Hyperoxaluria is one of the well-established risk factors for stone formation; this certainly makes sense as the most common type of kidney stone is of the calcium oxalate variety. Interestingly, having high levels of serum oxalate does not appear to have as strong an effect as hyperoxaluria on the rate of stone formation in epidemiologic studies. However, a low oxalate diet may still be recommended to stone formers with hyperoxaluria, defined as greater than roughly 45 mg oxalate/day. The classic oxalate-rich food is spinach, though oxalate is also found in high abundance in most nuts (e.g., peanuts, almonds, cashews), baked potatoes, rhubarb, beets, and oranges.
Hyperoxaluria may occur via several scenarios. Various mutations in genes necessary for oxalate metabolism can cause primary hyperoxaluria, in which there may be recurrent stone formation at a pediatric age which may lead to end-stage renal disease. GI malabsorptive syndromes (e.g., chronic diarrhea, gastric bypass surgeries) also commonly lead to hyperoxaluria. One hypothesis states that individuals likely to form oxalate stones due to hyperoxaluria have a relative deficiency of the gut bacterium Oxalobacter formigenes, part of the normal GI flora which naturally digest oxalate.