Stone metabolic clinic · KIMS Secunderabad
Hyperoxaluria — When Excess Oxalate Destroys the Kidneys
Oxalate is a metabolic end-product produced by the liver and absorbed from the diet. In healthy physiology, urinary oxalate excretion is below 40 mg/day (0.44 mmol/day). When urinary oxalate rises above this threshold — hyperoxaluria — calcium oxalate crystals precipitate in the renal tubules and collecting system, causing recurrent calcium oxalate kidney stones, nephrocalcinosis (calcium oxalate deposits throughout the renal parenchyma), tubular damage, and in severe cases, progressive oxalate nephropathy leading to end-stage renal disease.
Hyperoxaluria is classified by its primary mechanism: primary hyperoxaluria (an inborn error of metabolism causing massively elevated endogenous oxalate production), enteric hyperoxaluria (increased oxalate absorption from the gut in malabsorption conditions — particularly after bariatric surgery, Crohn's disease, or short bowel syndrome — the most common type in India), and dietary hyperoxaluria (excessive intake of oxalate-rich foods — spinach, nuts, tea, chocolate — usually contributing to borderline hyperoxaluria rather than severe oxalate nephropathy in isolation).
Three types of hyperoxaluria — mechanism and severity
Primary hyperoxaluria (PH1, PH2, PH3)
Mechanism: Inborn error of glyoxylate metabolism — genetic mutation causes the liver to produce excess oxalate from glyoxylate. Severe, begins in childhood. PH1: above 100 mg/day (often above 200 mg/day) urinary oxalate. Recurrent calcium oxalate stones from early childhood. Nephrocalcinosis. ESRD before age 20–30 in severe PH1. Oxalate deposition in bone, heart, eyes, and blood vessels (systemic oxalosis). Treatment: B6 (pyridoxine) for PH1-responsive cases · High fluid intake · Oxalate-restricted diet · Combined liver-kidney transplant (the only cure — the liver harbours the enzyme defect) · Lumasiran (RNA interference therapy — approved for PH1, dramatically reduces hepatic oxalate production).
Enteric hyperoxaluria
Mechanism: Fat malabsorption in the small bowel — unabsorbed bile salts and fatty acids bind calcium in the gut lumen, leaving oxalate free to be absorbed in the colon. Moderate to severe. Urinary oxalate 30–100 mg/day. After Roux-en-Y gastric bypass, ileal resection, Crohn's disease, chronic pancreatitis. Dramatic increase in urinary oxalate post-bariatric surgery. Recurrent CaOx stones. Nephrocalcinosis and CKD in severe cases. Treatment: low-oxalate, low-fat diet · Oral calcium supplements with meals (bind oxalate in the gut) · Oral magnesium · Treat underlying malabsorption.
Dietary hyperoxaluria
Mechanism: Excessive intake of oxalate-rich foods (spinach, almonds, cashews, peanuts, tea, chocolate, beets). Mild. Urinary oxalate 40–80 mg/day. The most common 'hyperoxaluria' — usually contributes to stone risk as a factor alongside other metabolic abnormalities. Rarely causes nephrocalcinosis alone. Treatment: dietary oxalate restriction · Increased calcium intake with meals (calcium binds oxalate in the gut) · Adequate fluid intake.
Dietary oxalate content — the most common patient question
Categorising common foods by oxalate content:
Very high oxalate — avoid or strictly limit: Spinach, rhubarb, beets, Swiss chard · Chocolate and cocoa powder · Wheat bran · Nuts in very large quantities · Star fruit — extremely high, can cause acute oxalate nephropathy in CKD.
High oxalate — limit: Almonds, cashews, peanuts · Tea (black) · Soy products · Sweet potatoes · Berries (moderate).
Low oxalate — safe: Cauliflower, broccoli, cabbage · Rice, pasta, white bread · Meat, fish, eggs · Dairy products — protective (calcium binds oxalate) · Most fruits (moderate to low oxalate — safe in moderation).
Star fruit (carambola) contains very high oxalate concentrations and has caused acute oxalate nephropathy — severe AKI from massive acute oxalate precipitation in the renal tubules — in patients with CKD who consume it. Star fruit should be avoided by all patients with CKD, recurrent kidney stones, or any degree of renal impairment.
Diagnosis at KIMS
24-hour urine oxalate
The definitive measurement. Above 40 mg/day (0.44 mmol/day) = hyperoxaluria. Severity and type guide the investigation.
Plasma oxalate
Elevated in primary hyperoxaluria and severe enteric hyperoxaluria. Provides a more accurate assessment of total body oxalate burden than urine alone.
Genetic testing for AGXT, GRHPR, HOGA1
For suspected primary hyperoxaluria. PH1 (AGXT mutation) accounts for 70 to 80% of primary hyperoxaluria.
Liver biopsy with enzyme assay
Historically used to confirm PH1 (AGXT deficiency). Increasingly replaced by genetic testing.
Renal ultrasound and CT KUB
Nephrocalcinosis, stones.
Malabsorption evaluation
For suspected enteric hyperoxaluria: fat malabsorption assessment, small bowel imaging, history of GI surgery.
Primary hyperoxaluria — the transplant dilemma
Primary hyperoxaluria Type 1 (PH1) is unique among hereditary kidney diseases: the enzyme defect is in the liver (not the kidney), yet the kidney suffers the damage. Kidney transplant alone does not cure PH1 — the transplanted kidney is exposed to the same hepatic overproduction of oxalate and will develop oxalate nephropathy and fail within years. The definitive treatment for PH1 is combined liver-kidney transplant (replacing the enzyme-deficient liver with a normal liver and replacing the damaged kidney simultaneously). At KIMS, PH1 patients with ESRD are evaluated for combined liver-kidney transplant in collaboration with the liver transplant team.
Lumasiran (Oxlumo) — an RNA interference (RNAi) therapy approved for primary hyperoxaluria Type 1 — silences the hepatic LDHA gene that produces the glyoxylate substrate, dramatically reducing hepatic oxalate synthesis. Clinical trials show up to 65% reduction in urinary oxalate with monthly subcutaneous injection. This has transformed the outlook for PH1 patients and may prevent ESRD if started early enough. At KIMS, PH1 patients are assessed for eligibility for lumasiran therapy.
Book a Hyperoxaluria and Metabolic Stone Assessment at KIMS
Frequently Asked Questions — Hyperoxaluria
In people with a genetic predisposition to stone formation, regular high-intake of spinach — which has the highest oxalate content of any common food — significantly elevates urinary oxalate and promotes calcium oxalate stone formation. One cup of boiled spinach contains approximately 750 to 800mg of oxalate — 20 times the oxalate in the same volume of broccoli. In patients who have already had a calcium oxalate stone or who have been found to have hyperoxaluria on metabolic evaluation, spinach, rhubarb, and beets should be significantly restricted or avoided. However, dietary oxalate alone (without other metabolic risk factors) rarely causes severe hyperoxaluria — total dietary context, calcium intake, and fluid intake all matter.
Yes — high fluid intake is the single most important universal intervention for all kidney stone types, including calcium oxalate stones. The goal is a urine output above 2 to 2.5 litres per day — achieved by drinking 2.5 to 3 litres of fluid daily (more in hot weather and with physical activity). High urine volume dilutes oxalate, calcium, and other crystallisation promoters — reducing the supersaturation that drives crystal formation. In hyperoxaluria specifically, high urine flow also dilutes the oxalate concentration in the tubular lumen, reducing the probability of calcium oxalate precipitation in the renal parenchyma.
This is one of the most important misconceptions in stone medicine. Low dietary calcium intake does not reduce calcium oxalate stone risk — it increases it. When dietary calcium is restricted, the calcium that normally binds oxalate in the gut lumen (forming insoluble calcium oxalate that is excreted in the stool) is no longer available. Oxalate is instead absorbed freely from the gut and appears in the urine — dramatically increasing urinary oxalate. Studies consistently show that a normal dietary calcium intake (800 to 1,000mg/day from food sources, not supplements) reduces recurrent stone risk in calcium oxalate stone formers. Calcium supplements taken between meals (rather than with meals) may increase urinary calcium without the beneficial oxalate-binding effect and should be avoided in stone formers.
Yes — enteric hyperoxaluria after Roux-en-Y gastric bypass (RYGB) is a well-documented cause of dramatically increased calcium oxalate stone risk. The mechanism: fat malabsorption after RYGB causes unabsorbed fatty acids and bile salts in the colon to bind calcium, leaving oxalate free to be absorbed — oxalate absorption increases by 2 to 3 times, and urinary oxalate can rise to 60 to 100mg/day. Stone formation and nephrocalcinosis occur in 7 to 10% of RYGB patients. Prevention: low-oxalate, low-fat diet after bariatric surgery; oral calcium citrate with meals (binds oxalate in the gut); high fluid intake. Patients planning bariatric surgery should have a pre-operative metabolic stone risk assessment at KIMS.
KIMS Secunderabad — Dr. V. S. Reddy (Senior Consultant Nephrologist, transplant programme), 24-hour urine oxalate measurement, plasma oxalate, genetic testing for PH1/PH2/PH3, metabolic stone clinic with dietary counselling, lumasiran assessment for PH1, combined liver-kidney transplant for PH1 ESRD. NABH and NABL accredited. Call 040-4488-5000.