What causes calcium oxalate in urine?

Causes of Calcium Oxalate in Urine

Calcium oxalate appears in urine due to supersaturation from multiple dietary and metabolic factors, with high sodium intake, excessive dietary oxalate, low fluid intake, high animal protein consumption, vitamin C supplementation, and low urinary citrate being the primary modifiable causes. 1, 2

Primary Dietary Causes

High Sodium Intake

• Excessive sodium consumption is one of the most important dietary contributors to calcium oxalate crystalluria, as it reduces renal tubular calcium reabsorption and increases urinary calcium excretion 3, 4
• Urinary calcium increases by approximately 0.77 mg per mmol of sodium excreted, making sodium restriction to 2,300 mg daily a critical intervention 1, 4

Dietary Oxalate Sources

• Consumption of high-oxalate foods directly increases urinary oxalate excretion, particularly from spinach, rhubarb, beets, nuts, chocolate, tea, wheat bran, and strawberries 3, 5
• These eight foods are the only ones definitively shown to cause significant increases in urinary oxalate 5
• Patients with hyperoxaluria should specifically limit almonds, peanuts, cashews, walnuts, pecans, and rice bran 3, 2

Vitamin C Supplementation

• Vitamin C is metabolized to oxalate in the body, with 1,000 mg consumed twice daily increasing urinary oxalate excretion by 22% 3
• Supplementation of 1,000 mg or more per day increases stone formation risk by 40% compared to consuming less than 90 mg/day 3, 1
• Calcium stone formers with hyperoxaluria should discontinue vitamin C supplements 3, 2

Animal Protein Consumption

• Metabolism of sulfur-containing amino acids in animal flesh generates sulfuric acid, which increases urinary calcium excretion and reduces urinary citrate excretion 3
• This acid load also potentially increases calcitriol production, further promoting calcium excretion 3
• Limiting intake to 5-7 servings of meat, fish, or poultry per week is recommended 1

Metabolic and Physiologic Causes

Inadequate Fluid Intake

• Dehydration concentrates stone-forming substances in urine, creating supersaturation conditions that promote calcium oxalate crystallization 1, 6
• Fluid intake should achieve at least 2-2.5 liters of urine output daily to dilute stone-forming substances 1, 6

Low Urinary Citrate (Hypocitraturia)

• Citrate is a critical inhibitor of calcium oxalate stone formation that complexes with calcium and inhibits spontaneous nucleation of calcium oxalate crystals 7
• Low potassium intake stimulates tubular citrate reabsorption, decreasing urinary citrate excretion 3
• Potassium citrate supplementation increases urinary citrate by approximately 400 mg/day at a dosage of 60 mEq/day 7

Paradoxical Effect of Low Calcium Intake

• Restricting dietary calcium paradoxically increases urinary oxalate by reducing gastrointestinal binding of oxalate, leading to increased oxalate absorption 1, 2, 8
• Normal dietary calcium intake of 1,000-1,200 mg per day should be maintained 1, 6
• Calcium consumed with meals enhances gastrointestinal oxalate binding 1, 6

Low Dietary Potassium

• Potassium restriction increases urinary calcium excretion and decreases citrate excretion 3
• Potassium-containing foods like fruits and vegetables provide an alkali load that increases urinary citrate excretion 3

Underlying Pathophysiology

Crystal Formation Process

• Calcium oxalate crystallization occurs through a multistep process including supersaturation, nucleation, crystal growth and aggregation, and crystal retention after cellular adhesion 9
• High oxalate concentrations damage renal tubular cells and cause oxidative stress, supporting crystal precipitation and cellular adhesion 9

Primary Hyperoxaluria

• In primary hyperoxaluria type 1, finding >200 pure whewellite (calcium oxalate monohydrate) crystals per cubic millimeter is highly suggestive of the diagnosis, especially in young children 6
• These patients require more aggressive hydration (3.5-4 L/day in adults) and may benefit from pyridoxine therapy 2

Enteric Hyperoxaluria

• Patients with chronic diarrheal syndrome or intestinal disease have increased oxalate absorption, leading to elevated urinary oxalate 2, 7
• These patients may require more restrictive oxalate diets and higher calcium intake specifically timed with meals 2

Common Pitfalls to Avoid

• Never restrict dietary calcium, as this worsens oxalate absorption and increases stone risk 1, 6
• Avoid calcium supplements unless specifically indicated, as they increase stone risk compared to dietary calcium when not taken with meals 1
• Do not use sodium citrate instead of potassium citrate, as the sodium load increases urinary calcium excretion 1, 6
• Avoid recommending oxalate restriction to patients with pure uric acid stones or low urinary oxalate excretion, as this is inappropriate 1, 6