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 Table of Contents  
REVIEW ARTICLE
Year : 2021  |  Volume : 8  |  Issue : 1  |  Page : 18-22

The modest hormonal effects of soy isoflavones: What do we really know? A narrative review


1 Dietetics and Nutrition Department, Hamad Medical Corporation, Doha, State of Qatar
2 Columbus County Department of Public Health, Whiteville, NC, USA

Date of Submission26-Aug-2020
Date of Decision01-Dec-2020
Date of Acceptance03-Dec-2020
Date of Web Publication29-May-2021

Correspondence Address:
Lubna A G Mahmood
Dietetics and Nutrition Department, Hamad Medical Corporation, Doha.
State of Qatar
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jhrr.jhrr_30_20

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  Abstract 

Background: Soybean is a species of legume native to Southeast Asia. It is widely grown as it is widely used in producing foods and food additives. The categories of soy protein differ according to their production methods, which ranges from isolated to concentrated and textured soy protein. The direct effect of soy bean on health and its mechanism of work are yet unclear and is still a controversial issue that needs to be investigated. Aim: To provide an overview of existing soybean and health-related research studies and to identify the positive and negative effects if found especially for heart diseases and cancers. Materials and Methods: A search of periodical literature by the author involving dietary therapy and soybean was carried out. Items were identified initially through health-oriented indexing services such as Medline, Health STAR, and Cinahl, looking up for articles published in English language, from 2010 to 2020. Keywords included “Isoflavones,” “soybean,” “cancer,” “Tofu,” and “heart disease.”Results: Researches are still controversial, some studies acknowledged the soybean product’s efficacy in preventing diseases such as heart diseases and cancers, while others found no or negative effects on health. Conclusion: The risk/benefit status of soy products as beneficial supplements continues to be controversial. To conclude, it can be confirmed that 12–25 gm of soy with 75 gm of isoflavones daily (2–4 servings of soy foods per day) can improve the estrogen level along with lipid profile and offer benefits to the cardiovascular system and estrogen deficit symptoms. Most populations should not exceed the recommended amount and the effectiveness of using soybeans should be matched with tolerance and compliance to reach the desirable improvements.

Keywords: Cancer, heart diseases, isoflavones, soybean, tofu


How to cite this article:
Mahmood LA, Matthews L. The modest hormonal effects of soy isoflavones: What do we really know? A narrative review. J Health Res Rev 2021;8:18-22

How to cite this URL:
Mahmood LA, Matthews L. The modest hormonal effects of soy isoflavones: What do we really know? A narrative review. J Health Res Rev [serial online] 2021 [cited 2021 Dec 4];8:18-22. Available from: https://www.jhrr.org/text.asp?2021/8/1/18/317214


  Introduction Top


Soybeans are from the pea family, which grows mainly in tropical climates. It grows mostly in China, Southeast Asia, and the United States. Soybeans contain significant amounts of oil, carbohydrates, dietary fiber, protein, vitamins, isoflavones, and minerals. Soybeans can be provided in different forms and products including soy milk, tofu, and soy flour.[1] Scientists investigate the role of soybeans on hormone-related effects for treating cardiovascular diseases and cancers.[2] In China, it has been found that risk of breast cancer appears to have decreased by half among women who consume soybeans.[3] Whereas, the American Heart Association (AHA) soy products had no effects on lipid profile.[4]

The aim of this narrative review is to provide an overview of existing research studies and their approaches concerning the linkage between soybean and its effect on health (i.e. heart diseases and cancers).


  Materials and Methods Top


Search criteria and methodology

A search of periodical literature by the author involving dietary therapy and soybean was carried out. Items were identified initially through health-oriented indexing services such as Medline, Health STAR, and Cinahl, looking up for articles published in English language, from 2010 to 2020. Keywords included “Isoflavones,” “soybean,” “cancer,” “Tofu,” and “heart disease.” An extensive search was also carried out on educational database ERIC. Through an electronic search, 55 studies were identified [Figure 1].
Figure 1: Flow diagram of article selection

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  Result Top


The result analyzed based on the PICO elements

  • P (Problem or Patient or Population): population with heart disease or cancer


  • I (intervention/indicator): soybean-enriched diet


  • C (comparison): soybean-enriched diet; no soybean-enriched diet


  • O (outcome of interest): reduced risk of heart disease or cancer


  • Risk of bias

    A systematic review consists of primary studies, published and unpublished, meeting the criteria of inclusion and exclusion determined a priori, and must be representative of all the results of the primary studies available at the time the review was conducted. However, if only published studies with language restriction (e.g. English) are used from certain electronic databases, as is the case with many published reviews, a large number of eligible studies may be missed, resulting in a substantial bias on selection. In order to minimize such errors, we had two reviewers independently gathering information from primary studies, and resolving disagreements. In addition, since a systematic review of observational studies uses information presented in primary studies, any measure taken or errors reported in later studies may invalidate the findings of the review. In addition, assessing the quality of primary studies to be included in a systematic review is of the utmost importance. Only studies with reasonably valid association results were included.


      Discussion Top


    What are soybeans?

    Soybean or “Glycine max” is classified as oilseeds not pulses.[3] Today, the US produces over half of the world’s supply.[4],[5] It can be provided in many products including soy milk, tofu, natto, soy sauce, and tempeh. The soybeans extracted oil is used in several industrial applications with the main producers being India (4%), China (6%), Argentina (19%), Brazil (27%), and the United States (35%). Soy contains many nutrients, phospholipid, isoflavones, protein, lecithin, essential amino acids, and fiber.[1],[6]

    The bioavailability of soy’s bio-actives

    Soy beans (Glycine max) contain considerable amounts of nutrients and bio-actives. They contain the highest amount of Isoflavones in the range of 0.1–0.4% dry weight.[7],[8] According to the US Food and Drug Administration (FDA), “bioavailability” has been defined as the rate and extent to which the active ingredient of moiety is being absorbed from a product and becomes available at the action site.[9] Dietary Isoflavones have been the focus of many recent research projects that looked at the nutritional benefits of soy products. These substances accrue in relatively high concentration in soy beans along with purified soy proteins.[10] Compounds that are considered flavonoids include diadzein, genistein, and glycitein. When it comes to the bioavailability, isoflavone glucosides are less well absorbed than aglycones.[10]The bioavailability of isoflavones can be influenced by the food matrix, their hydrophobicity and susceptibility to degradation, and their chemical form in foods.[10]

    Research has found that the fermentation of soybeans can modify the amounts of isoflavones in the fermented soy. Also, fermented soy bean products are significantly different than those in unfermented soy products.[11] The amount of genistein in the fermented form of soy products, such as miso and natto, is higher than that in unfermented ones such as tofu and soy milk.[12] The proportion of the diadzein and genistein combined and presented as aglycones is less than 15% in soy milk while it can reach 50% in tempeh. Therefore, isoflavones are expected to be higher in fermented soy products compared with the unfermented ones.[10] Studies have indicated that diadzein and genistein are absorbed more quickly, while the bioavailability of glucosides is greater than aglycones. Also, the liquid matrix, such as soy milk, achieves a faster absorption rate than a solid matrix.[12] Low bioavailability of some flavonoids needs to be enhanced for full exploitation of their therapeutic benefits in preventing and treating diseases. More investigation is required to enhance the bioavailability and efficacy of some flavonoids using related techniques.[13]

    Soy and health

    Most of soybean’s contents have been studied, some of these contents had strong effects on health, thus its effects remain as a controversial issue.[1]

    The recommended serving of soy have been confirmed to be 1–2 servings/day.[14] This serving differs from one soy product to another as the amount of isoflavones varies from one product to another. One hundred grams of soybeans, tofu, and soy milk contain 2–221 , 4–67 , and 1–12  of isoflavones, respectively. Thus, the recommended amount of 25g/day of soy can be applied to its different products as 1 cup of tofu, 2 cups of soy milk, or 1 ounce of soy flour.[15] Many studies have been conducted to identify the effect of soy on health. For example, a significant improvement was noticed in the lipid profiles of Indian women taking soy protein; however, there was also an improvement in serum triglyceride level in those who were taking soy isoflavones. It has been observed that both soy protein and isoflavones are helpful in decreasing the postmenopausal symptoms among Indian women, but soy protein offers greater health effects due to its highly beneficial effect on lipid profiles.[15],[16]

    The effect of soy on heart

    Diet is considered as one of the modifiable factors that have a major impact on cardiovascular diseases [Table 1]. A low-fat high-protein diet has been shown to help in reducing the risk of heart disease.[8] In 1999, the FDA approved the health claim based on the role of soy protein in reducing the risk of heart diseases.[17] Studies indicate that soybeans are able to improve lipid profile by lowering serum cholesterol.[9] The insoluble fibers found in soy help in reducing the serum total cholesterol (TC) and LDL cholesterol by 88 and 84%, respectively. In addition, clinical studies found that when fibers are consumed. In addition, clinical studies found that when fibers are consumed, they bind to cholesterol in the small intestine preventing it from entering the bloodstream and then facilitate the excretion of the cholesterol through faces.[4] An intervention study group receiving 25 gm of isoflavones over a period of 6 months showed a reduction in cholesterol and LDL-cholesterol by 7.7 and 14%, respectively; however, no effect was found on HDL-cholesterol.[18] Soybeans are also a unique source of protein. Different biological functions are found in soy proteins such as daidzein and genistein. Moreover, the isoflavones serve as endocrine disruptors and phytoestrogens. They select the estrogen receptor modulators and help in the estrogen metabolites production in females. Structurally, these compounds are similar to the hormone estrogen, which are able to bind and cooperate with estrogen receptors for decreasing the concentration of serum cholesterol. This process happens through blocking some of those effects in some cells and stimulating the effects of estrogen in other cells.[10]
    Table 1: Types of cholesterol. (Rizzo and Baroni)[31].

    Click here to view


    Isoflavones help in reducing the level of both TC and LDL-cholesterol. Cholesterol is sensitive to estrogen, when the concentration of TC is increased, it means that the estrogen level is decreased. However, the mechanism is yet unclear. Furthermore, isoflavones proved its ability in decreasing the development and formation of a “plaque.”[10]

    Antioxidants also helps in reducing the risk of cardiovascular diseases among postmenopausal women with low levels of estrogen as soy products can counteract increased levels of serum cholesterol.[11] One study determined that isoflavones have significant effects on lowering, triglyceride, LDL and TC and increasing HDL cholesterol.[4] However, one study found that the high consumption of soy had no effect on the lipid profile.[5] According to the FDA, soybeans do not appear to have significant benefits, but, in general, soybeans are still beneficial for good health. While, regarding fibers, consuming 25g of soybeans daily may show some positive protection against cardiovascular health. But, in reality 25g of fiber cannot be achieved easily.[13]

    In contrast, a review of earlier studies found that the daily consumption of soy (32–200 ) had no effect on hormones; however, isoflavones have a small effect on estrogen. It appears that the major effect is in their ability in making anti-estrogenic-like effects.[18] Whereas, it remains uncertain toward the real mechanism behind the effect of soybeans on lowering cholesterol, especially that increased consumption of soybeans could lead to toxicity because of the increased amounts in antioxidants that serve as pro-oxidants and destroy body cells.[12]

    The impact of soy product intake on cancer

    A healthy diet plays an important role in decreasing the risk of cancer. Epidemiologic evidence indicates that in women who consumed soybean products, the risk of breast cancer declined almost by half.[19]

    It has been found that in postmenopausal women, the risk of breast cancer is increased because of high levels of estrogen[20] and its ability facilitating the growth of estrogen-sensitive neoplastic tumor cells. Whereas, the isoflavones content of soybean has a similar structure to estrogen (phytoestrogens) and able to do some estrogen-like effects as an alternative to estrogen therapy.[21],[22]

    Moreover, the fiber content of soy also helps in breast cancer prevention, since these fibers are able to bind to excess estrogen and get excreted out of the body. Whereas, fibers are also able to improve serum cholesterol levels.[23],[24],[25]

    On the other hand, researchers from the University of Hawaii have found no difference between decreased level of breast cancer and the high intake of soy products.[26],[27] Regardless of its ability to fight free radical molecules, they still can alter cancer cells which may lead to increasing the process of cancer cell division and growth.[28],[29] Another study showed that soybeans are not pure, there are still other undefined and unreliable components to consume. Since the mechanisms of these components are unknown, and there have not been enough published data, more researches are needed to determine the safety of regular consumption of soybeans.[22],[30]


      Conclusion Top


    The risk/benefit status of soy products as beneficial supplements continues to be controversial. To conclude, it can be confirmed that 12–25 gm of soy with 75 gm of isoflavones daily (2–4 servings of soy foods per day) can improve the estrogen level along with lipid profile and offer benefits to the cardiovascular system and estrogen deficit symptoms. Most populations should not exceed the recommended amount and the effectiveness of using soybeans should be matched with tolerance and compliance to reach the desirable improvements.

    The research report analyses and provides the historical data along with current performance of the global PP Pipe industry, and estimates the future trend of Soy Sauce on the basis of this detailed study. The data provided in this study can be used as a sole base for the upcoming researches.

    Ethical approval and consent to participate

    Not applicable.

    Acknowledgement

    This research was supported/partially supported Hamad Medical Corporation, dietetics department. We are thankful to our colleagues who provided expertise that greatly assisted the research.

    Financial support and sponsorship

    None.

    Conflict of interest

    There are no conflicts of interest.

    Data availability statement

    The data that support the findings of this study are available from the corresponding author (LM) upon reasonable request.



    [31]

     
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