Dietary oxalate and kidney stone formation

Abstract

Dietary oxalate is plant-derived and may be a component of vegetables, nuts, fruits, and grains. In normal individuals, approximately half of urinary oxalate is derived from the diet and half from endogenous synthesis. The amount of oxalate excreted in urine plays an important role in calcium oxalate stone formation. Large epidemiological cohort studies have demonstrated that urinary oxalate excretion is a continuous variable when indexed to stone risk. Thus, individuals with oxalate excretions >25 mg/day may benefit from a reduction of urinary oxalate output. The 24-h urine assessment may miss periods of transient surges in urinary oxalate excretion, which may promote stone growth and is a limitation of this analysis. In this review we describe the impact of dietary oxalate and its contribution to stone growth. To limit calcium oxalate stone growth, we advocate that patients maintain appropriate hydration, avoid oxalate-rich foods, and consume an adequate amount of calcium.

Keywords: calcium oxalate; crystalluria; kidney stones; oxalate.

Fig. 1.

Reduction of urinary oxalate excretion of 12 healthy subjects consuming a diet lacking oxalate. Relative amounts of urinary oxalate derived from intestinal absorption and endogenous synthesis are shown. Values are means (SD). [Data were calculated from Holmes et al. (21).]

Fig. 2.

Urinary oxalate excretion of healthy subjects on oxalate- and calcium-controlled diets: constellations of 39 individuals (19 male, 20 female, 22–43 yr old) from 4 previously published dietary-controlled studies in our laboratory (28, 32, 33, 35). Dotted line represents line of best fit for 1,000-mg calcium diets (■) and 50- to 750-mg dietary oxalate intake. Urinary oxalate excretion increases 2.7 mg for every 100 mg of dietary oxalate ingested. At 250 mg of oxalate intake, data points demonstrate urinary oxalate excretion at 400 mg of calcium in Oxalobacter formigenes-colonized (○) and -noncolonized (∆) individuals. □, Urinary oxalate excretion of individuals on an oxalate-free formula diet (21). Values are means (SD).