Myths About Soy

Recent research has proven the health benefits of soy, but misconceptions remain among many health professionals and the public. Learn the truth behind some common soy myths.

Myth: Large amounts of soy food must be consumed to get health benefits.

The U.S. Food and Drug Administration (FDA) recommends 25 g/day of soy protein as the threshold intake required for cholesterol reduction. This amounts to about three to four servings of soy foods since, on average, each serving of traditional soy foods (e.g., 1 cup of soy milk or 3 oz of tofu) contains roughly 6 to 8 grams of protein.

However, the soy protein recommendation applies only to reduction of heart disease risk and not other possible benefits of soy or isoflavones.

Using Asian diets for guidelines, particularly those in Japan and China, mean soy food consumption ranges dramatically, from as little as one to two servings of soy food per day to eight or more.^1,63—72

While the mean intake serves as a guideline, Asian epidemiologic studies still show that individuals eating above average amounts of soy have lower disease rates than those consuming the average or less.^{63, 73, 74}

In fact, clinical data suggests that 50 mg to 100 mg of isoflavones per day is necessary to derive health benefits. Two large, long-term osteoporosis trials funded by the U.S. government are studying the use of 80 and 120 mg/day of isoflavones.

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Myth: All soy foods are healthy.

Soy protein can be a healthy alternative to animal protein, but some foods contain more soy protein than others. Deep fried tofu may contain just as much fat as a lean cut of grilled meat. Soy sauce can be a good substitute for fatty condiments, but it should be a low-sodium version. As with all snack foods, remind your clients that soy ice cream and soy chips are a treat to be eaten in moderation.

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Myth: Soy foods contain estrogen compounds that may cause hormonal disturbances.

The isoflavones found in soy foods are sometimes called phytoestrogens (plant-estrogens) because they have a chemical structure similar to the hormone estrogen. Also, isoflavones can bind to estrogen receptors and exert some estrogen-like effects in cells.^{18, 19}

That said, isoflavones are actually quite different from estrogen. Estrogen-like effects are rarely observed in clinical studies measuring the ingestion of soy foods or isoflavones.^20-22

This is not surprising since receptor binding – a common in vitro measure of estrogenicity – is a poor predictor of in vivo activity.²³

Compounds that bind to estrogen receptors often have different, and sometimes opposite, physiological effects depending upon how the isoflavone and receptor interact within different cells.^24-26

Clinical studies do show that neither soy foods nor isoflavones affect serum levels of testosterone or estrogen.^27-39 In fact, several studies have found no effects on testosterone despite ingestion of isoflavones at levels that were 20- to 30-times higher than the typical Japanese intake.^{40, 41}

No hormone-related abnormalities, such as thelarche or precocious puberty, have been ascribed to soy formula use in infants.

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Myth: Fermented soy foods are better for you than nonfermented soy foods since traditional Asian cuisine uses fermented soy products.

It's true that many traditional Asian soy foods (miso, natto and tempeh) undergo fermentation while tofu and soy milk do not. It's also true that fermented and non-fermented soy foods have some nutritional differences.

In fermented soy foods, microbial hydrolysis causes a greater proportion of the isoflavones to occur as aglycones (without an attached sugar molecule), while the isoflavones of unprocessed soy beans appear as glycosides (with the sugar molecule attached).¹ However, due to in vivo hydrolysis there is little difference in overall bioavailability between isoflavone aglycones and glycosides.^{2, 3}

Fermentation reduces the phytate content of soy foods and, therefore improves mineral absorption somewhat, but the extent of this effect in unclear.^4-16 Some research suggests that the allergenicity of soy protein may be reduced by fermentation.¹⁷

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Myth: Soy foods cause thyroid problems.

A review of clinical data provides no evidence that soy foods or isoflavones adversely affect thyroid function.

The idea that soy and isoflavones exert anti-thyroid effects is based primarily on several cases of goiter found in infants ingesting soy formula in the early 1960s and more recent research in rodents.^{42, 43}

Due to those early cases of goiter, iodine was added to soy infant formula and the formulation of these products was changed in the mid-1960s, replacing soy flour with isolated soy protein.

Since that time, no cases of goiter have been reported among the millions of infants using soy formula.^{44, 45} Even in the rat, an animal sensitive to anti-thyroid effects,⁴⁶ normal thyroid function is maintained in response to the consumption of isoflavones.⁴²

A recent review of 14 clinical trials found that with only one exception (a study published in Japanese that was poorly designed⁴⁷), either no effects or only modest changes in healthy thyroid function were noted.⁴⁸ In the only trial that specifically examined isoflavone (90 mg/d) supplements in postmenopausal women, no changes were seen in levels of serum thyroid-stimulating hormone over a 6-month period.⁴⁹

Some evidence indicates that soy foods may reduce absorption of medication in hypothyroid patients.^50-53 Such an effect is common with many herbs, medications and even fiber supplements.^54-62 In any event, hypothyroid adults can still enjoy the heart healthy benefits of soy foods by working with their doctor to adjust their dosage if necessary.

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Myth: Tofu and soy milk are only for vegetarians.

Once considered a food for vegetarians, soy has broken the taste barrier and emerged onto the mainstream aisles in grocery stores. Although considered a health food, it's no longer hidden in the health food section. Your clients can now find soy products like soy milk in the dairy cooler next to dairy milk. All types of clients, including non-vegetarians, can enjoy the health benefits of soy.

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Myth: Soy foods are bland.

It's true that many early soy-based foods lacked flavor, tasted unfamiliar or had a strange texture to people who hadn't eaten it before. Today, manufacturers have greatly expanded their product lines and launched soy-based products that consumers find more approachable.

Soy milk now comes in popular flavors such as vanilla, chocolate and chai, and tofu appears in easy-to-cook forms for the unadventurous cook. Greater interest in tofu has generated new, healthier recipes for old, fatty favorites that once called for meat. Encourage your clients to try soy again.

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References

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