New study shows soy products may be harmful to some
WEDNESDAY, Sept. 26, 2001 (HealthDayNews) --Got kidney stones? Don't drink soy milk or eat tofu, soy energy bars or just about anything else containing the otherwise healthful soybean. New research shows soy-based products could increase the risk of developing this painful urinary tract condition.
The culprit is oxalate, a compound in plants that recently was discovered to be abundant in the soybean.
"We have long known that legumes are high in oxalate and that oxalate can increase the risk of kidney stones. Now we know that soybeans, a legume, and foods made from soy are also high in oxalate, with levels far above what is recommended for those who suffer with kidney stones," says study author Linda K. Massey, professor of human nutrition at University of Washington at Spokane.
Indeed, while the American Dietetic Association (ADA) suggests that patients with calcium oxalate kidney stones limit oxalate levels to no more than 10 milligrams per serving, Massey found soy products can have more than 50 times that amount. Texturized soy protein alone weighs in at a whopping 638 milligrams of oxalate per average serving of almost 3 ounces. Soy cheese, with 16 mg of oxalate per serving, still is considered high.
"Under the current guidelines, there isn't a single soy-based food that would be appropriate for people who suffer with any condition for which oxalate represents a problem," says Massey.
Other high-oxalate foods include legumes like refried beans, lentils and peanuts, each containing between 100 and 200 milligrams of oxalate per serving. The only food other than soy that far exceeds the suggested ADA oxalate levels is spinach, with 548 milligrams per 2-ounce serving.
For most folks oxalate, which has no nutritive value of its own, is not a problem. Massey says healthy people usually pass it from their body in urine with no consequence.
For some, however, oxalate combines in the body with calcium to form the hard mass of a kidney stone that can painfully block the urinary system. The stones sometimes pass on their own, but many times require surgery to provide relief.
While no one is certain why some of us get kidney stones while others do not, for those who do, "Oxalate can be a factor that increases that risk," says Massey.
Other nutritionist agree but say the cautions about soy apply only to those who are oxalate sensitive and not the general population.
"If you do not have this problem, don't be afraid to eat soy. It is a healthy, non-animal source of protein that is low in fat and rich in fiber, and it is a healthy food for most people," says Barrie Wolfe, a dietician and nutritionist at New York University Medical Center.
In addition to affecting stone formation, other research has shown that high oxalate foods can irritate a chronic gynecological condition called vulvar vestibulitis, a problem marked by extreme vulvar pain during intercourse, and sometimes even when walking or sitting.
Massey's discovery about soy products also may be significant for women just before menopause, when the risk of vulvar pain increases and when many women turn to soy-based products for hormonal support.
"We plan to study isoflavones, a component of soy often found in many menopause-related products, to check oxalate levels here as well," says Massey.
Massey's study analyzed and measured oxalate in a dozen varieties of soybeans and 13 different types of soy-based foods, including tofu, soy cheese and soy drinks.
Eleven of the soybean varieties contained significantly high levels of oxalate, as did all the soy-based food products.
In addition, the study showed that converting soy beans into various food forms does not alter oxalate levels. "They remain high regardless of cooking or other processing," says Massey.
The age of the soy products also didn't seem to matter. Levels in texturized soy protein, for example, were nearly identical in batches purchased nearly three years apart.
The study appears in the September issue of the Journal of Agricultural and Food Chemistry, a publication of The American Chemical Society.
What To Do
For Massey the message is clear: "If you suffer from any oxalate related condition, don't eat soy-based foods."
For Wolfe, the take-home message is that variety is not only the spice of life, it's also the basis of a healthy diet.
"Balance and variety of all food groups is key. You don't want to have an excess of any one food in your diet, no matter how healthy a food it is, and you don't want to focus on only one food, even a food as healthy as soy," says Wolfe.
For information on the health benefits of soy, click here.