In the meantime, soya has been linked to a lower risk of heart disease compared to other diets. As a good source of protein , unsaturated fatty acids, B vitamins, fibre, iron, calcium and zinc, it is becoming increasingly popular in the West as a healthy substitute for meat. But despite more people associating soya with health over the last decade, one fear has come to the fore: the idea that soya can disrupt our hormones.
The controversy around soya comes down to its uniquely high content of isoflavones. These compounds have oestrogenic properties , which means they act like oestrogen, the primary female sex hormone, and bind to oestrogen receptors in the body — and oestrogen can fuel the growth of some types of breast cancer.
And often, it seems soya protects against cancer risk — rather than making it worse. To start, there are the observational findings.
Soya is often consumed as part of a healthy diet and as a substitute for red meat, which is associated with a higher risk of heart disease and cancer. Soya products often replace foods like red meat, which could be why soya intake is associated with healthier outcomes Credit: Getty Images. This means a reduced risk of breast cancer could have been due to lower BMI, not to soya consumption. Without this process, damaged cells can form into cancer.
If products like soya milk lower breast cancer risk, it may be by bolstering the mechanism that tells cells to self-destruct when they get DNA damage Credit: Getty Images. Whether reading a popular press article or a well-designed clinical study about soy, some debate remains.
As a species within the legume family , nutrition scientists often label soy as a food with potential for significant health benefits. However, due to contrary research that suggests possible negative effects of soy in certain situations, there has been a hesitancy to wholeheartedly promote soy.
Soy is unique in that it contains a high concentration of isoflavones, a type of plant estrogen phytoestrogen that is similar in function to human estrogen but with much weaker effects. Soy isoflavones can bind to estrogen receptors in the body and cause either weak estrogenic or anti-estrogenic activity.
The two major soy isoflavones are called genistein and daidzein. Soy isoflavones and soy protein appear to have different actions in the body based on the following factors:. Thus, there are many factors that make it difficult to construct blanket statements about the health effects of soy. That said, aside from their isoflavone content, soy foods are rich in nutrients including B vitamins , fiber , potassium , magnesium , and high-quality protein. Unlike some plant proteins, soy protein is considered a complete protein, containing all nine essential amino acids that the body cannot make which must be obtained from the diet.
Soy foods are also classified as fermented or unfermented see table with examples, below. Fermented means that the soy food has been cultured with beneficial bacteria , yeast, or mold. Soy protein took center stage after research showed that it might lower levels of harmful cholesterol.
However, a number of studies since have tempered that finding. Even though soy protein may have little direct effect on cholesterol, soy foods are generally good for the heart and blood vessels if they replace less healthful choices, like red meat, and because they deliver plenty of polyunsaturated fat, fiber, vitamins, and minerals, and are low in saturated fat. Hormone replacement therapy has traditionally been used as an effective treatment for hot flashes and other unpleasant symptoms that accompany menopause, but its long-term use carries concerns of an increased risk of some diseases including breast cancer and stroke.
Soy has been a popular alternative treatment but not clearly supported by research; in theory the potential estrogenic effects of soy isoflavones could help to tame hot flashes by giving an estrogen-like boost during a time of dwindling estrogen levels. In Asian countries where soy is eaten daily, women have lower rates of menopausal symptoms, although research is conflicting as to whether soy is a primary contributor.
Yet several meta-analyses and carefully controlled clinical studies have not found strong evidence of a link. A review of 43 randomized controlled trials have examined the effects of phytoestrogens on hot flashes and night sweats in perimenopausal and postmenopausal women. Four trials found that extracts of 30 mg. Other trials that used dietary soy or soy extracts suggested a reduced frequency and severity of hot flashes and night sweats when compared with placebo, but these trials were small with a possible strong placebo effect.
Another meta-analysis of 16 studies found that soy isoflavone supplements had a small and gradual effect in weakening menopausal hot flashes compared with estradiol human estrogen. However, the authors noted weaknesses in the analysis due to a small number of participants and high variability in study design. This area needs further research as questions remain about a possible benefit of soy.
Results are conflicting, potentially due to variation in the types of soy preparations used, the quantities given, and for how long they are used.
In some tissues and in some people, they may block the action of estrogen. Studies so far have not provided a clear answer. Some have shown a benefit with soy consumption and breast cancer while others show no association. In animal and cell studies, high dosages of isoflavone or isolated soy protein extracts tend to stimulate breast cancer growth.
Women from Asian countries appear to receive greater protective benefit from breast cancer with high soy intakes than American and European women, but this may simply be a difference in the amount of soy consumed. The Breast Cancer Family Registry was a prospective study following 6, women diagnosed with breast cancer and living in the U.
Another prospective study followed 1, American women who were breast cancer survivors for six years. The incidence of prostate cancer is highest in Western countries and lowest in Asian countries, where soy foods are a regular part of the daily diet.
In addition, observational studies have found an increased risk of prostate cancer in Chinese and Japanese men who move to Western countries and adopt a Western diet, but not in those who continue eating a traditional diet.
Natural soy foods contain isoflavones, which are similar to estrogen; thus my clients often think soy will raise their risk for hormone-sensitive cancer, such as breast cancer. Studies exploring the relationship between soy intake and breast cancer have been mixed, but recent research suggests soy does not promote the development or progression of breast cancer.
According to the American Institute for Cancer Research, consumption of soy among breast cancer survivors, at levels of 1 to 2 serving per day, did not increase their risk for poorer outcomes.
Research on soy and heart health is promising. Consider a soy-based meal once a week in place of meat, but use caution with meat alternatives such as soy crumbles, burgers, or sausage, as these often have a lot of added salt.
Food and Drug Administration promotes it in foods to reduce the risk of heart disease. School lunch programs across the country are even adding soy to hamburger patties. But animal studies suggest that eating large amounts of those estrogenic compounds might reduce fertility in women, trigger premature puberty and disrupt development of fetuses and children.
Although most studies looking at the hormone-disrupting properties of genistein , the main isoflavone in soy, have been conducted in rodents, many scientists believe the findings may be relevant to humans as well. More definitive answers, she said, may lay ahead in future long-term human studies. Soy consumption in the U. Clinical studies have shown that eating soy can lower cholesterol as well as the risk for certain types of breast and prostate cancer.
But Newbold and other researchers are not convinced that eating more soy is healthy for everyone. Infants fed soy formula ingest six to 11 times more genistein on a bodyweight basis than the level known to cause hormonal effects in adults. In some lab studies, animals were fed doses similar to what people might get from a high-soy diet, which would be roughly 25 or more grams per day.
Blood levels of genistein in people eating a lot of soy are generally in the range of one to five micromoles, or about one milligram of genistein circulating in the body of an average adult.
One study showed that genistein led to reduced fertility and abnormal embryo development in female mice. They were fed one to ten micromoles in their drinking water for four days. The highest doses were associated with fewer eggs that were successfully fertilized and increased cell death in developing embryos.
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