Dietary influences on urinary oxalate and risk of kidney stones.

Calcium oxalate is the most common constituent of kidney stones. Increases in urinary oxalate increase risk of calcium oxalate supersaturation more than increases in urinary calcium, as the physiological level of oxalate is about one-fifth to one-tenth that of urinary calcium. Urinary oxalate derives from two sources: endogenous synthesis and diet. Endogenous synthesis is proportional to lean body mass, and cannot be altered by any current treatment. Dietary oxalate is found in all plant foods. A single food may vary 2-15 fold in oxalate content, depending on variety and growth conditions. The salt form of oxalate, whether sodium, potassium, calcium or magnesium is likely to affect absorption, but has been little studied. Absorption of oxalate from food sources typically is 3-8% of its total oxalate in non-stone-forming individuals. Recent research shows that 40-50% of urinary oxalate comes from the diet of healthy individuals consuming typical diets with 150-250 mg/d dietary oxalate. However, a subpopulation of oxalate "hyperabsorbers" is found in most studies of stoneforming patients. It is likely that all stone formers will benefit from reduction of dietary oxalate, but especially hyperoxaluric stone formers.