Soy and Your Health: An Update on the Benefits

Confused about soy? Here’s more solid evidence on the health benefits from a man who has been studying the topic for over 20 years- Mark Messina, PhD

About the Author:

Dr. Mark Messina is an adjunct associate professor at Loma Linda University and the Executive Director of the Soy Nutrition Institute. He has been studying the health effects of soy for more than 20 years and has published more than 60 scientific papers and given more than 500 presentations on soyfoods to health professionals.

Soyfoods have been a part of Asian diets for centuries. Today, there is a growing interest in these foods among westerners because of their proposed health benefits and also because their versatility makes them valuable for replacing meat and dairy foods in the diet. Soybeans provide excellent nutrition and contain a number of biologically active components that collectively may be responsible for a variety of health benefits. However, most of the interest in soy is due to their isoflavone content. Isoflavones have been rigorously studied for their protective effects against several chronic diseases including osteoporosis, coronary heart disease and certain forms of cancer.

Soy Isoflavones

Isoflavones are essentially unique to soyfoods; no other commonly-consumed foods contain enough to impact health [1]. Although they are among a group of naturally-occurring compounds known as phytoestrogens (plant estrogens), isoflavones are much different from the hormone estrogen. In fact, they are most accurately classified as SERMs (selective estrogen receptor modulators) [2]. Other examples of SERMs are the breast cancer drug tamoxifen and the breast cancer and osteoporosis drug, raloxifene. The effects of SERMs vary depending upon a variety of circumstances. SERMs like isoflavones may have estrogen-like effects, but depending on a number of factors, they may also have effects opposite to those of estrogen or no effects at all in tissues that are affected by estrogen. Therefore, looking at the health effects of estrogen doesn’t provide much information about how isoflavones act. The only way to learn about the effects of isoflavones is to look directly at their biological activity in studies.

Soybean Nutrition

Soybeans are unique among legumes, a group of foods that includes beans, peas, and lentils, because they are much higher in protein and fat than other beans, and lower in carbohydrate [3]. The fat in soybeans is primarily a combination of heart-healthy essential polyunsaturated omega- 6 and omega-3 fatty acids [4]. This makes soybeans one of the few plant foods to provide both of the essential fatty acids. The carbohydrate in soy is comprised primarily of simple sugars that have been shown in some studies to act as prebiotics, thereby stimulating the growth of healthy bacteria in the colon. Soyfoods are also good sources of B vitamins and minerals such as potassium, iron and sometimes calcium.

Soybeans are perhaps best known for their high protein content. Although soybeans contain trypsin inhibitors, compounds which can interfere with the digestion of protein, the normal processing used to produce soyfoods inactivates these compounds. As a result, protein in soy is very well digested; digestion typically exceeds 90 percent. For this reason, and because of its

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excellent amino acid profile, soy protein is comparable in quality to the protein in animal products [5].

Soyfoods and Heart Disease

Research suggests that incorporating soyfoods into the diet may decrease LDL-cholesterol (the bad cholesterol) by as much as 8 percent [6]. When soyfoods replace conventional sources of protein in western diets, saturated fat intake is reduced and polyunsaturated fat intake is increased. As a result, blood cholesterol levels will be lowered. That soyfoods contain a combination of omega-6 and omega-3 fatty acids is especially important for reducing risk of heart disease [7]. In addition to the healthy fat found in soy, soy protein has been shown to directly reduce levels of blood cholesterol. The Food and Drug Administration awarded a health claim for soyfoods and coronary heart disease on this basis in 1999. The effects of soy protein are comparable to cholesterol-lowering benefits of soluble fiber, the kind found in oat bran.

In addition to lowering LDL-cholesterol, soyfoods give a modest boost to HDL-cholesterol, which is protective against heart disease, and reduces levels of triglycerides (another fatty compound in the blood that can raise heart disease risk). Finally, soyfoods may reduce heart disease risk in ways that are independent of their effects on cholesterol. For example, soyfoods may lower blood pressure [8] and research indicates that isoflavones directly improve the health of the arteries [9]. Therefore, even people with normal cholesterol levels can benefit by consuming soyfoods.

Soyfoods and Breast Cancer Risk

In Asian countries, where soyfoods are a usual part of the diet, breast cancer rates are much lower than in western countries. This observation helped fuel speculation that soyfoods reduce breast cancer risk. However, after years of research, it is not clear that women who begin to consume soyfoods in adulthood will lower their risk of cancer. Rather, the protective effects appear to be related to early soy consumption. That is, women who consumed these foods in childhood and/or the teen years may have a lower risk for breast cancer later in life. Protective effects of soy are thought to be due to actions of soy isoflavones on the developing breast in ways which make breast cells more resistant to being transformed into cancer cells later in life [10, 11].

Studies conducted in China and the United States show that the consumption of modest amounts of soy—1 to 1 1⁄2 servings per day—is associated with a 25 to 50% reduction in risk. Although the hypothesis that early soy intake is protective against breast cancer remains speculative, because the amount of soy needed for benefit is modest and soyfoods provide good nutrition, there is no reason to wait for the results of future research before encouraging young girls to consume soy.

Soyfoods and Male Reproduction

The estrogen-like effects of isoflavones have led to concerns that soyfoods may exert feminizing effects in men, but a wealth of human data show these concerns to be without merit. Although two reports published in the scientific literature describe feminizing effects of soy in two individuals, these men consumed as many as 14 to 20 servings per day [11, 12]. Excessive intake of nearly any food can be expected to have negative effects on health.

In contrast to these two reports, a comprehensive analysis of the human data, which involved more than 30 individual studies, found that neither soyfoods nor isoflavones affect levels of the male sex hormone, testosterone [13]. In many of these studies, soy intake was far beyond what is typical for Asian men, so the lack of effect is very reassuring. In addition, a recent review of nine studies found that soy has no effect on estrogen levels in men [14]. Men in these studies consumed as much as six servings of soy per day. The clinical studies also show there is no effect of soy on sperm or semen. In fact, there is one report describing a male with low sperm count whose sperm concentration normalized after taking isoflavones for 6 months [15]. Thus, the evidence clearly shows that soyfoods do not exert feminizing effects.

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Thyroid Function

Although soy isoflavones can interfere with thyroid function in laboratory animals, they have no effect on thyroid activity in healthy people. More than 20 studies have examined thyroid function in response to the consumption of soyfoods, soy protein, or isoflavones [16]. One limitation of this research is that most of the studies were conducted for 6 months or less. However, recently, two three-year studies found no effects of large amounts of soy on thyroid function [17]. As a result, the evidence overwhelmingly supports the safety of soy. In people who need to take synthetic thyroid hormone, soyfoods are among many foods that can reduce absorption of this medication. However, these individuals can safely consume soyfoods as long as they are consistent in the daily amounts of soy in their diet [18].

Finally, approximately 5 to 10% of the population has subclinical hypothyroidism, which can progress to hypothyroidism (low thyroid function) over time. A small number of these people may be sensitive to soyfoods and therefore, they should be monitored by their physician if they begin consuming soy [19]. However, even in these individuals, evidence indicates that consuming soyfoods may markedly lower risk for chronic diseases like diabetes and heart disease. In one study involving subclinical hypothyroid patients, soy protein dramatically reduced blood pressure and levels of inflammation and increased the ability of the subjects to use the hormone insulin.

Cognitive Function

Nutrition epidemiologic studies record the dietary habits of subjects and then determine whether these habits are associated with higher or lower rates of a particular disease. In one epidemiologic study conducted in Hawaii, people who ate the most tofu had poorer cognitive function [20]. This study had some important limitations however. For example, the main focus of the study was actually heart disease, not cognition. Also, the researchers only assessed the intake of 26 foods; in studies initiated today, it would be customary to assess the intake of more than 100 foods. In addition, the way in which tofu intake was assessed changed throughout the course of the study. Furthermore, in contrast to the Hawaiian study, in a study conducted in Hong Kong [21], tofu had no effect on cognition, and in Indonesia, intake of the widely consumed soyfood tempeh, was linked to better cognitive health whereas tofu was unrelated [22]. Even more important, the clinical studies, which are studies in which subjects are actually given the test product, suggest that soy may improve cognitive function [23]. These studies have involvedprimarilypostmenopausalwomen. Atthispoint,thetotalityoftheevidencesuggests soy may actually improve cognitive function, not impair it but the data are far too limited for any claims about the impact of soy on cognition to be made.

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Childhood Development

Asian women have been consuming soyfoods for centuries without any apparent adverse effects in the offspring. In pregnant women who consume soy, isoflavones are transferred to the fetus. However, levels of the hormone estrogen are incredibly high compared to the small amounts of isoflavones in the womb or amniotic fluid. As a result, it’s not likely that isoflavones would have any type of estrogenic effect on the growing fetus.

Very little soy research has involved children or young people although several studies have found that adding soyfoods to the diet allows for normal growth and development. Two studies, one in teenage boys [24] and another in children 5 to 12 years of age, found that isoflavones did not exert estrogen-like effects or produce any abnormalities [25]. Also, according to the American Academy of Pediatrics, soy infant formula leads to normal growth and development; furthermore, their position is that there is negligible (the safest possible rating) concern about the safety of soy infant formula. Finally, one advantage of consuming soy early in life may be protection against breast cancer. Epidemiologic studies show that consuming approximately one serving of soy per day during childhood and/or adolescence reduces breast cancer risk later in life by as much as 50 percent [26, 27].

Allergies

Allergy to soy is fairly rare. According to one US study, around 1 out of every 2,500 adults has been diagnosed with an allergy to soy protein. For comparison, allergy to cow’s milk protein is around 40 times higher [28]. Approximately 70% of children with soy allergy outgrow their intolerance by age 10 [29]. And in formula-fed infants who are allergic to cow’s milk, research suggests that a switch to commercial soy infant formula can often alleviate symptoms [30].

Mineral Absorption

Soybeans are high in compounds that can inhibit absorption of minerals such as calcium, iron and zinc. However, recent research shows that the type of iron found in soybeans is absorbed at higher rates than previously believed. In fact, two studies in women found iron absorption from soy was excellent [31, 32]. Also, in a just-published study, replacing animal products in the diet with two to three servings of soyfoods per day for 10 weeks did not affect iron status [33]. Calcium absorption from soyfoods is also very good. In fact, calcium absorption from soybeans is much better than from other legumes [34] and calcium absorption from a variety of calcium- fortified soy products is excellent, being similar to the absorption of calcium from cow’s milk. This includes for example different types of calcium-fortified soymilks [35, 36] and calcium-set tofu [37].

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Fermented versus Unfermented Soyfoods

Fermented soyfoods were the first forms of soy to be consumed, but historical records indicate that unfermented soy has been consumed for at least 1000 years. Commonly-consumed fermented soyfoods include natto, miso and tempeh; unfermented soyfoods include edamame (green soybeans), tofu, soymilk and isolated soy protein. In Japan, tofu, miso, natto, and fried tofu account for about 90% of total soy protein and isoflavone intake [38, 39] whereas in Shanghai, soymilk, tofu, and processed soy products other than tofu accounted for about 80% of total soy protein intake [40]. Most soy consumed in Chinese cities such as Shanghai is in unfermented form whereas in Japan roughly half comes from fermented foods. Interestingly, even in Indonesia, the birthplace of the fermented soyfood tempeh, unfermented tofu accounts for about 40% of soy intake.

Fermented and unfermented soyfoods have similar protein and isoflavone contents. Fermentation does reduce the amount of protease inhibitors in soybeans, compounds which can inhibit the digestion of protein. However, the protein digestibility of protein from unfermented soyfoods is excellent, typically exceeding 90 percent. Epidemiologic studies from Asia generally show intake of both fermented and unfermented soyfoods to be associated with benefits although a few studies have found unfermented soyfoods to be superior. Overall however, the evidence indicates that both forms of soy can make important contributions to a healthy diet.

Safe Intake Levels

There is a considerable range of soyfood intake among Asian countries. In Japan and some urban areas of China, average consumption is around 1 1⁄2 to 2 servings per day, but older people with more traditional diets may consume as many as 3 servings per day. Both fermented and unfermented soyfoods are popular throughout Asia. Based on an understanding of the health of Asian people and traditional soyfood consumption, as well as the clinical studies conducted in Asia and the West, those who enjoy soyfoods can feel comfortable eating as much as three or four servings of these foods per day. This amount provides around 25 grams of protein and about 100 milligrams of isoflavones. Exceeding these amounts is not harmful, but eating more than four servings of soy per day is inconsistent with the principles of variety and moderation as no food should place too large a role in the diet, no matter how healthy it may be.

 

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