The primary goals of this study are to determine the contribution of dietary oxalate absorption, renal oxalate handling, and endogenous oxalate synthesis to urinary oxalate excretion in normal Body Mass Index (BMI) and obese calcium oxalate kidney stone formers.

Inclusion Criteria:

• Normal Body Mass Index (≥ 18.5 to < 25); Obese Body Mass Index (≥ 30 to ≤ 45)
• Able to provide informed consent
• Willingness to consume controlled diet
• Composition of most recent stone > 50% calcium oxalate, no uric acid component
• First time or recurrent calcium oxalate stone former
• 24-hour urine collections with creatinine values within 20% of appropriate ratio of creatinine (mg)/body weight (kg) for gender, and with creatinine values that are consistent between collections (within 20% of each other)
• Willingness to stop supplements [vitamins, Ca (citrate or carbonate) and other minerals, herbal supplements, nutritional aids, probiotics] for 14 days before start and during study
• Willingness to not undertake vigorous exercise during the controlled dietary study
• Normal fasting blood Comprehensive Metabolic Panel
• Hemoglobin A1c < 6.5%
• No food allergies or intolerance to any of the food in study menus
• Permitted/willing to discontinue medications for kidney stone prevention, including thiazides, allopurinol, and febuxostat and citrate preparations, for 14 days before start and during the studies. They will be instructed to restart these medications at the conclusion of the study

Exclusion Criteria:

• Diabetes
• Abnormal fasting comprehensive metabolic panel (CMP)
• Hemoglobin A1c (HbA1c) result ≥ 6.5%
• Gout
• Estimated Glomerular Filtration Rate (eGFR) <60 ml/min/1.73m^2
• Primary hyperoxaluria
• Cystic fibrosis
• Cystinuria
• Uric acid stone former
• Utilization of immunosuppressive medication
• Nephrotic syndrome
• Enteric hyperoxaluria
• Gastrointestinal disorder that could impact oxalate transport
• Sarcoidosis
• Uncontrolled hypertension
• Renal tubular acidosis
• Primary hyperparathyroidism
• Liver disease
• Neurogenic bladder
• Urinary diversion
• Chronic diarrhea
• Bariatric surgery
• Active malignancy or treatment for malignancy within 12 months prior to screening
• Pregnancy
• Breast feeding/nursing
• Females of child bearing age who are not able to use an effective method of birth control during the study
• Mental/medical condition that is likely to impede successful study completion
• Illness including flu / common cold / fever 14 days before study and during study
• Diarrhea or other abnormal gastrointestinal event (e.g. abnormal bowel movements) 14 days before study or during study
• Antibiotic use within last 6 months (based on recommendations of the NIH Human Microbiome Project, Protocol A)
• Inability or unwillingness to undergo MRI