Exploring the sense and nonsense of food and health

Soy is Healthy – Fact or Fiction?

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Soy is everywhere!!  I am not a big fan of soy foods and there is a tremendous controversy over soy these days.

  • In America, soy is an ingredient in processed foods in the form of soy protein isolates, soy isoflavones, textured vegetable protein, and soybean oils.
  • It is estimated that these forms of soy account for a fifth of the calories in our diets and that does not include whole soy foods and soy drinks.
  • According to the USDA, soy plays a major role in agriculture.  In 2008-09, the farm value of soybean production was $29.6 billion, the second highest among U.S crops.
  • Soybeans are now often genetically modified

Since soy foods are an important part of the vegan diet and have been added to many processed foods, we should look at some of the claims made against them.  Exactly what is the evidence against them?   I found the best sources for the evidence pro and con from Mark Messina, MD, an adjunct associate professor at Loma Linda University and expert on soy and chronic disease and Virginia Messina, MPH, RD an adjunct assistant professor at Loma Linda University and a consultant on vegetarian nutrition.

Claim #1 Soy foods affect thyroid function

Goitrogens are compounds found in soy and cruciferous vegetables that interfere with thyroid function, causing the thyroid gland to enlarge forming a goiter.  Iodine is necessary for thyroid function.  A problem arose when soy formulas were given to infants in the 1950’s.  Some cases of goiter developed in infants fed soy formulas made with soy flour.  Since then, the formulas have been changed to employ soy isolate that do not contain goitrogens and are now fortified with iodine.  Consequently, no cases of goiter in infants have been diagnosed since the change.

There is also no evidence that consuming soy causes thyroid problems in healthy, well-nourished people who have adequate iodine in their diet.  The problem could possibly affect vegans who have low iodine levels since good sources of this mineral are fish and milk.  However, the simple solution in the U.S. is just to get adequate iodine by using iodized salt and fortified foods.

Claim #2  Soy foods cause dementia.

The Honolulu Heart Study came up with an alarming finding. The study looked at Japanese men living in Hawaii in order to compare diet to risk of dementia.  The intake of 26 food foods, including tofu, was studied in 3,000 men between 1965 and 1967 and again in 1971 to 1974.  Cognitive test performance was assessed between 1991 and 1993 and the researchers also looked at brain shrinkage through autopsy data of the men who died during the study. Men consuming just two to four servings of tofu per week performed poorer than those who consumed fewer servings and also had more brain loss.  Interestingly, the wives of these men also showed more signs of poor cognitive function.

This result was questionable due to the fact that dementia rates in Asian countries are lower than those in western countries.  It is important to note that this was an epidemiological study that raises questions but does not show cause and effect.  These studies merely show two things that occur that occur together.

It is also important to look at other studies that have assessed soy consumption and dementia.  Although another epidemiological study, a small Indonesian study was conducted in 2 rural sites and 1 urban site among 719 Javenese and Sundases elderly.  High tofu consumption (at least 9 times/week) was associated with worse memory.  The researchers attributed this effect to isoflavones.

However, high tempeh (fermented soy product) consumption was independently related to better memory in people over 68 years old. Tempeh is also high in isoflavones.  A reason for this paradox may be that that formaldehyde is added to tofu, but not to tempeh.  Formaldehyde causes memory problems in animals.  Fortunately, formaldehyde is not used as a food preservative in the United States.

Another study showed no effect of soy on brain function.  This study included nearly 4,000 people aged 65 and over from Hong Kong.  The results indicated that isoflavone intake was unrelated to cognitive function.  Another study (a clinical trial) suggests, however, that isoflavone-rich products favorably affected some aspects of cognitive function in younger postmenopausal women.

So, as so often in nutrition studies, the results are unclear and no conclusions can be drawn.  There should be more intervention studies, i.e. clinical trials rather than reliance on epidemiology.  At this point, there is no hard evidence of harm with a few indications of benefit. Moderation is the key until research can clarify the confusion

Claim #3  Soy inhibits mineral absorption

Critics of soy point out that soy is high in phytates that inhibit mineral absorption (iron, zinc, calcium).   Studies actually show that calcium absorption from soy foods is very good in spite of the high phytate content and that isoflavones support positive bone health.

Soy protein does inhibit iron and zinc absorption.  Since soy is not high in iron or zinc, vegans must be careful to consume adequate iron and zinc from other food sources like whole grains, beans, dried fruits, and nuts.  Otherwise, healthy non-vegans have no more risk of developing these deficiencies when iron and zinc are adequate in the diet.

Claim #4  Soy formula should not be used for infants.

The primary isoflavones found in soy products, including tofu, soy milk and soy formula, are genistein, daidzein, and to a smaller extent, glycitein. These isoflavones are referred to as phytoestrogens because they are found in plants (phyto) and because of their ability to act like the hormone estrogen in the body.

Although there have been no specific health problems documented in human infants receiving soy formula, it is recognized that infants go through developmental stages that are sensitive to estrogens. Therefore, infants are more likely than adults to be vulnerable to the estrogen-like effects of the phytoestrogens in soy. In some cases, the health effects resulting from a soy-based diet may not be apparent until years later when the concern may be early puberty or reproductive development problems.

Claim #5  Soy reduces coronary heart disease.

On the positive side, on October 26, 1999, after a review of over 50 studies, the Food and Drug Administration authorized a health claim that links the consumption of soy protein with a reduced risk of coronary heart disease. The health claim states that  “25 grams or 4 servings of foods with 6.25 grams of soy protein daily to lower cholesterol.” Is supported by further studies since the original approval.

Bottom Line:

  • Fermented soy products and whole soy foods eaten in Asian cultures may be the best choice for soy foods.
  • One serving of soy protein powder a day probably is not harmful for anyone.
  • Most “soy products”, soy chips, soy milk, soy ice cream, soy burgers, soy cheese are probably no healthier than other “junk” foods they claim to replace.
  • Soy formulas for infants and small children should be used with caution due to allergic reactions and estrogen-like isoflavones.
  • Soy supplements are not recommended – research has shown that these widely used products for menopause- related symptoms have not been very helpful.
  • To avoid genetically modified soy products, choose organic soy.



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