Identify individuals with greater absorption of oxalate based on increase in urinary oxalate excretion in response to a controlled oxalate-rich test meal.

This was a prospective 4-day study of oxalate absorption in subjects with secondary hyperoxaluria. Subjects visited the research center three times for outpatient visits (Screening, Test Day 3, Day 4).

Subjects were instructed to follow a low-oxalate diet, consuming only study-specified food choices that had low or no oxalate and controlled calcium content Days 1 through 4 (Baseline Period through the Day 4 clinic visit) and, on Test Day 3, consumed an oxalate-rich test meal.

Subjects performed three 24-hour urine collections during Screening, Baseline Day 2, and Test Day 3. Test Day 3 included the oxalate-rich test meal and the subject collected a 24-hour urine separated into 4 urine collection intervals. (Pre-test meal, post-test meal to 4 hours, 4-6 hours post test meal and remaining time to complete 24 hours).

Inclusion Criteria:

• Males or non-pregnant and non-lactating females
• History of hyperoxaluria secondary to a known underlying enteric disease (e.g., Crohn's disease, bariatric surgery, and others) or without a known underlying cause (idiopathic), and kidney stones.
• Urinary oxalate ≥ 40 mg of oxalate/24 hr at screening.
• If taking drugs for the prevention of stone disease, including pyridoxine, thiazides (chlorthalidone, hydrochlorothiazide, and indapamide), citrate supplements, and allopurinol, there must have been no changes in these medications for at least one month prior to screening and no changes are anticipated for the duration of the study.
• Able to understand and provide written informed consent.

Exclusion Criteria:

• Under or over-collection on screening 24-hour urine collection (mg creatinine/kg body weight outside of gender-specific range).
• Estimated glomerular filtration rate (eGFR) <40 mL/minute/1.73 m^2 or acute renal failure.
• Primary hyperoxaluria.
• Allergy, intolerance, or any other factors limiting ability to consume the study foods or adhere to the study meal schedule.
• Positive pregnancy test during Screening.
• Clinically significant findings (including acute renal colic), an ongoing clinically significant illness requiring changes in management, or any planned medical/surgical procedure during the study.
• Malignancy undergoing active therapy; patients in remission on chronic suppressive or maintenance therapies are not excluded.
• Investigational compound within 30 days prior to screening.
• Investigator determined that the subject would not be able to adhere or to complete the study procedures , or could not discontinue Vitamin C supplements, calcium supplements, or other medications that could interfere with oxalate absorption and/or excretion.

34 days (adverse events were recorded from the time of informed consent until the last study visit, and the screening period was approximately 30 days)

Since no investigational product was administered; no drug-related adverse events (AEs) occurred. The investigator assessed AEs by intensity (severity) and causality in relation to the study intervention, which was the study diet. Any untoward symptoms or worsening of medical conditions reported during the study were recorded as AEs.