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New research has indicated that soybeans and soy-based foods, a staple in the diets of many health-conscious consumers, may promote kidney stones in patients with a history of the disease.

No one has previously examined soy foods for oxalate, thus the researchers are the first to identify oxalate in store-bought products like tofu, soy cheese and soy drinks. Other foods, such as spinach and rhubarb, also contain significant oxalate levels, but are not as widely consumed for their presumed health benefits.

Researchers measured nearly a dozen varieties of soybeans for oxalate, a compound that can bind with calcium in the kidney to form kidney stones. They also tested 13 types of soy-based foods, finding enough oxalate in each to potentially cause problems for people with a history of kidney stones, according to experts at Washington State University in Spokane.

The amount of oxalate in the commercial products easily eclipsed the American Dietetic Association's 10 milligram-per-serving recommendation for patients with kidney stones. Some foods even reached up to 50 times higher than the suggested limit.

During the tests, researchers found the highest oxalate levels in textured soy protein, which contained up to 638 milligrams of oxalate per 85-gram serving. Soy cheese had the lowest oxalate content, at 16 milligrams per serving. Spinach, measured during previous research, has approximately 543 milligrams per one-cup (2 oz. fresh) serving.

Soy, a natural source of protein, fiber and healthy oils, is used to enhance a myriad of foods, ranging from hamburgers to ice cream. It also can be ground into flour and used in a variety of grain products, or formed into chunks and ground like meat, according to the Journal of Agricultural and Food Chemistry (September 2001).