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BACKGROUND: Observational studies in obese adults have shown abnormal urinary metabolic indices that predispose to nephrolithiasis. Few studies of this nature have been performed in severely obese adolescents. The purpose of this study was to assess urinary stone risk factors in severely obese adolescents and in those undergoing two types of weight loss surgery.
METHODS: A prospective cross-sectional study was performed to assess urinary metabolic profiles in severely obese adolescents who have either not undergone any gastrointestinal surgery or who have undergone Roux-en-Y gastric bypass (RYGB) or vertical sleeve gastrectomy (VSG). Exclusion criteria included previous urologic surgery or a history of urinary stone disease. A family history of stone disease was elicited. 24-hour urine collections were performed at home and evaluated at a central laboratory. Established normal reference ranges for adults were used in the analysis. A linear regression analysis was performed assessing the relationship of the study group with each of the outcomes.
RESULTS: A total of 55 samples were analyzed from 14 non-operative, 10 RYGB, and 7 VSG patients. Demographic data are presented in Table 1. Median BMI was similar between the RYGB and VSG groups. Median urinary metabolic indices are presented in Table 2. The median 24 hour excretion of oxalate was markedly higher in the RYGB group . Calcium and uric acid indices as well as the median supersaturation (SS) of calcium oxalate, calcium phosphate, and uric acid were similar among all groups.
CONCLUSIONS: In severely obese adolescents after RYGB but not VSG surgery, an elevated excretion of oxalate in the urine may portend an increased risk for kidney stone formation. Larger longitudinal studies are needed to verify these findings and to determine the clinical risk of developing stone disease in these patient populations.
Table 1. Patient Demographics

Demographic	Non-operative	RYGB	VSG
Patients, N Male	14 6	10 0	7 1
Female	8	10	6
Samples, N	25	18	12
Age (years), Median	17.9	18.6	17.6
Weight (kg), Median	133	105	94
Current BMI (kg/m2), Median Pre-op BMI (kg/m2), Median	50 N/A	36 52	32 51
Time since surgery (years), Median	N/A	1.0	0.8
Family history of nephrolithiasis	6	2	2

Table 2. Comparison of urinary metabolic indices

Median values	Normal Range	Non-operative	RYGB group	VSG group	P-value1
Volume (liters)	0.5 - 4	1.31	1.49	0.95	0.74
pH	>5.8	6.2	6.2	6.1	0.87
Calcium (mg/day)	<200	124	84	120	0.32
SS Calcium oxalate	6 - 10	7.21	5.02	5.23	0.88
Oxalate (mg/day)	20 - 40	34	42	26	0.01
SS Calcium phosphate	0.5 - 2	0.82	0.64	1.07	0.61
SS Uric acid	0 - 1	0.75	0.49	0.83	0.76
Uric acid (g/day)	<0.75	0.61	0.57	0.40	0.24
Citrate (mg/day)	>450	590	646	687	0.94

¹ Results of a linear regression analysis between study groups