturation of calcium oxalate , which was only 10% higher .
this finding is in contradiction to a study in the early 1980s that looked at 24-h urine collections in obese patients before and both 2 and 5  years after jejunoileal bypass surgery .
following surgery , urinary calcium decreased from baseline , and oxalate excretion was threefold higher , despite calcium supplementation .
however , in this study , the relative ss product ( by robertson s equation ) increased by a full 40% following surgery . however , the low supersaturation of calcium oxalate in our bariatric patients with hyperoxaluria is even more surprising given recent data published by asplin .
this found a ss caox in gastric bypass and jejunoileal bypass patients that was nearly twice as high as our value of 6.7 .
although our patients had higher urinary oxalate ( 130 vs. 100  mg / day ) , higher urinary calcium ( 160 vs. 140  mg / day ) , and lower citrate ( 390 vs.~400  mg / day ) , our patients urinary volume was significantly greater ( 2.9 vs. 1.8  l ) .
this may be the key difference between our bariatric stone formers and those of asplin .
thus , many of the recommendations that are applicable to the stone - forming public in general , i.e. , increased fluid intake , can have a significant impact on stone - forming risk even in this highly selected population .
bariatric surgery can have profound effects on urinary citrate levels due to chronic metabolic acidosis .
a study in jejunoileal bypass patients found a mean urinary citrate of 22  mg / day .
these patients subsequently underwent bypass reversal with an increase in urinary citrate to 230  mg / day .
citrate supplementation was initiated , and urinary levels rose further to 550  mg / day .
in contrast , we did not see a significant degree of hypocitraturia in our studied bypass patients .
this was likely influenced by the fact that all of the bariatric surgery patients were taking potassium citrate at the time of 24-h urine collection .
however , urinary citrate levels were still significantly lower in the bariatric patients than non - bariatric patients , despite their higher rate of citrate supplementation .
additionally , compliance is supported by the mean urinary potassium level ( 100  mg / day ) since for every one meq of potassium intake ; approximately one meq of potassium appears in the urine .
the influence of inhibitors of urinary stone formation also has been studied in weight reduction surgery patients .
high oxalate levels in the intestine bind not only calcium , but also magnesium ( a stone inhibitor ) .
high calcium oxalate stone indices following jejunoileal bypass have been attributed to both to high urinary oxalate and low urinary magnesium , and 80% of our bariatric patients were on oral calcium , which likely lowered both oxalate absorption and oxalate available for magnesium binding .
therefore , it is not surprising that our study found no difference in magnesium levels between hyperoxaluric patients with or without a history of bariatric surgery .
our study included a large number of patients with hyperoxaluria , but was limited by its retrospective nature and small size of specific patients with a history of bariatric surgery . since most patients were seen after evaluation and treatment by an outside urologist , we were not able to examine an individual patient s response to any given therapy , i.e. , a change in 24-h urine over time .
furthermore , although we were able to determine from chart review that all of the bypass procedures had taken place over a year prior to 24-h urine collection , the exact dates of surgery could not be determined . additionally , the comparison groups differed in distribution of gender , as more of the bariatric patients were women .
men in general have higher urine ss caox , which could have brought the results closer to the null hypothesis .
while it is reassuring that none of our bariatric patients had renal insufficiency to suggest oxalate nephropathy , a recent study reported on two patients ( 8.7% of the study cohort ) with renal failure presumed to be due to hyperoxaluria after roux - en - y gastric bypass .
future studies could focus on 24-h urine collections in patients before and after bariatric surgery .
this kind of internal control could factor in each patient s inherent stone - forming risk , with the additional risks of obesity and subsequent bariatric surgery and weight loss .
this could be performed in a prospective fashion in coordination with a bariatric surgery center .
these patients undergo extensive evaluations with sometimes multiple follow - up visits prior to surgery , and the opportunity likely exists for enrollment in a clinical trial . because of the dramatic increase in the numbers of these procedures , as well as the potential difficulties in performing stone operations in these patients , we need to know significantly more about the exact risk and nature of stone formation in these patients .
despite the anticipated significantly higher oxalate levels , compared with other hyperoxaluric patients , modern bariatric surgery patients had a dramatically lower supersaturation of calcium oxalate , predominantly due to higher urinary volume and lower urinary calcium excretion .
appropriate medical management , in particular oral calcium supplementation and aggressive fluid intake may mitigate some of the effects of enteric hyperoxaluria after modern bariatric surgery .
 introductiontwo recent studies have shown that modern bariatric surgery leads to significant hyperoxaluria and risk of nephrolithiasis . however 
 , neither report evaluates the use or effects of stone risk modifying agents in these patients . 
 we sought to determine the impact of medical management on stone risk profile in patients who have undergone roux - en - y gastric bypass.materials and methodstwenty - four - hour urine collections of all patients referred to a tertiary clinic for nephrolithiasis in the past 4  years were reviewed . 
 those patients with severe ( > 75  mg / day ) hyperoxaluria were identified . 
 retrospective chart review was performed to identify those patients with a history of bariatric surgery . 
 student s t - test was used to compare mean urinary parameters between bariatric and non - bariatric patients.resultsout of all stone formers within our 24-h urine collection database , 39 patients had severe hyperoxaluria ( oxalate > 75  mg / day ) . 
 twenty - six patients had complete information for review . 
 five patients had a history of bariatric surgery . compared with non - bariatric patients , those with a history of bariatric surgery 
 had increased use of oral calcium ( 80 vs. 28% ) and citrate supplementation ( 100 vs. 47% ) , higher urinary oxalate ( 129 vs. 91  mg / day ) and volume ( 2.9 vs. 2.4  l / day ) , lower urinary citrate ( 390 vs. 800  mg / day ) and calcium ( 155 vs. 235  mg / day ) , and a decreased supersaturation of calcium oxalate ( 6.7 vs. 11).conclusionsappropriate medical management , in particular oral calcium and citrate supplementation , and perhaps most importantly aggressive fluid intake can mitigate some of the effects of enteric hyperoxaluria caused by fat malabsorption after modern bariatric surgery . 
Introduction
Materials and methods
Results
Discussion
Conclusions