Can soy reduce mineral absorption?

Soy foods contain a substance called phytic acid, as do all grains, legumes, nuts and seeds. It is found in the bran and hull. Phytic acid impairs mineral absorption, namely calcium, zinc and iron. However, phytic acid also serves as an antioxidant.¹ Sprouted and fermented soy products, such as tempeh and miso, do not pose a problem because the phytic acid is neutralized during these processes.² In addition, sprouting and fermenting start digesting this bean, which makes it more digestible and absorbable.

Can soy cause hypothyroidism?

Some types of foods, including soy, are reputed to be goitrogenic, which means they create problems with thyroid hormones. Other common foods with this reputation are members of the brassica family (which includes cauliflower, Brussels sprouts, cabbage and broccoli).³ This goitrogenic property is often inactivated when these foods are cooked.^4,5 Nevertheless, those who have thyroid problems and concerns may want to avoid soy or limit their intake to occasional use of fermented forms, such as miso, soy sauce, tempeh and natto.

Soy and breast cancer risk?

In 1998, researchers at the Paterson Institute for Cancer Research in England found that a daily dose of 60 grams of soy protein, containing 45 mg of the plant estrogens called isoflavones, spurred the growth of healthy breast cells. This study was published in the American Journal of Clinical Nutrition.⁶ Researchers theorized that, if the isoflavones also stimulate the growth of cancerous breast cells, they could make matters worse for some women who have breast cancer.⁷

This study and its possible ramifications can be viewed in light of two facts. First, populations that eat large amounts of soy, like the Japanese, have lower rates of breast and prostate cancer. ⁸ It is important to note that the Japanese consume soy mainly in traditional fermented forms. The isoflavones are part of the food and not isolated. Secondly, no one can say with any certainty how large doses of isolated soy isoflavones will affect the body. However, when isoflavones are part of the food, it is unlikely that you will ingest too many from your diet. On the other hand, there is a potential to consume too much if you take soy supplements in pill form. The whole food behaves very differently in the body than do isolated parts. Take this into consideration when you are choosing your supplements.⁸

Is soy infant formula safe?

Thyroid problems are more prevalent among infants who consume soy formula than in those who consume breast milk. The mechanism behind this disparity is not clear. This may be due to phytochemicals in soy that alter thyroid hormone metabolism or possibly to a soy allergy. When adults eat soy products, their thyroid hormone levels can be negatively altered, so there may be substances in soy that directly influence thyroid activity.³

The preliminary studies that suggest babies fed soy formulas face a risk for increased thyroid and hormonal problems later in life has raised concern among some doctors. Britain and New Zealand have issued warnings that the phytoestrogens in soy-based infant formulas could adversely affect infant health. They recommend that parents use soy formulas only on the advice of a doctor. If you have concerns, it is best to talk to your pediatrician.

Soy and GMOs

Somewhere between 80-90% of all soy grown in the United States is genetically modified. While we don’t yet know the full effects of ingesting genetically modified organisms (GMOs), there is growing evidence to suggest that they are not as safe as industry would have us believe. One study found that GMO soy had seven times the amount of a known soy allergen as its conventional counterpart.⁹ Of even more concern is the fact that the only published human feeding experiment showed that genetic material that had been inserted into genetically- modified soy could survive digestion and transfer to the DNA of intestinal bacteria.¹⁰ The best way to avoid GMO soy is to always buy organic soy products. Beware though that soy is ubiquitous in our food supply and hides in places like soy lecithin in processed foods and soy oil used in restaurants. Soy is commonly fed to conventionally raised cattle and chickens and farmed fish; we do not yet know what GMO residues may remain in their meat.

Can I eat too much soy?

Yes, as with any food, overindulgence in soy may be detrimental. We would be wise to take a lesson from traditional Asian practices, which used soy more as a condiment, rather than as a centerpiece of the meal. In a healthy individual, consuming small amounts occasionally can be perfectly healthy. Emphasize your fermented options, which are tempeh, miso, traditionally brewed soy sauce and natto. Tofu, while not fermented, is also probably acceptable in small amounts and sprouted varieties are available now, which should help to improve digestion.

Limit processed forms like soy protein isolate, soy milk, soy-based imitation meat and dairy products and isoflavone powders and pills.

Summing it Up

Avoid getting caught up in the hype and hysteria surrounding soy. It is not a matter of all or none – moderation is key. Consuming this bean in moderation and as a whole food is healthy, particularly in the fermented forms. Limit your intake of processed soy found in numerous packaged foods, energy bars and baked goods. Including some soy as part of a balanced whole food diet is fine for most people, so go ahead and enjoy!

References

¹ Kamen, Betty, Ph.D. 1991. New Facts about Fiber. Nutrition Encounter, Inc., Novato, California.

² Obstet Gynecol, 1998.

³ Bland, Jeffery, Ph.D. 1997. Keep your Thryoid Healthy for Peak Energy. Delicious Magazine.

⁴ Lieberman, Shari, PhD & Bruning, Nancy. The Reach Vitamin & Mineral Book. Avery Pub. 1997.

⁵ Murray, Michael, N.D. Encyclopedia of Nutritional Supplements. Prima Pub. 1996.

⁶ DF McMichael-Phillips, C Harding, M Morton, SA Roberts, A Howell, CS Potten and NJ Bundred American Journal of Clinical Nutrition, Vol 68, 1431S-1435S.

⁷ Jaret, Peter. 2000. Should anyone take soy pills? Health Magazine.

⁸ Weil, Andrew, Dr. 2000. Weighing the benefits of soy. Found at www.pathfinder.com/drweil.

⁹ A. Pusztai and S. Bardocz, “GMO in animal nutrition: potential benefits and risks,” Chapter 17, Biology of Nutrition in Growing Animals, R. Mosenthin, J. Zentek and T. Zebrowska (Eds.) Elsevier, October 2005.

¹⁰ Netherwood et al, “Assessing the survival of transgenic plant DNA in the human gastrointestinal tract,” Nature Biotechnology

22 (2004): 2.