The Soy Controversy.
The soybean is a hotly debated plant in the field of nutrition. Nutritionist, researchers, and industry experts argue about the various nutritional components of soy and their relation to human health. In recent decades, soy has become the protein of choice for many vegetarians and other health conscious individuals. However, there is growing evidence that many soy products are not as healthy as we thought them to be and in fact may be very damaging.
The soybean is a legume native to East Asia. The ancient Chinese cultivated the soybean as part of a crop rotation pattern due to the nitrogen fixing properties of the plant. Interestingly, the ancient Chinese did not eat the plant until around 2500 years ago. Since then many soy products have been developed and soy has become a huge industry. In 2009, soybeans were planted on 77.5 million acres and produced 211 metric tons; 38 percent of these soybeans were produced in the United States. US soybean production has risen from 52 million metric tons in 1989, to 72 million metric tons in 1999, to 91 million metric tons in 2009 (American Soy Association, 2010).
There are two broad categories of soy products: fermented and not fermented. Around 2500 years ago, the Chinese began to ferment the beans into chiang, similar to miso, to preserve animal foods. Soy sauce was a biproduct of chiang. Other fermented soy products were invented years later: natto was developed circa 1000 AD and tempeh sometime after the 1600.
Most of the soy controversy is focus on the non-fermented soy products. Soy oil was introduced around 1300. The first references to soymilk were from the 1800s and it was popularized in the early 1900s. Soy formula was developed by a Baltimore pediatrician in 1909. Soy nuts and butter are also a product of the 1900s. Other highly processed products were being distributed globally after World War II and were popularized in the 1960s. These recent additions to the soy family include soy protein isolate (SPI), textured
soy protein (TSP) and soy protein concentrate.
In the 20th century soy became quite popular for its protein content. Diets are based on it and the protein is isolated for specific uses. Compared to most plants, soy has a decent protein profile. Although low in the sulfur-containing amino acid methionine, the amino acids are more available than other legumes and most other plant sources (Lappe, 1991). Soybeans also contain more proteins that most other plant sources. This said, soy still falls short of animal proteins in both quantity and availability. Traditional soy products are a decent protein source as long as other protein sources are also part of the diet. Modern protein isolate products are highly processed, which ensures a long shelf life, however the high heat used in processing has reduced the vitamin, mineral, and protein quality.
Unrefined soy oil is a high-quality oil containing essential fatty acids. While traditional low heat processes preserve essential fatty acids, most commercially available soy oil is refined and partially hydrogenated which destroys essential fatty acids.
Soy contains the oligosaccharides raffinose and stachyose. Mammals don’t have the alpha-galactosidase enzyme to properly digest these carbohydrates so they pass into the large intestine where they become food for bacteria. This process produces carbon dioxide, hydrogen, and methane and can cause uncomfortable flatulence in many people. Tofu is low in raffinose and stachyose because they are usually lost in the whey during production, germination removes them in fermented soy products, and in products like soy protein isolate the carbohydrates have been removed entirely.
The debates regarding macronutrients in soy are minimal compared to the debates raging over isolated components of soy such as protease inhibitors, phytates, saponins, and phytoestrogens.
Protease inhibitors inhibit digestion enzymes. In soy we are primarily dealing with the Bowman-Birk inhibitor (BBI), trypsin inhibitors, and chymotrypsin inhibitors. BBI contains both a trypsin inhibitor and and a chymotrypsin inhibitor. Promising research has shown the chymotrypsin inhibitor part of BBI to prevent cancer. On the other hand, trypsin inhibitors have been linked to malnutrition, pancreatic disease, intestinal disorders and cancer, specifically pancreatic cancer (Daniel, 2005). Fermentation deactivates the protease inhibitors and cooking deactivates most, but not all, of them.
Phytic acid is the principle storage method of phosphorus in beans, grains, and other seeds and readily bonds to important minerals such as calcium, magnesium, zinc, and iron as well as toxic chemicals such as cadmium. In the digestive tract it keeps these nutrients insoluble and therefore they cannot be absorbed into the body. Phytic acid is also an antioxidant and therefore helpful in the prevention of dietary colon cancer (Head & Jurenka, 2006). Phytates are deactivated by soaking, fermenting, and germination. So fermented soy foods will have low levels of phytic acid. Tofu, soymilk, soy protein isolate, and other non-fermented soy foods can still have high levels of phytic acid.
Saponins are bitter compounds found in plants. Soy enthusiasts claim that their antioxidant and antimutagenic properties also make them anticancerous (Head & Jurenka, 2006). On the other side of the fence, saponins may cause leaky gut (Daniel, 2005). Fermented soy products are fairly low in saponins, but other soy products retain most of the saponins through processing.
Phytoestrogens are plant compounds that are structurally similar to estrogen and can bind with estrogen-receptor sites in the body. The effects of phytoestrogens on the body are not fully understood: at times they act like estrogens and at other times they seem to block the effects of estrogen. Much of the current research on soy is focusing on phytoestrogens, specifically the isoflavones genistein and diadzein. Researchers are
looking at the effects of those isoflavones on cancer, cardiovascular disease, menopausal symptoms, osteoporosis, and thyroid function amongst other conditions. Studies have shown conflicting results. For example, lets take a look at the cardiovascular research. The American Heart Association (2006) reviewed recent studies published on the affect of soy protein and isoflavones on cardiovascular health. They reviewed 22 randomized trials involving isolated soy protein with isoflavones and decided that although the research found a small improvement in decreased LDL cholesterol, they thought the effect to be minimal and found no significant effects on HDL cholesterol, triglycerides, lipoprotein, or blood pressure. After reviewing 19 studies of soy isoflavones, they found the weighted average effect on LDL cholesterol and other lipid risk factors to be insignificant. They did add that many soy products should be beneficial to cardiovascular health due to their high content of polyunsaturated fats, fiber, vitamins, and minerals and low levels of saturated fat, but not the isolated constituents.
Cancer research has shown similar results. While some studies have demonstrated that soy isoflavones can prevent cancer, other studies have demonstrated the opposite or have shown results to be inconsistent. According to Head and Jurenka (2006), many epidemiological, human, animal, and in vitro studies have demonstrated that genistein and diadzein are useful in preventing certain types of cancer. However, they also recognizes that, "the effects of phytoestrogens vary greatly according to the species of animal, the particular phytoestrogen compound being tested, the age of the animal
, the duration of ingestion, the presence or absence of exogenous estrogen, the target tissue in questions, and the dosage used." Everyone agrees that much research still needs to be done.
So can we safely consume soy products? Yes and no. Fermented soy products have a much longer history and most will agree that fermented soy products are nutritious. The fermentation process deactivates protease inhibitors, digests oligosaccharides, and produces the enzyme phytase, which breaks down phytic acid.
Additionally microorganisms increase levels of some B vitamins and vitamin K (but reduce levels of thiamine), and traditional low heat processes preserve essential fatty acids. Highly processed soy products should be avoided and others consumed in moderation. We should certainly not rely on soy as a dietary base as is done with soy-based infant formula.
We may yet find promise in the health benefits of isoflavones and other soy constituents. However their effects seem to be hard to pin down and hard to control. I submit that the future of many of these constituents lies in pharmaceuticals not in soy as a food.
American Soybean Association. (2010). Soystats 2010. Retrieved September 12, 2010, from http://www.soystats.com/2010/Default-frames.htm
Daniel, Kaayla T. (2005). The Whole Soy Story. The dark side of America’s favorite health food. Washington DC: New Trends Publishing.
Head, Kathleen A., & Jurenka, Julie S. (2006). Soy Isoflavones and Other Constituents. Textbook of Natural Medicine, Third Edition. St. Louis: Churchill Livingstone Elsevier.
Lappe, Frances Moore. (1991). Diet for a Small Planet. New York: Ballantine Books.
Sacks, Frank M. MD, Lichtenstein, Alice DSc, Van Horn, Linda, PhD, RD, Harris, William, PhD, Kris-Etherton, Penny, PhD, Winston, Mary, EdD. (2006). Soy Protein, Isoflavones, and Cardiovascular Health. An American Heart Association Science Advisory for Professsionals From the Nutrition Committee. Circulation, 113,1034-1044.