Introduction
Recently, benefits of soy foods have been reported by media outlets, prompting more people to explore and ingest soy products. Consumption of soy foods has risen in recent years, bolstered by scientific studies proclaiming all kinds of health benefits related directly to soy consumption. These benefits include lowering cholesterol and reducing a person's risk of heart disease, preventing hot flashes, osteoporosis, and even cancer among a few(4). But are soy foods really worth all the hype and should we all increase our consumption of soy foods based on studies published? Is the research behind these benefits sound, and is the FDA approval of health claims for soy products warranted? Is consumption of soy safe for men and women alike? Could there be adverse effects to consuming soy products? We will discuss sources of soy foods in our diets, examine studies and research that show positive and negative health effects of soy consumption, and finally determine if soy foods are worth including in every sensible diet.
Soy and its food sources
Soy is a versatile bean which is found in many foods that Americans already consume. For example, soybean oil accounts for 79 percent of the edible fats used annually in the United States(4). Everyday things like commercial mayonnaises, margarines, salad dressings, or vegetable shortenings all contain soybean oil.
The health claims only include soy protein, however. The most common sources of soy protein include:
Tofu: made from cooked soybeans processed into a custard-like cake. It has a neutral flavor and can be stir-fried, mixed into "smoothies," or blended into a cream cheese texture for use in dips. It comes in firm, soft and silken textures.
Soymilk: produced by grinding dehulled soybeans and missing them with water to form a mil-like liquid. It can be consumed as a beverage or used in recipes as substitutes for cow's milk. Soymilk, sometimes is fortified with calcium, comes plain or in flavors such as vanilla, chocolate and coffee. It is appropriate for lactose-intolerant individuals.
Soy flour: created by grinding roasted beans into a fine powder.
Tempeh: made from whole, cooked soybean formed into a chewy cake and used as a meat substitute.
Miso: fermented soybean paste used for seasoning and in soup stock.
Soy protein intake from the diet should equal to at least 25 grams to be considered beneficial. Since not all foods that contain soy ingredients will meet the required conditions for the health claim, consumers should check the labels to see if the product contains enough soy protein.
Soyfoods are rich in compounds called phytochemicals. One particular family of phytochemicals, isoflavones, may fight cancer in a variety of ways. Isoflavones are chemically similar in structure to estrogen. Isoflavones are being studied for a wide range of physiological effects due to their estrogenic activity. The two primary isoflavones in soybeans are daidzein and genistein(4,1).
Genistein, has captured special attention as to its health benefits. When genistein is added to live cancer cells in laboratory test tubes, they stop growing. Genistein is thought to act against cancer in several ways, some similar to common cancer-treating drugs. For example, scientists believe certain enzymes in the body convert normal cells to cancer cells. Some cancer drugs simply inhibit these enzymes. In cancer cells, genistein has been shown to do the same(4).
Health Claims(FDA)
In October 1999, FDA approved a health claim that can be used on labels of soy-based foods to tout their heart-healthy benefits(1). The agency review showed soy protein's value in lowering levels of total cholesterol and low-density lipoprotein (LDL, or "bad" cholesterol). Food marketers can now use the following claim, or a reasonable variation, on their products: "Diets low in saturated fat and cholesterol that includes 25 grams of soy protein a day may reduce the risk of heart disease. One serving of (name of food) provides __ grams of soy protein." To qualify for the claim foods must contain per serving:
- 6.25 grams of soy protein
- low fat (less than 3 grams)
- low saturated fat (less than 1 gram)
- low cholesterol (less than 20 milligrams)
- sodium value of less than 480 milligrams for individual foods, less than 720 milligrams if considered a main dish, and less than 960 milligrams if considered a meal.
Other health claims
While the FDA looked at the health claim of soy-based foods may reduce the risk of heart disease, other health claims abound. Soy-rich diets are linked to cancer prevention. Residents of countries where soyfoods are regularly consumed are less likely to develop certain cancers. Research indicates that soyfoods help protect against several types of cancer, including lung, colon, rectal, stomach and prostate(4).
Soybeans and soyfoods have been found to help prevent and treat osteoporosis, a disease that weakens bones and often results in bone fractures. Research into soy's effect on diabetes found less glucose in the urine of diabetics who consumed soybeans. This could indicate that their cells were able to absorb more glucose. Soyfoods may reduce menopause symptoms along with hot flashes(4).
Supportive Research
Cholesterol and heart disease
A yearlong review of the available human studies in 1999 prompted FDA to allow a health claim on foods containing at least 6.25 grams of soy protein per serving and fit other criteria, such as being low in fat, cholesterol, and sodium. The FDA allowed the health claim for soy protein in response to a petition by Protein Technologies International, Inc., a leading soy producer that tracks its origins to soybean studies sponsored by Henry Ford in the early 1930s. The company was company was acquired by E.I. du Pont de Nemours & Company (DuPont) in 1997. In considering the petition, FDA reviewed data from 27 clinical studies submitted in the petition, as well as comments submitted to the public record and studies identified by FDA. The available research consistently showed that regular soy protein consumption lowered cholesterol to varying degrees(1).
One of the studies, conducted over nine weeks at Wake Forest University Baptist Medical Center and reported in the Archives of Internal Medicine in 1999, found that soy protein can reduce plasma concentrations of total and LDL cholesterol but does not adversely affect levels of HDL, or "good" cholesterol, which at high levels has been associated with a reduction in heart disease risk(2). Another often-quoted study, published in the New England Journal of Medicine in 1995, examined 38 separate studies and concluded that soy protein can prompt "significant reductions" not only in total and LDL cholesterol, but also in triglycerides, another fat linked to health problems when present at elevated levels(3).
A meta-analysis conducted by Tufts University that involved 52 trials found the reduction to be slightly less - approximately 3 percent. These reported decreases are similar to the 1995- meta-analysis referred to in the previous section. In that analysis, the overall reduction in response to 47 g/d soy protein was 12.9 percent but the estimated decrease in response to 25 g/d (the amount established by the FDA as the threshold intake for cholesterol reduction) was approximately 5 percent(1).
Cancer
Two studies have utilized changes in serum prostate specific antigen (PSA) levels to assess reduction, stabilization, or progression of prostatic cancer. The first study fed 38 grams of soy protein with 70 or 4 mg of total isoflavones for six-week periods in a crossover-study to 34 men with elevated PSA(7). Half of the subjects had pre-study biopsies and none of these had prostate cancer. The other half declined to have biopsies. Neither of the soy preparations altered blood PSA levels. A second study enrolled 41 patients with confirmed prostate cancer (8). All patients had progressive or recurrent cancer based on increasing concentrations of serum PSA or PSA concentrations greater than 10 micrograms per liter. The patients consumed pills containing 100 mg of soy isoflavones twice daily for a median time period of six months. Overall, soy isoflavone treatment slowed prostate cancer growth as assessed by a slowing of the rise in PSA levels. Four patients had not received treatment prior to the study. In three of these men, PSA levels were stabilized by consuming soy isoflavones, indicating a slowing of the prostate cancer growth. Eighteen patients had previously had surgery or radiation therapy, but serum PSA was increasing before the study. Consumption of soy isoflavones resulted in 15 of these patients having stabilized PSA levels. Nineteen men had previously received hormone therapy, but had recurrent or progressive disease at the beginning of the study. Six of these men's PSA levels stabilized after consuming soy isoflavones.
Osteoporosis
Study conducted at the University of Illinois at Urbana-Champaign, involved 66 post-menopausal women, who consumed 40g per day of isolated soy protein containing either 55mg or 90mg of isoflavones for a six-month period(10). Those women consuming the higher level of isoflavones had an increase in bone mineral density and bone mineral content in the lumbar spine.
In the second study, done in Australia and involving 52 post-menopausal women, those who consumed 45g of soy grits per day for a 12-week period also had a significant increase in bone mineral content in the lumbar spine(11).
Messina and colleagues reviewed 15 studies examining skeletal effects of soybean isoflavones(9). Most of the studies were small (fewer than 30 participants per group) and lasted one year or less. Though the findings were somewhat inconsistent, the data generally suggested that isoflavones can reduce bone loss in younger menopausal women. The limited epidemiological data demonstrated that isoflavone intake among Asian populations was generally associated with higher bone mineral density. The authors concluded that though soyfoods and isoflavones should not be considered substitutes for anti-osteoporotic medications, health professionals could consider encouraging postmenopausal women who are concerned about osteoporosis to incorporate soy into their diets.
Menopause
A recent meta-analysis conducted by Messina and Hughes examined these studies(12). Thirteen of the 19 trials were parallel studies and involved more than 1,700 women. The studies ranged in length from 1 to 24 months, and isoflavone dose ranged from 34 mg to 100 mg isoflavones per day, primarily >=70 mg/day or more. By comparison, Japanese adults typically consume 35-40 mg of isoflavones per day. Data from six studies were excluded from the analysis for the following reasons: two involved breast cancer patients, two reported on hot flash severity but not frequency, one was not blinded, and one failed to have a control group. Regression analysis of the data from the remaining 13 trials revealed a statistically significant relationship (P=0.01) between the frequency of hot flashes and treatment efficacy. Frequency of hot flashes decreased by five percent (taking into account placebo or control effects) for every additional hot flash per day in subjects who initially reported five or more hot flashes per day (study baseline).
Counter Research
Heart Disease and Cholesterol
In a transgenic mouse model of hypertrophic cardiomyopathy, switching from a standard soy-based lab diet to a similar one in which milk is the major protein source radically improves disease indicators and cardiac function(13). The male mice had a mutation in the alpha-myosin heavy chain gene. On a soy diet, they progress to dilation and heart failure.
However, when the animals were switched to a casein-based diet they no longer developed severe dilated cardiomyopathy. Their left ventricular size and contractile function were preserved and there was an absence of pathological indicators including fibrosis and caspase-3 activation.
A study reviewed 22 such studies and concluded that eating soy-based foods has only minimal impact on cholesterol and other heart-disease risk factors(14).
The studies indicated that people who ate about 50 grams of soy protein a day, which represented about half of the usual total daily protein intake, reduced LDL, or "bad," cholesterol by only about 3%. Eating large amounts of soy had no effect on other risk factors such as triglycerides or HDL "good" cholesterol.
Combined data from 19 studies assessing isoflavones -- the plant estrogen derived from soy --suggested that isoflavone supplements had no effect on cholesterol at all.
Menopause
A study found that cancer survivors who took soy pills did not experience any noticeable changes in stopping or preventing hot flashes(15). Researchers started their randomized, double-blind study of female breast cancer survivors in March 1998, reporting data regarding 177 patients who entered this study. One half of the study's participants received the soy pills for 4 weeks while the other half received placebo. Each group switched regimens after 4 weeks and completed another 4 weeks under the new regimen. Participants kept detailed diaries on their reactions each day. The soy product that study participants consumed had 50 milligrams of soy isoflavones in each tablet. Patients were instructed to take 1 tablet 3 times a day, which would result in 150 milligrams of isoflavones per day, an amount similar to what would be consumed with 3 glasses of soy milk.
Cancer and osteoporosis
Study authors reviewed 22 randomized trials comparing consumption of isolated soy proteins with other proteins(16). The range of soy consumption was 25 to 135 g/day. Some epidemiologic studies have demonstrated a protective effect of soy isoflavones against breast cancer, whereas other studies have failed to demonstrate a benefit. While some research has suggested that increased consumption of soy isoflavones during adolescence can reduce the later risk for breast cancer, isoflavones can also stimulate breast epithelial cell proliferation in premenopausal women, which is a potential precancerous condition. Overall, there is insufficient evidence to recommend soy isoflavones as a means to prevent breast cancer the study concluded.
There is less evidence for the efficacy of soy isoflavones in the prevention of endometrial cancer. However, one placebo-controlled trial demonstrated that soy isoflavones can increase the rate of endometrial hyperplasia in postmenopausal women.
There is insufficient evidence to recommend soy isoflavones in the prevention of prostate cancer.
Soy safety
In one study, Barry Delclos, a researcher at FDA's National Center for Toxicological Research (NCTR), is overseeing a long-term, multigeneration study in rats of the soy component genistein(1). Early data using rats suggest that genistein alone may prompt undesirable effects such as the growth of breast tissue in males. The National Institutes of Health is sponsoring a long-term follow-up study on the safety of soy infant formula(1). The study is a "longitudinal retrospective epidemiological" assessment in which young adults who consumed soy formula as infants will be compared with young adults who consumed milk-based formulas as infants. They will be evaluated for any adverse effects from infancy into their childbearing years.
Conclusion
The American Heart Association (AHA) nutrition committee found a lack of benefit from soy and therefore does not recommend isoflavone supplements in food or pills(16). However, they do advocate consumption of foods containing soy, as these foods generally have high contents of polyunsaturated fats, fiber, and vitamins.
As presented in the paper, there is research that proves and disproves benefits associated with soy food consumption such as reducing risk of cancer, improving heart health, and improving osteoporosis. Some of the studies analyzed present better research methods then others, and it is important to weigh the reliability of each study when determining your own stance on benefits of soy foods. The recent AHA recommendation is a big blow to the soy industry, but the FDA health claim still stands. Some of the potential soy safety issues mentioned are a bit worrisome as well, especially for men. I will personally stay on top of the research, and will continue to consume soy products in moderation, just as I would with any other food group while maintaining a balanced diet.
References
(1) Soy: Health Claims for Soy Protein, Questions About Other Components. U.S. Food and Drug Administration. FDA Consumer Magazine, May-June 2000.
(2) Sophie D., et al. "Soy Protein Favorably Affects LDL Size Independently of Isoflavones in Hypercholesterolemic Men and Women," The American Society for Nutritional Sciences J. Nutr. 134:574-579, March 2004
(3) Anderson, J.W., et al. "Meta-Analysis of the Effects of Soy Protein Intake on Serum Lipids, " New England Journal of Medicine 333 (5):2 76-282, 1995.
(4) United Soybean Board, www.talksoy.com.
(6) Anderson JW, Johnstone BM, Cook-Newell ME. Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med 1995;333:276-282.
(7) Urban D, Irwin W, Kirk M, et al. The effect of isolated soy protein on plasma biomarkers in elderly men with elevated serum prostate specific antigen. Journal of Urology 2001;165:294-300.
(8) Hussain M, Sarkar FH, Djuric Z, et al. Soy Isoflavones in the Treatment of Prostate Cancer. Journal of Nutrition 2002; (in press).
(9) Messina M, Ho S, Alekel DL. Skeletal benefits of soy isoflavones: a review of the clinical trial and epidemiologic data. Curr Opin Clin Nutr Metab Care. 2004 Nov;7(6):649-58.
(10) Dalais FS, Rice GE, Bell RJ, Murkies AL, Medley G, Strauss BJG, Wahlqvist ML. Dietary soy supplementation increases vaginal cytology maturation index and bone mineral content in postmenopausal women.
(11) Erdman Jr. JW, Stillman RJ, Lee KF, Potter SM. Short-term effects soy soybean isoflavones on bone in postmenopausal women.
(12) Messina M, Hughes C. Efficacy of soy foods and soybean isoflavone supplements for alleviating menopausal symptoms is positively related to initial hot flush frequency. J of Med Food. 2003;6(1):1-11.
(13) Soy-Based Diet Worsens Heart Disease in Mice, Journal of Clinical Investigation, 2006; 116:16-19,209-216.
(14) Sacks, F.M. Circulation, Feb. 14, 2006; online edition.
(15) North Central Cancer Treatment Group Clinic Journal of Clinical Oncology, March 1. Online edition.
(16) Sacks, F. M. Rapid Access, Circulation, Vol. 113, Num.2. Online Edition.
