BACKGROUND AND OBJECTIVES: Bariatric surgery (BS) may be associated with 
increased oxalate excretion and a higher risk of nephrolithiasis. This study 
aimed to investigate urinary abnormalities and responses to an acute oxalate 
load as an indirect assessment of the intestinal absorption of oxalate in this 
population.
DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Twenty-four-hour urine specimens 
were collected from 61 patients a median of 48 months after BS (post-BS) as well 
as from 30 morbidly obese (MO) participants; dietary information was obtained 
through 24-hour food recalls. An oral oxalate load test (OLT), consisting of 
2-hour urine samples after overnight fasting and 2, 4, and 6 hours after 
consuming 375 mg of oxalate (spinach juice), was performed on 21 MO and 22 
post-BS patients 12 months after BS. Ten post-BS patients also underwent OLT 
before surgery (pre-BS).
RESULTS: There was a higher percentage of low urinary volume (<1.5 L/d) in 
post-BS versus MO (P<0.001). Hypocitraturia and hyperoxaluria (P=0.13 and 
P=0.36, respectively) were more frequent in BS versus MO patients. The OLT 
showed intragroup (P<0.001 for all periods versus baseline) and intergroup 
differences (P<0.001 for post-BS versus MO; P=0.03 for post-BS versus pre-BS). 
The total mean increment in oxaluria after 6 hours of load, expressed as area 
under the curve, was higher in both post-BS versus MO and in post-BS versus 
pre-BS participants (P<0.001 for both).
CONCLUSIONS: The mean oxaluric response to an oxalate load is markedly elevated 
in post-bariatric surgery patients, suggesting that increased intestinal 
absorption of dietary oxalate is a predisposing mechanism for enteric 
hyperoxaluria.