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The Science on Soy


Soy seems to have an equal number of promoters and detractors. Should we go out of our way to eat more soy or avoid it? One website touts it as a cure for hot flashes, while another warns it may bring on early puberty. One source says it’s good for your heart, and another says it’s bad for your thyroid.

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The naysayers may be prevailing among the public. A 2016 survey by the International Food Information Council Foundation revealed that, while 68 percent of participants said they were trying to eat more beans, nuts and seeds, 27 percent said they were trying to eat less soy (which is a bean) as a protein source.
 
Unlike some hot-button food trends, there is a lot of research on soy, allowing food and nutrition professionals to offer evidence-based answers to questions and concerns about soy-based foods and beverages, including soymilk, tofu, edamame and soy protein powder and fermented soy-based foods such as miso, tempeh and natto.
 
Focusing mainly on dietary soy (evidence for soy-based isoflavone supplements should be evaluated separately), here’s what the science says in answer to eight frequently asked questions:
 
Do soy foods contain estrogen?
Soybeans contain phytoestrogens, compounds that are similar in structure to the human hormone estrogen (estradiol). In vitro studies have shown that phytoestrogens can interact with estrogen receptors on the surface of human cells, giving them the potential to exert mild estrogenic effects. Phytoestrogens also may have anti-estrogenic effects by blocking the more active human estrogen from reaching the receptors.
 
Soy’s potential estrogen agonist and antagonist properties have led to questions about how eating soy might influence hormone levels in men, women and children. The answer: Evidence is mixed.
 
Does feeding soy to infants or children cause early puberty?
The average age of puberty onset has decreased in Western societies, and the popularity and consumption of soy has increased. Is there a connection? Evidence says probably not.
 
A case-control study found no difference in age of puberty between children who were given soy infant formula and kids who were breast-fed or given cow’s milk formula.
 
A cross-sectional analysis of pre- and post-pubescent girls found no relationship between reported soy consumption and reported age of menarche. A similar analysis of teenage boys found the reported age at puberty was an average of six months earlier for those who reported high soy consumption compared to those who consumed low amounts of soy.
 
However, a prospective study of pre-pubertal boys and girls found adding soy protein to the diet on a daily basis for a year did not affect sexual maturation or age of puberty.
 
Does soy consumption affect male fertility?
Among male patients at an infertility clinic, soy foods were associated with a reduction in sperm concentration but not motility, and was not associated with the success of fertility treatment. Soy isoflavone supplementation did not appear to affect sperm quality in healthy males. Limited evidence addresses this topic and more research is needed.
 
Does eating soy protect against breast cancer?
Epidemiological evidence has linked higher soy food consumption with reduced breast cancer risk. Women from countries such as Japan, where soy foods are central to the diet, experience much lower incidence of breast cancer than Western women who traditionally eat less soy. It appears that eating soy foods early in life may deliver the most protective benefit.
 
Is soy safe for those who have been diagnosed with breast cancer?
The potential estrogenic effects of soy have been a source of concern for those with hormone-sensitive cancers such as breast cancer. The concern is that even weak phytoestrogens in soy might promote the growth of cancer or its recurrence. However, a 10-year follow-up study of thousands of women with previous breast cancer diagnosis found that soy isoflavone intake from foods did not increase their risk of recurrence. To the contrary, it was associated with reduced all-cause mortality, although this association was most significant in women with hormone-negative cancers and those who did not receive hormone treatment.
 
Can soy relieve hot flashes?
Hot flashes and other menopausal symptoms caused by waning estrogen levels can be affected by many factors, including diet, body weight, exercise and stress. Estrogen replacement therapy often is prescribed to women experiencing hot flashes; could the phytoestrogens in soy foods be potent enough to reduce the hot flashes? Although a prospective study of Japanese women found that hot flashes were inversely associated with soy consumption, a review of 10 intervention studies found that incorporating more soy foods into the diet did not reliably reduce hot flashes and other symptoms of menopause.
 
Although evidence does not support recommending soy foods for preventing or alleviating hot flashes, soy can be recommended as a nutritious food.
 
How might soy affect thyroid function?
Some warn that soy may negatively affect thyroid function. As long a person’s iodine intake is adequate, this is unlikely to be a concern. A review of 14 trials found that soy and soy isoflavone from dietary sources and supplements had either no effect or minimal effect on thyroid function in healthy people with normal iodine status.
 
Soy foods may affect the absorption of synthetic thyroid hormone. This does not necessarily mean patients on thyroid replacement need to avoid soy. If their soy food consumption is reasonably consistent, their medication dose can be calibrated to accommodate typical eating patterns: for example, a slightly higher dose to compensate for any absorption-inhibiting effects of soy. Patients with thyroid conditions should discuss this with their health care provider.
 
Do soy foods lower cholesterol or reduce the risk of coronary heart disease (CHD)?
There are at least two potential mechanisms by which soy foods might reduce cholesterol and thereby heart disease risk. Soy contains phytosterols and stanols, plant compounds that have been shown to lower LDL cholesterol in a dose-dependent manner. (These fat-soluble compounds are concentrated in soybean oil.) Whole soybeans also contain soluble fiber, which has been shown to reduce LDL cholesterol.
 
Earlier studies suggested that replacing animal protein with soy protein foods reduced total and LDL cholesterol levels. In 1999, the Food and Drug Administration approved the claim that “soy protein included in a diet low in saturated fat and cholesterol may reduce the risk of CHD by lowering blood cholesterol levels.”
 
However, in 2017, the FDA proposed a rule to revoke this claim, citing “numerous studies published since the claim was authorized in 1999 have presented inconsistent findings on the relationship between soy protein and heart disease.”
 
But in 2017, the FDA also approved a qualified health claim that “supportive but inconclusive scientific evidence suggests that eating about 1½ tablespoons of soybean oil daily may reduce the risk of coronary heart disease.”

Soy foods may play a role in a heart-healthy diet, especially to the extent that they replace foods higher in saturated fat. However, soy does not appear to be a silver bullet against high cholesterol or heart disease. Read More
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