How soy works in the body by Julie Deardorff

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How soy works in the body



December 17, 2006

Here's a look at soy's role in various health conditions.


Soy and cardiovascular health

The American Heart Association endorsed soy as a way to reduce "bad" cholesterol, or lipid levels, in 2000, a year after the Food and Drug Administration approved the health claim. But after reviewing more studies in the last several years, it reversed its position.

Bottom line: Soy isoflavones, plantlike compounds that have a chemical structure similar to estrogen, can lower lipid levels but not by as much as was once thought.

The heart association says that "taking soy or isoflavone supplements is unlikely to reduce your risk of heart disease."

But don't rule it out. Eating foods that contain soy protein to replace food high in animal fats may prove beneficial to blood cholesterol levels.



Soy and breast cancer

Asian women have a lower incidence of Breast Cancer compared with Western Caucasian women, until they immigrate to Europe or North America. Some believe it's because Asians are protected by soy's isoflavones.

The isoflavones bind to both estrogen receptors and exert estrogenlike effects in some experimental conditions. They also possess non-hormonal properties that are associated with decreased Breast Cancer risk. The concern is that isoflavones, especially genistein, may stimulate the growth of estrogen-sensitive tumors.

Bottom line: Both epidemiological evidence and animal studies show that consuming soy foods during adolescence can reduce breast-cancer risk in adulthood.

"The Breast Cancer issues remain complicated; it may protect from breast cancer but only if exposure takes place early in life, maybe even in utero," said Christopher Gardner, an assistant professor at the Stanford Prevention Research Center. "It doesn't appear to confer risk similar to the increased risk that estrogen- or hormone-replacement therapy does for certain types of breast cancer."

More research about the impact of isoflavones on breast tissue in high-risk women is needed.



Soy and bone health

At menopause, the ovaries produce less estrogen, so estrogen levels in the blood and tissues decrease, leading to a loss of bone. Some research has shown that isoflavones, with estrogenlike activity, may counteract post-menopausal bone loss or even build bone density.

Bottom line: The benefits might be life-stage specific and depend on the number of estrogen receptors you have; that means perimenopausal and early menopausal women may be more receptive to the therapeutic effects. The degree of protection and the best time to take isoflavones aren't yet known.



Soy and menopause

Menopause, with its telltale hot flashes, night sweats and vaginal dryness, is associated with decreased estrogen production and an increased risk of cardiovascular disease. So when women learned about the dangers of hormone-replacement therapy--higher rates of heart attack, stroke, breast cancer and deep-vein thrombosis--they turned hopefully to soy.

Bottom line: Soy protein isolate with isoflavones just might offer premenopausal benefit. The problem is that the timing for intervention remains uncertain, according to Jay Kaplan, director of the Department of Comparative Medicine at Wake Forest University in Winston-Salem, N.C., who is studying soy, stress and cardiovascular disease in women.

Also, although hot flashes are the easiest symptom to measure, clinical trials have been of poor quality and shown mixed results. Though soy has been shown to reduce the frequency of weekly hot flashes by as much as 40 percent, the effect could be a placebo effect.



Soy and thyroid

Isoflavones have hormonal, or estrogenlike, effects but also non-hormonal properties, meaning they can affect factors that control cell growth and other characteristics. Concerns have been raised over whether soy adversely affects thyroid function and can interfere with the absorption of synthetic thyroid hormone.

Bottom line: There is little evidence that soy foods or isoflavones adversely affect thyroid function in those with normal thyroid functions who get an adequate amount of iodine, which is found in fortified table salt .



Soy and the brain

In one of the more hopeful areas of research, soy might have positive effects on cognition because in some conditions isoflavones and estrogens have been shown to be beneficial for certain brain functions, according to Pauline Maki, associate professor of psychiatry and psychology at the Center for Cognitive Medicine at the University of Illinois at Chicago.

"In women under age 65, small studies uniformly show soy aids frontal-lobe functions, which include the ability to plan, shift mentally and to be flexible in they way we're thinking," Maki said. "We need larger studies to more firmly support this potential benefit."

Bottom line: Although a recent study in the Journal of the American Medical Association found no benefit in cognition in women in their late 60s, Maki said the time at which a woman takes soy might be the critical factor.

"For a young, healthy brain, soy could be beneficial," Maki said. "We don't exactly know why, but in the estrogen world, there might be a similar effect with cardiovascular health: There may be a period in which if a woman initiates estrogen, her heart is protected against cardiovascular disease. But if she's older, it doesn't."

This early benefit could also apply to estrogen and cognitive function, Maki said.

Unfortunately for men, there is little evidence that the potential benefits of soy isoflavones apply to them.





Soy formula for children

Soy-based formula, which is different from soy milk, often is used as an alternative for infants who have trouble breaking down the protein found in cow's milk or by vegan or vegetarian parents. Concerns have been raised over whether the isoflavone called genistein can lead to breast enlargement in baby girls and adult infertility.

Bottom line: It hasn't been adequately studied, but no such effects have been reported after more than 40 years of soy formula use in the U.S., according to a 2006 draft report by the Center for the Evaluation of Risks to Human Reproduction. The federal National Toxicology Program says there is "minimal concern for adverse effects" in neonates and infants who consume up to 8 milligrams a day of total genistein in soy formula.

It's also unclear whether soy can help prevent allergy or food intolerance in children who show no clinical signs of problems.

Soy formula is not recommended as a prevention strategy to combat food allergies or intolerances in high-risk infants, according to a 2006 review of studies by the international Cochrane Collaboration, an international, independent group that reviews evidence-based health-care interventions.

On the other hand, Duke University researchers have found that prenatal genistein can reduce obesity in animal studies.

Copyright © 2006, Chicago Tribune


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