The Effects of Soy Isoflavones on Breast Cancer Risk
Abstract
In the last three decades, there has been conflicting evidence on the health benefits and risks of soy intake, specifically concerning breast cancer. Because the isoflavones in soy have a similar structure to estrogen, there is fear that consuming soy could raise one’s risk of developing cancer. The low rate of breast cancer in Asian countries has frequently been related to a greater consumption of soy, although it might be necessary to consume soy from childhood to produce these preventative results. By comparing meta-analyses and examining literature on the topic, this research aims to demonstrate that soy isoflavone intake does not have a negative effect on breast cancer development or recurrence. There is substantial evidence that moderate amounts of soy consumed on a regular basis will either reduce or have no impact on breast cancer development and recurrence, and that the fears of soy isoflavones causing breast cancer are unfounded.
Introduction
The isoflavones found in soy have a similar structure to estrogen, which causes fear that consuming soy could raise one’s risk of developing breast cancer (Messina, 2016). Ju et al. (2001) released a study that soy isoflavones could stimulate the growth of estrogen-dependent human breast cancer (MCF-7) tumors implanted in mice. While this worried many about the safety of consuming soy for breast cancer survivors, there have now been many studies that show that it does not negatively effect breast cancer development or recurrence and that it may actually lower one’s risk of contracting the disease. Low rates of breast cancer is Asian countries has frequently been related to high consumption of soy (Chi et al., 2013). However, current research has been inconclusive in relating this association to Western populations. This could be due to the higher amounts of soy consumed in Asian populations or the early age from which Asian women typically begin eating soy. While studies have not found that soy intake definitively lowers breast cancer risk in Western women, the researcher found no evidence of a negative association either, suggesting that soy is a safe food for women with breast cancer and those at high risk of developing it.
Methods
Using the search parameter “breast cancer AND soy AND isoflavones,” the researcher identified studies that address the effect of soy isoflavones on breast cancer risk. To narrow the results, the researcher limited the articles used to those no more than seven years old, aside from one older article used for comparison. This study includes three meta-analyses that seek to compare isoflavone intake and breast cancer risk. Another article specifically shows the difference between soy isoflavone metabolism for rodents and humans, which is relevant to the older study that claims that isoflavones caused tumor growth in mice. Several other review articles are included that contrast data from Japanese and Western women and examine the effect of soy isoflavones on breast cancer risk.
Results
There are many health benefits to consuming soy. It can alleviate menopause symptoms, reduce the chance of prostate cancer in men, and reduce the risk of coronary heart disease (D’Adamo & Sahin, 2014). Additionally, soy is an excellent source of protein that is often an important component of vegetarian and vegan diets. In the early 1990s, researchers also began to document the chemopreventative and potentially positive effects of soy isoflavones on breast cancer risk (Wu, Lee, & Vigen, 2013). However, because the isoflavones found in soy have properties similar to estrogen, these beneficial concepts were soon after replaced with fears that soy could have negative effects on women with estrogen-sensitive breast cancer.
Early studies on rats and mice fueled these concerns. Ju et al. (2001) conducted a study that showed that isoflavones stimulated the growth of estrogen-dependent human breast cancer implanted in mice. Healthcare providers began recommending that breast cancer patients avoid consuming soy for fear that it could cause tumor growth and recurrence. However, more recent studies have shown that these ideas may have been misguided. For example, Setchell et al. (2011) conducted a study to see if any differences existed in how humans and rodents metabolize soy isoflavones, and determine whether or not mice were a representative depiction of how soy affected human breast cancer. They found significant differences in the metabolism of the two species, casting “doubt on the value of the use of these rodents for gaining insight into the effects of isoflavones in humans, especially with regard to the effects on breast tissue.” They determined that it was no longer necessary for high-risk women and women with breast cancer to avoid consuming soy.
Soy has also been a healthy staple in the Asian diet for centuries, and many studies have found that soy consumption lowers breast cancer risk in Asian populations. The meta-analyses that this researcher reviewed, however, do not provide evidence to support the same beneficial properties of soy in Western women. Chen et al. (2014) determined that soy intake did lower breast cancer risk for both pre- and post-menopausal women in Asian populations, and found a weak protective effect in Western populations. Yet the evidence was not strong enough to definitively determine that a relationship existed. Similarly, Dong and Qin (2011) found that “soy isoflavones intake is associated with a significant reduced risk of breast cancer incidence in Asian populations, but not in Western populations.” Although it is generally believed that soy consumption lowers the risk of breast cancer in Asian women, current research has not been able to determine that the same is true for women in Western countries.
There are a few possible explanations for this discrepancy. First, the amount of soy consumed is much higher in Asian populations than in Western countries. For instance, data from the UN Food and Agriculture Organization in 2003 found soy consumption in most European and North American countries to be less than one gram of soy protein per day, whereas the Japanese were consuming an average of 8.7 grams each day (Barrett, 2006). Also, soy is eaten from a much younger age in Asian countries. It is possible that if soy is consumed from childhood throughout adulthood, it could enhance the preventative qualities of isoflavones and reduce breast cancer development later in life (Messina, 2016). Furthermore, Asian populations typically consume a higher percentage of soy from whole foods like tofu or miso, rather than from processed soy products or supplements. The type of soy consumed may have an impact on the positive effects that it produces (Nagata, 2010). Less wholesome forms of soy may also be less beneficial to one’s health. Highly processed soy can lose up to 80% of its isoflavone content (Messina, 2016). Asian populations typically eat a higher percentage of whole soy foods, from an earlier age, and in much larger quantities, all of which may produce more beneficial effects.
It is still possible that soy intake could have positive effects for adult women in Western countries, although the results are not conclusive. Messina (2016) cites studies of over 11,000 women with high levels of soy intake who were followed for several years and showed reduced rates of recurrence and mortality compared to those who consumed less soy. These results could lead one to believe that soy intake is inversely related to breast cancer risk. In contrast, Braakhuis et al. (2016) found only “65% of studies reporting no effect or slightly protective against breast cancer risk,” and that the protective effects of soy may be more pronounced in post-menopausal women. While soy isoflavones may help prevent breast cancer recurrence in Western women, more research is needed.
Menopausal status might have an impact on the positive effects of soy. When comparing mortality and recurrence with menopausal and estrogen receptor (ER) status, one meta-analysis found that soy intake was associated with lower mortality regardless of menopausal status, but only “associated with lower recurrence in ER negative, ER+/ PR+, and postmenopausal patients (Chi et al., 2013).” Therefore, soy intake may be more likely to reduce mortality from breast cancer, but not necessarily recurrence for pre-menopausal women. Dong et al. (2011) found similar results that the positive effects of soy consumption on breast cancer development and recurrence were stronger for post-menopausal women. Conversely, another review found that soy consumption was actually more likely to decrease the risk of breast cancer in pre-menopausal women, and that it consistently decreased both the risk of mortality and recurrence (D’Adamo & Sahin, 2014). While these studies may not reach a definitive conclusion, they did not find that soy intake increased the risk of breast cancer.
Discussion
While the studies on the positive effects of soy are sometimes contradictory and inconclusive, the researcher has found no current evidence showing that soy has negative consequences on breast cancer development or recurrence. The original studies that showed that soy caused tumor growth in rats should be interpreted with caution. Rats are not a representative tool to measure the effect of soy on breast cancer, as they metabolize isoflavones differently than humans (Setchell et al., 2011). It is possible that soy may lower one’s risk of developing breast cancer, as seen in Asian populations. However, current research has not found the same beneficial effects in Western women.
Post-menopausal women already diagnosed with breast cancer have sometimes shown that soy can lower the risk of recurrence or mortality. The results for healthy women at risk of developing breast cancer are less conclusive. Soy intake has not necessarily been found to be beneficial to women pre-diagnosis, although recent studies have not found it to be harmful either. It has been suggested that consuming soy early in life could lower one’s risk of developing breast cancer, yet the results are still speculative (Messina, 2016). Further research is necessary to determine if this correlation exists, and would require long-term studies following women with varying levels of soy intake over many years. At this time, soy consumption does not appear to have a negative effect for adolescent women and can most likely be consumed safely in moderation.
These findings support the research thesis that dietary soy intake does not have negative consequences on breast cancer development or recurrence and may possibly lower one’s risk of contracting the disease. Even though the current research does not definitively prove that soy consumption lowers one’s risk of breast cancer development, it does show the absence of negative effects. Soy is a nutritious food that can be beneficial to women’s health in many ways. There is substantial evidence that moderate amounts of soy consumed on a regular basis will either reduce or have no impact on breast cancer risk, and that the fears of soy isoflavones causing breast cancer are unfounded. Consumed in moderation, soy can be a safe and healthy food for women.
Conclusion and Recommendations
Although it is still unclear whether or not soy intake lowers one’s risk of breast cancer, the current research does not show that soy has negative effects on breast cancer development or recurrence. If this research continues and soy intake is definitively found to inversely affect breast cancer risk, high soy consumption could be promoted in the future for breast cancer patients and survivors. However, the acceptance of soy in the healthcare system is not quite at that level yet and more research is required. The inconclusive nature of these findings demonstrates the need for future research before any changes to breast cancer treatment or prevention could be made. While these findings show that women can safely consume soy without increasing their chances of breast cancer, the evidence does not currently exist to promote soy consumption for preventative effects, or demonstrate that high amounts of soy intake are healthy for women.
To determine whether or not soy intake in childhood could prevent breast cancer development in adulthood, future research could study high-risk young women with varying levels of soy intake over many years to see if their breast tissue changed and whether or not they developed the disease. This would not be an easy study to carry out. However, it could answer the question as to whether or not early consumption accounts for the difference in breast cancer risk between Asian and Western populations. If this hypothesis were true, it could potentially change the way our children eat. Soy consumption could be encouraged from a very young age, similar to Asian populations, and perhaps meat intake could even decrease.
In the last three decades, there has been a fear among women with breast cancer and women at high risk of developing breast cancer about consuming soy. Current research is proving that soy intake does not have negative effects on breast cancer recurrence and development. Soy isoflavones may even lower one’s risk of contracting the disease. It is unfortunate that women with and without breast cancer avoid soy unnecessarily. Hopefully this research can dissuade these fears and allow women to confidently consume moderate amounts of soy, which will be beneficial to their health in many ways.
References
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