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). Their results clearly indicated that diet-ary 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 dif-ferences 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 (gene-tic 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.
My colleague Susan Kynast-Gales, PhD, RD, and I compiled an online database with more than 1,200 published food oxalate values (http://www.spokane.wsu.edu/research%26service/HREC/FoodOxalateOverview.asp). Foods from diverse countries are included to allow application of the database by international health professionals and scientists. Values in the aggregate database are referenced to original source. The Excel spreadsheet may be printed or downloaded.
The American Dietetic Association Care Manual recommendation is to restrict dietary oxalate to less than 40 to 50 mg per day. Because patients do not like to consult a long list of foods or do not care to take the time needed to calculate actual intake in milligrams, the first step in dietary oxalate restriction should be to avoid foods highest in oxalate, such as spinach, rhubarb, beets (roots and leaves), black teas (not green or herbal), chocolate, some tree nuts, bran concentrates and cereals, and legumes (beans, peanuts, soybeans and some soyfoods). All plant foods contain some oxalate, but these foods have been shown to increase urinary oxalate after eating and have high amounts of oxalate. When these foods are avoided, oxalate content of the other low and moderate oxalate foods typically eaten will often only add up to the 40-50 mg daily dietary target.
Second, patients should add calcium to each meal to bind oxalate and prevent it from being absorbed. The total calcium intake for the day should be divided between as many eating occasions as possible. Patients should include about 150 mg calcium in each meal. This is the amount found in one half cup of milk, ice cream, yogurt, or pudding, or a three-quarter ounce slice of cheese. Studies looking at the addition of calcium carbonate support the use of calcium supplements with meals for patients who cannot or will not eat dairy products.
Finally, patient-friendly food oxalate tables have been recently updated from the aggregate database and can be found on the website of the Oxalosis and Hyperoxaluria Foundation (www.ohf.org/docs/Oxalate2004.htm).