Kidney stone patients should avoid foods with high oxalate content, add calcium to meals.
Until recently there was little interest in food oxalate values, as the dominant paradigm was that dietary oxalate contributed only 10% of daily oxalate excretion. This changed when Ross P. Holmes, PhD, of Wake Forest University School of Medicine in Winston-Salem, N.C., and his colleagues reported that 24% to 53% of urinary oxalate originated from dietary oxalate at typical intakes of 10-250 mg/day (Kidney Int. 2001;59:270-276).
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Their results clearly indicated that dietary oxalate makes a much greater contribution to urinary oxalate than was previously recognized. Dr. Holmes and Dean G. Assimos, MD, of the same institution, reviewed evidence that the absorption and excretion of dietary oxalate can be an important factor in calcium oxalate kidney stone formation (Urol Res. 2004;32:311-316).
Although urinary oxalate concentration is only one tenth that of calcium, calcium oxalate in most human urine is near its saturation limit; therefore, even a small increase in oxalate concentration may increase risk of crystal precipitation. Drs. Holmes and Assimos concluded that to avoid the possibility of stone growth in calcium oxalate stone formers, the transient increase in urinary oxalate following an oxalate-rich or calcium-poor meal should be suppressed in patients.
Advice to reduce dietary oxalate intake requires knowledge of food oxalate values, but there are differences in published values for some foods. I recently reviewed the sources of variation (J Am Diet Assoc. 2007;107:1191-1194) and found that differences in oxalate values for a single food may be due to biological variation from several sources including cultivar (genetic variant less than a species), time of harvest and growing conditions, as well as analytical differences.
Although recent reports use reliable methods for analyzing oxalate extracted from foods, controversy continues with respect to the ex-traction method. Ruth Hönow, MD, and Albrecht Hesse, MD, showed that hot acid generated oxalate in cherry juice, primarily from ascorbate (Food Chem. 2002;78:511-521). Their analysis of extraction techniques showed that oxalate extraction from cherry juice with room temperature 2N hydrochloric acid was complete and without generation of new oxalate.
Even if the food oxalate value is known, the bioavailability of the food oxalate, and thus urine oxalate, also will be affected by several factors. The major one is the salt form of oxalate, with calcium oxalate being very poorly soluble. Soluble oxalate is potassium or sodium oxalate, which is absorbed better. Methods of processing and cooking that include water immersion will reduce oxalate.
The presence of calcium or magnesium in a meal will reduce oxalate absorption. Finally if the patient’s gut contains oxalate-degrading bacteria such as Oxalobacter formigenes there will be less oxalate to be absorbed and excreted in the urine.
