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New research indicates that soybeans and
soy-based foods, a staple in the diets of many health-conscious
consumers, may promote kidney stones in those prone to the
painful condition.
The 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 Linda Massey, Ph.D., 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, with some foods reaching up to 50 times higher
than the suggested limit, she noted.
"Under these guidelines, no soybean
or soy-[based] food tested could be recommended for consumption
by patients with a personal history of kidney stones,"
she said.
No one had 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, Massey said.
During their testing, the researchers
found the highest oxalate
levels in textured soy protein, which contains
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 can be ground into flour
and used in a variety of grain products, or formed into chunks
and ground like meat.
Soy is also being studied for its potential
to lower cholesterol, reduce bone loss and prevent breast
cancer. The U.S. Food and Drug Administration recently approved
a new label on foods containing at least 6.25 grams of soy
protein per serving that boasts of a reduced risk of cardiovascular
disease.
Oxalate, however, cannot be metabolized
by the body and is excreted only through urine, Massey said.
The compound has no nutritional value, but binds to calcium
to form a mass (kidney stones) that can block the urinary
system, she said.
Further research is needed to find types
of soybeans with less oxalate, or to develop a processing
method to remove the compound before it reaches consumers,
she added.
No one knows precisely why kidney stones
occur in particular individuals.
But Massey said high
levels of oxalate in the urine increase the risk and those
with a family history of the ailment are more likely
to suffer from the condition; individuals with a low probability
of kidney stones are unlikely to be affected by oxalate in
soy-based foods.
More than one million people were diagnosed
with kidney stones in the United States in 1996, the most
recent available data, according to the National Institutes
of Health.
Stones can range in size from the diameter
of a grain of rice to the width of a golf ball. An estimated
10 percent of the U.S. population, mostly men, will develop
a kidney stone at some point in their lives, according to
the NIH.
Journal
of Agricultural and Food Chemistry September 2001
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