Nutrition Matters by Rebecca Kourmouzi

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Is soy safe to consume? Positions of the scientific community

is-soy-good-for-you

Soybean is a type of legume that has East Asian origins, and its products have long been recognized as a source of high biological value protein and good fats. Over the last 25 years, soy has been investigated for its role in the prevention and treatment of chronic diseases including coronary heart disease and breast and prostate cancers. One reason why soy has stimulated the interest of both researchers and consumers is the fact that it is a rich source of isoflavones. Isoflavones are classified as both phytoestrogens and selective estrogen receptor modulators. This, coupled with certain results of studies, has led to concerns that soy consumption may have adverse effects on some individuals. However, these concerns are mainly based on in-vitro or animal studies, and clinical trials conducted in human subjects support the safety and benefits of soybeans in humans.

Why is there concern about the action of isoflavones?

Isoflavones have an estrogen-like chemical structure which allows them to bind to estrogen receptors and to exert an estrogen-like activity under certain experimental conditions. This is why they are also referred to as phytoestrogens. Although isoflavones are believed to provide certain health benefits, concerns have arisen that they may exert negative side effects in some individuals such as postmenopausal women, due to their estrogen-like properties. However, recently, and following a comprehensive, multi-year evaluation of the literature, the European Food Safety Authority (EFSA) concluded that isoflavones do not adversely affect the breast, thyroid and uterus in postmenopausal women. Recently, the North American Menopause Society concluded that isoflavones do not increase the risk of breast or endometrial cancer.

In any case, it is clear that isoflavones should not be equated with the hormone estrogen. This is because the literature is replete of clinical examples of differences between these two molecules. For example, unlike estrogen, isoflavones do not stimulate the vaginal maturation index or increase C-reactive protein (CRP). In addition, the classification of isoflavones only by their hormonal activity is an incomplete characterization, since they can also exert other physiological effects. Finally, not only isoflavones should not be equated with estrogen, but also soy must not be equated with isoflavones, since soy, like all other foods, is a collection of many biologically active molecules.

Positive effects of soy on human health

Effects on risk factors for cardiovascular disease

is-soy-safe-to-consume

Elevated LDL (bad) cholesterol is an established risk factor for coronary heart disease (CHD), and LDL-cholesterol lowering is the primary strategy to prevent CHD. The benefit of soy protein was initially recognized by the US Food and Drug Administration (FDA) in 1999. In particular, the FDA has determined that the consumption of 25g /day soy protein may result in a reduction in blood cholesterol by 4-6%. According to two systematic reviews and meta-analyses, in addition to lowering LDL cholesterol, soy protein can also lower blood triglycerides (approximately -7%) and increase HDL (good) cholesterol (approximately +1% -3%). In addition, some studies have also shown that soy protein can probably reduce blood pressure and that isoflavones improve the health of the arteries. In addition to the direct effects of soy protein on blood lipids, soy products can indirectly lower cholesterol as they can be used to substitute foods that increase LDL cholesterol and triglycerides, such as products of animal origin high in saturated fat.

Possible reduction in the risk of breast cancer

It is widely accepted that breast cancer rates in soyfood-consuming countries are much lower than in Western countries. Related are Asian epidemiological studies showing that higher soy consumption is associated with a reduction in breast cancer risk by one-third. However, important data suggest that, to reduce the risk, soybean consumption should start from childhood or adolescence. Specific studies show that high intake of soy from the early stages of life is associated with a reduction in the risk of breast cancer by 25% to 60%. In fact, it appears that the protection provided by isoflavones may be similar to the observed protective effect of early pregnancy against breast cancer. However, due to the different DNA but also the overall lifestyle, it is difficult to determine if soy is the only protective factor against breast cancer. Although the indications so far are encouraging, further and longer clinical studies in humans are necessary to give more accurate results.

Male fertility and possible reduction in the risk of prostate cancer

As with breast cancer, prostate cancer rates in Asian countries are lower than in Western countries. Studies in Asian populations show that higher soy consumption is associated with a reduction in the risk of prostate cancer by up to 50% of. However, most of these data come from case-control studies rather than longitudinal studies, so the positive effect of soy on prostate cancer cannot be confirmed with certainty yet. It is worth noting that neither soybean nor isoflavone exposure affects testosterone levels in men. All three clinical trials conducted so far show that isoflavones have no effect on sperm concentration or quality, so soybeans cannot be accused of causing infertility in men. 

Favors good kidney function

The potential benefits of soy are of great importance for public health due to the growing global prevalence of kidney disease, which is largely a consequence of the rising incidence of diabetes. There is preliminary evidence that soy protein can reduce kidney pressure as compared to animal proteins, which over time could reduce the risk of developing renal failure in susceptible individuals, such as those with diabetes. A recent meta-analysis of 12 clinical studies in chronic renal disease patients showed that soy protein significantly reduced creatinine, phosphorus, C-reactive protein and proteinuria. In individuals whose renal function is compromised, blood phosphorus levels are often abnormally high, so replacement of animal protein with soy protein may appear helpful.

Menopausal Symptoms

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Hot flashes are one of the main symptoms reported by menopausal women. A meta-analysis published in 2012 provides strong support for the effectiveness of isoflavones in reducing these symptoms. This analysis found that soy isoflavones statistically significantly reduced the frequency and severity of hot flashes by 20.6% and 26.2% respectively, compared to control groups. Approximately 40mg of isoflavones derived from soy beans provide the amount of genistein that appears to be effective in reducing these symptoms.

What should you pay attention to when it comes to scientific research results?

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There is a large volume of research, and widely divergent research results from in-vitro and animal studies, making it challenging for anyone to interpret the soy literature. However, it must be recognized that these two types of studies are of doubtful relevance to understand the effects of soybeans on humans. Obviously, the conditions in an in-vitro study (a process taking place in a test tube, culture dish or elsewhere outside a living organism) do not represent the complexity of living organisms such as humans. Also, in-vitro studies necessarily examine the effects of isolated compounds, such as isoflavones in soybeans, which may be quite different from the effects observed when these compounds are tested in their natural environment. In addition, the biological effect of a food ingredient may be affected by the presence of other ingredients in the same food, which in in-vitro studies cannot be examined.

Additionally, studies in mice and rats are often of limited value for predicting effects in humans due to the many physiological and anatomical differences between rodents and humans. Indeed, in the case of soybean, there is an additional difference. Most animals, including rodents, metabolize isoflavones very differently from humans. Consequently, the results of animal-based researches cannot give meaningful answers to the effect of soybean on humans.

For all the above reasons, consumers should be cautious when interpreting the results of any research, taking into account all conflicting factors.

Key points

  • Soy protein is higher in quality than other legume proteins, and soy is a good source of monounsaturated and polyunsaturated fatty acids.
  • Replacing animal protein sources with soy may result in a favorable change in the saturated fat content of the diet.
  • Only 25g / day of soy protein can directly lower LDL-cholesterol levels in the blood (4-6%), and possibly moderately lower blood pressure.
  • There is scientific evidence that isoflavones in soy relieve hot flashes and improve arterial health in menopausal women, as well as preliminary indications that they can reduce the risk of breast and prostate cancers.
  • Concerns that the estrogenic properties of isoflavones cause undesirable effects in some individuals, such as postmenopausal women, are not supported by clinical and epidemiological research.
  • The evidence so far suggests that soy can be safely consumed by all individuals except those who are allergic to soy protein, something that is relatively unusual.
  • When consuming soy products, it is important to take into account the overall nutritional quality of a food product, as several products include a variety of ingredients other than soybeans.

Bibliography:

  1. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature

  2. Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile.
  3. Systematic review, meta-analysis and regression of randomised controlled trials reporting an association between an intake of circa 25 g soya protein per day and blood cholesterol.
  4. Soy protein effects on serum lipoproteins: A quality assessment and meta-analysis of randomized, controlled studies.
  5. Childhood soy intake and breast cancer risk in Asian American women.

  6. Phytoestrogens and risk of prostate cancer: A meta-analysis of observational studies.

  7. Beneficial effects of soy protein consumption for renal function. 

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