Effect of Weight Loss on Urinary Oxalate Excretion in Obese Calcium Oxalate Kidney Stone Formers

Inclusion Criteria: * First time or recurrent calcium oxalate stone formers defined as composition of most recent stone \>50% calcium oxalate without uric acid component * Last stone passage or removal greater than 3 months prior to screening * Body Mass Index (BMI) \>35 kg/m2 * Able to provide consent * Hemoglobin A1C \<6.5% * Normal fasting blood CMP: Normal serum Na, K, Cl, CO2, Calcium, Albumin, AST, ALT \<2 times upper limit of normal, Glucose \<126 mg/dl, eGFR \>60 ml/min/1.73 m2. * 24-hour urine collection with creatinine within 20% of gender appropriate (mg creat/kg body weight) and consistent between collections (within 20% of each other) * Willing to ingest menus prepared at the University of Alabama and University of Texas Southwestern Clinical Research Units * Willing to consume the Optifast VLCD diet * Willing to ingest menus prepared in Metabolic kitchen. No food allergies or intolerance to any of the foods in study menus. Substitutions for lactose intolerance will be made. * Willing to avoid vigorous exercise during eucaloric, low-oxalate dietary phase * Willing to stop use of weight-loss medications * Willing to stop for 14 days before and during study: medications for stone prevention including diuretics, potassium citrate, allopurinol, febuxostat, and calcium, nutritional, or herbal supplements * Those with controlled hypertension (systolic blood pressure \<160 mmHg, diastolic \<90 mmHg) will be included. * Those without severe dyslipidemia (LDL \<200 mg/dL…