Table of Contents
Soy is a general term for products made from soybeans. Soy products include tofu, tempeh, soy oil, natto, miso, soymilk, and edamame.
Soybeans are the most widely used beans in the world. They are a good source of protein and contain no cholesterol. Soy is a complete protein. It contains all the essential amino acids that the body needs, and in this sense is different from most vegetable proteins and nutritionally equivalent to animal protein. Unlike animal protein, soy contains no cholesterol and is low in saturated fat. Soy is a heart-healthy choice and has met the United States Food and Drug Administration (FDA) requirements to make that claim on certain soy product labels.
Soy is believed to promote cardiovascular health, but many other health claims are also made for soy. Some of these claims remain unsubstantiated, are under review, or are in dispute. These health claims include that soy:
- promotes weight loss
- helps prevent certain cancers
- helps slow bone loss
Soybeans are the seeds of the plant Glycine max. This plant is native to China, where it has been cultivated
(Illustration by GGS Information Services/Thomson Gale.)
for about 13,000 years. From China, soybeans gradually spread to other areas of Asia, where soy is now a major part of the diet of millions of people. Intense breeding has produced a number of variants (cultivars) of the original plant, some which have a higher oil content and others which have a higher protein content. Soybeans may be green, yellow, brown, or black in color, but all variations are edible.
Soybeans were introduced into the United States in the mid-1700s. George Washington Carver (1864– 1943) experimented with them before he began his famous nutrition research on peanuts. Today the United States is the world’s largest grower of soybeans, producing almost 84 million metric tons in 2005. However, most soybeans grown in the United States are pressed to make soy oil. After the oil is extracted the beans are ground into meal and used as livestock feed.
Soy products are part of the daily diet of many Asians. However, soy has only become readily available in mainstream food stores in the United States since the 1990s. In 1979 the first major company, Vitasoy, introduced soymilk into the United States. Since then, the number of soy products has soared. The Soyfoods Association of North America estimates that sales of soy products in the United States increased from $300 million in 1992 to $3.9 billion in 2004, and sales were expected to continue rising through the end of the decade. Between 2000 and
2006, 2,500 new food products containing soy were introduced to the U.S. market.
Nutritional value of soy
Soy is a nutrient dense food, and it is the least expensive source of complete dietary protein. It is relatively low in calories and contains no cholesterol, saturated fat, or trans–fat. One cup (172 g) of cooked soybeans has about 300 calories and contains the following nutrients. The percentage DV is the percent of the daily requirement that 1 cup of cooked soybeans meets for the average adult.
- protein 28.6 g: 57% DV
- dietary fiber: 10.3 g; 41% DV
- total fat: 15.4 g; calories from fat 139
- molybdenum: 129 mcg; 172% DV
- manganese: 1.4 mg; 71% DV
- iron: 8.8 mg: 49% DV
- vitamin K: 33.0 mg; 41.3 57% DV
- omega-3 fatty acids: 1.03; 41.3 57% DV
- magnesium: 147.9 mg: 37% DV
- vitamin B2 (riboflavin): 0.5 mg: 29 % DV
- potassium: 886 mg: 25% DV
Fresh soybeans can be cooked briefly in boiling water and then eaten, or they can be toasted. Dried beans need to be soaked overnight before cooking and require relatively long cooking times. Soybeans can also be pressed to make soy oil, but the most familiar soy products come from soybeans that are processed in various ways that give them a variety of textures and make them easier to use in cooking. These include:
- Tofu: Tofu is made of cooked, pureed, soybeans that are processed and then formed into soft slabs that must be kept wet until they are used. The slabs are produced with consistencies that vary from very soft or ‘‘silken’’ to firm or extra firm. Other tofu variations include reduced-calorie tofu and tofu fortified with calcium. Tofu is used to make cheese substitutes, blended into smoothies, and stir fried. It has a bland taste and tends to take on the flavors of the foods it is cooked with.
- Tempeh: Tempeh is made from partially cooked soybeans that are then fermented in a controlled environment. Tempeh is chewier than tofu and is often used as a meat substitute.
- Miso: Miso is a fermented soybean paste that is used as a soup base and for seasoning.
- Soymilk: Soymilk is a soy beverage made by grinding soybeans and mixing them with water. Soymilk can be flavored (chocolate, vanilla, coffee) or sold plain. Some soymilk is fortified with calcium. People who are lactose intolerant often use soymilk as a substitute for cow’s milk, and soy is also used in formula for infants who cannot tolerate lactose.
- Soy flour: Soy flour comes from roasted, ground soybeans. It can be used in baked goods, cereals, and many other foods. Soy flour contains more moisture than wheat flour. People with celiac disease who cannot tolerate wheat, barley, or rye products can use soy flour.
- Textured soy protein: This product is used most often as a meat substitute in processed foods such as soy burgers or home-cooked foods such as meat-loaf. It is made by defatting soyflour, which is then compressed into clumps and dehydrated.
The role of soy in health
In October 1999, the FDA decided that well-designed, well-controlled, repeatable research studies had shown that soy was a heart healthy food that could help decrease the risk of developing cardiovascular disease. Since that date, the FDA has allowed products that contained at least 6.25 g of soy per serving to make the following health claim on their label: ‘‘25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.‘‘ This endorsement applies only to complete soy products, not to soy-based dietary supplements. The American Heart Association (AHA) also gave its approval to soy as a food that can reduce the risk of heart disease.
Soy is a food that can also help in weight loss because it can be used as a substitute for higher calorie meat. If soy is substituted for meat on a regular basis, the reduction in calories can be significant. For example:
- A soy burger patty has about 100 fewer calories than an equivalent-sized beef burger patty.
- Two links of soy breakfast sausage have about 90 fewer calories than two links of pork breakfast sausage.
- A soy veggie dog has about 70 fewer calories than a beef hotdog.
Some health controversies about soy center on compounds called isoflavones that are found in abundance in soybeans. These compounds have a chemical structure similar to the female hormone estrogen. Several health effects, both positive and negative, have been attributed to isoflavones. In 2006, the American Heart Association concluded that isoflavones are not the cause of the cholesterol-lowering, heart-healthy properties in soy and that dietary supplements containing soy-derived isoflavones do not have the same cardiovascular benefits as whole soy.
Another claim is that isoflavones can improve bone health in women. This claim appears plausible because of the chemical similarity between isoflavones and estrogen. Estrogen is known to increase the amount of calcium deposited in bones, and the lack of estrogen in post-menopausal women is linked to decreasing estrogen levels. However plausible the connection between bone health and isoflavones in soy may be, studies have produced inconclusive results. As of 2007, any effect that soy may have on bone health appears to be weak. Also, because of their estrogen-like structure, isoflavones from soy have been touted as a dietary supplement that will help prevent symptoms of menopause such as hot flashes. A committee of the AHA that investigated isoflavones found that they had no effect on hot flashes.
A far bigger health question concerns the relationship between isoflavones and cancer. The AHA committee found that despite claims that soy isoflavone supplements can treat and prevent breast, endometrial, and uterine cancer in women and prostate cancer in men, there was no evidence to suggest that this treatment was safe or effective. On the other hand, there was also no evidence that, as some experts have suggested, soy increases the chance of post-menopausal women developing breast cancer. A large number of federally sponsored clinical trials are underway to investigate these and other effects of isoflavones and soy.
Although soy is often thought of as a benign food, some people are allergic to soy.
Soy contains compounds called goitrogens. Goitrogens interfere with the body’s ability to absorb or use iodine. The goitrogens in soy should not cause problems with iodine uptake in healthy people, but people with thyroid deficiencies should discuss with their healthcare provider whether they should limit soy in their diet.
No complications are expected from eating soy products.
One long-term study is underway to investigate the effect of increased concentrations of isoflavones in the blood of children who drink soy formula. The study plans to look for potential effects across a period of about 20 years, so no results are available yet.
GeniSoy Products. The Magic of Soy: Healthy Cooking with Soy Protein Summertown, TN: Book Pub. Co.,2000.
Hagler, Louise. Soyfoods Cookery: Your Road to Better Health. Summertown, TN: Book Pub. Co., 1996.
Riaz, Mian N. Soy Applications in Food Boca Raton, FL: CRC, 2006.
Sears, Barry. The Soy Zone. New York: ReganBooks 2000.
Shurtleff, William and Akiko Aoyagi. Tofu & Soymilk Production: A Craft and Technical Manual Lafayette, CA: Soyfoods Center, 2000.
American Heart Association Science Advisory Board. ‘‘Soy Protein, Isoflavones, and Cardiovascular Health.’’ Circulation 113(2006):1034-44. <http://circ.ahajournals.org/cgi/content/abstract/113/7/1034>
Henkel, John. ‘‘Soy: Health Claims for soy Protein, Questions About Other Components.’’ FDA Consumer (May–June 2000). <http://www.fda.gov/Fdac/features/2000/300_soy.html>
Sears, Barry. ‘‘The Soy Zone—Diet That Helps Balance the Body.’’ Vegetarian Times (September 2000). <http://findarticles.com/p/articles/mi_m0820/is_2000_Sept/ai_65802972>
Szalavitz, Maia. ‘‘How Healthy is Soy?’’ Psychology Today (May–June 2006). <http://psychologytoday.com/articles/pto-20060426-000001.html>
American Soybean Association. 12125 Woodcrest Executive Drive, Suite 100, St. Louis,MO 63141. Telephone: (800)688-7692.Website: <http://www.amsoy.org>
Soyfoods Association of North America. 1723 U Street NW, Washington, DC 20009. Telephone: (202) 986-5600. <http://www.soyfoods.org>
United Soybean Board. 424 Second Avenue West, Seattle, WA 98119. Telephone: (800) TALK-SOY (825-5769). Website: <http://www.talksoy.com> and <http://www.soyfoods.com>
Soyfoods Association of North America. ‘‘Soy Safety.’’
Whfoods.org. ‘‘Soybeans.’’ World’s Healthiest Foods, undated, accessed April 26, 2007. <http://www.whfoods.com/>
Helen M. Davidson