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, HDL \>30 mg/dL, and triglycerides \<250 mg/dL) will be included. Treated and controlled dyslipidemia is accepted * Smoking/chewing tobacco/vaping will be allowed Exclusion Criteria: * Renal colic or past stone surgery in the past 3 months * Estimated Glomerular Filtration Rate (eGFR) less than 60 ml/min/1.73m2 * History of type 2 diabetes or screening hgb A1c greater than or equal to 6.5% * Alcohol or drug abuse. Alcohol consumption \>12 units per week and/or unwillingness to abstain from alcohol consumption 2 days before and during the study. * Mental/medical condition likely to impede study completion * Current/planned pregnancy or breastfeeding in coming 6 months, unwillingness to use effective method of birth control during the study for individuals of childbearing potential. * Planned vacation in coming 4 months * Febrile illness including flu/common cold/COVID-19 within 21 days of study start * Primary or enteric hyperoxaluria * Gastrointestinal disorder or surgery that impacts oxalate transport * Cirrhosis, pancreatic or biliary disorder, porphyria, gout * Malignancy treated within the past 12 months * Nephrotic syndrome, neurogenic bladder, urinary diversion * History of primary hyperparathyroidism, cystinuria, renal tubular acidosis, sarcoidosis, cystic fibrosis * Uncontrolled hypertension, myocardial infarction in past 6 months, unstable angina, congestive heart failure * Medications: Topiramate, acetazolamide, zonisamide, diuretics, SGLT2-inhibitors, immunosuppressants