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.
Why is there concern about the action of isoflavones?
Isoflavones have an estrogen-like chemical structure which allows them to exert an estrogen-like activity under certain experimental conditions. This is why they are also referred to as phytoestrogens. Due to their estrogen-like properties, concerns have arisen that they may exert negative side effects in some individuals such as men and postmenopausal women.
However, 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 the human hormone estrogen, isoflavones do not stimulate the vaginal maturation index or increase C-reactive protein (CRP). It is now well established that in the human body, isoflavones do not behave like the human hormone estrogen, since they are weaker, they only have effects on some body tissues, and in some cases they can even have opposite effect to estrogen.
Recently, and following a comprehensive, multi-year evaluation of the literature, the European Food Safety Authority (EFSA), the World Cancer Research Fund and the World Health Organisation conclude that consuming soy foods as part of a healthy balanced diet is safe, and that soy does not adversely affect the breast, thyroid and uterus in postmenopausal women.
Soy and human health
Source of many essential nutrients
Soy beans contain a range of nutrients including high quality plant protein, fibre, the essential omega-3 and omega-6 fatty acids, and a range of vitamins and minerals including vitamin B6, iron, calcium and magnesium. The plant protein of both fermented and unfermented soy foods is high in a range of essential amino acids and similar to animal protein. Most commonly consumed soy foods include soy meat replacements (soy chunks or soy mince), and related products such as burgers and sausages, tofu, dairy alternatives to yogurts, milk and desserts fortified with calcium as well as vitamins B12 and D, and ice-cream.
Reduced cardiovascular disease risk
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 25gr of soy protein per day 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 may also lower blood triglycerides (approximately by 7%) and increase HDL (good) cholesterol (approximately by 1% -3%). Some studies have also shown that soy protein can probably reduce blood pressure, and that isoflavones can 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 breast cancer risk
It is widely accepted that breast cancer rates in soy food-consuming countries are much lower than in Western countries. Asian epidemiological studies have shown 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 in soy may be similar to the protective effect of early pregnancy against breast cancer. Although the indications so far are encouraging, further and longer clinical studies in humans are necessary to give more reliable results.
It is worth noting that a review by the American Institute for Cancer Research, the American Cancer Society and the World Cancer Research Fund indicates that soy foods as part of healthy balanced diet are perfectly safe for women with and without cancer diagnosis.
Male fertility and possible reduction in prostate cancer risk
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 prostate cancer risk by up to 50%. 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 or estrogen levels in men. All three clinical trials conducted so far show that isoflavones have no effect on sperm concentration or quality. No adverse effects on men’s fertility or sexual health have been reported. Controversy around this has been fueled by animal and laboratory studies, which cannot be compared to any human outcomes. Just two case studies have been conducted using human subjects, where men consumed high doses of soy foods and followed an unbalanced diet
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-derived protein, 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 or creatinine levels are often abnormally high, so replacement of animal protein with soy protein may appear helpful.
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 provide the amount of genistein that appears to be effective in reducing these symptoms. This amount is equivalent to 2 glasses of soy milk or 100gr soy mince. During the menopause, women’s risk of heart disease is doubled, and consuming soy foods may also help lower the risk.
A review of 14 studies has confirmed that there is no harmful effect of soy consumption in healthy humans with a normal functioning thyroid gland. Although soy foods do not affect thyroid function, they can interfere with the absorption of thyroid medication levothyroxine, just like many herbs, drugs, fibers and calcium supplements. For this reason, thyroid medication is always advised to be taken on an empty stomach. Soy should not be excluded by those with an under-active thyroid gland as their medication dose can easily be modified.
Well-planned completely plant-based, or vegan, diets need just one third of the fertile land, fresh water and energy of the typical British ‘meat-and-dairy’ based diet. With meat and dairy being the leading contributor to greenhouse gas emissions, reducing animal based foods and choosing a wide range of plant foods such as soy can be beneficial to the planet and our health.
The UK government’s new dietary recommendations, the Eatwell Guide (2016), clearly recommends a significant shift towards more plant-based eating both for the nation’s health as well as a more sustainable planet. Over 75% of the Eatwell Guide is dominated by plant foods with the most notable change to the protein section, where plant-based proteins are recommended over animal proteins.
What should you pay attention to when it comes to any study results?
It must be recognized that the controversy around isoflavones’ potential to have negative side effects in humans is fueled only by findings from in-vitro or animal studies using pure isoflavones or high doses. These types of studies are of doubtful relevance to understand the effects of isoflavones on humans. Obviously, the conditions in an in-vitro study (a laboratory process taking place in a test tube or culture dish, outside a living organism) do not represent the complexity of living organisms such as humans. Also, in-vitro studies examine the effects of isolated compounds, such as isoflavones, which may be quite different from the effects observed when these compounds are tested in their natural environment. Let’s also not forget that the biological effect of a food ingredient, isoflavones in this case, may be affected by the presence of other ingredients in the same food, something that in in-vitro studies cannot be examined.
Moreover, 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 isoflavones, there is an additional difference. Most animals, including rodents, metabolize isoflavones very differently from humans. Consequently, the results of animal-based studies cannot give meaningful interpretations to the actions of isoflavones on humans.
Finally, using high doses of pure isoflavones cannot be compared to consuming isoflavones from whole soy foods, as soy foods provide lower quantities and are a combination of many biologically active molecules.
Soy is a nutritious, safe and palatable part of the diet which fits well with healthy eating guidelines.
Substituting meat with plant proteins like soy can be beneficial for the planet and our health.
Soy provides high quality plant protein. Soy and soy products are a good source of omega-3 and omega-6 fatty acids, as well as calcium. Most soy products are also now fortified with calcium and vitamins B12 and D.
Replacing animal protein with soy can result in a favorable change in the saturated fat content of the diet. It is also thought that soy protein helps to lower the body’s natural LDL cholesterol producing capacity in the liver.
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 infertility or lower testosterone levels in men, 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.
Soy milk is suitable for children over six months of age and is a great alternative to cow’s milk for adults with lactose intolerance and with other adverse reactions to cow’s milk.