Is it safe to eat soy? Can children consume soy? Will soy make men feminine? Does soy cause breast cancer? These are all questions that you may be concerned about, due to having heard different information about whether it’s ok to eat soy foods.
Soybeans are a type of legume which are commonly consumed as either whole soybeans, edamame beans, or products made from soybeans including tofu, tempeh, and soymilk, as well as processed soy foods such as soy ice-cream and soy sausages.
Nutritionally, soybeans are a great source of protein and contain all essential amino acids. They are a good plant source of omega-3 fatty acids, and provide fibre, iron, zinc, potassium, vitamin E and folate. They also contain beneficial nutritional components including phytoestrogens, plant sterols, and prebiotics (a type of fibre that promotes good gut bacteria). In refined soy products however, such as isolated soy protein, most of the phytoestrogen content can be lost in processing. Fortified soy products, such as soymilk and some types of tofu, are high in calcium.
Soybeans contain phytoestrogens, mainly in the form of isoflavones, which are oestrogen-like substances. Phytoestrogens are similar to oestrogen but do not operate in the same way in the body. There are two oestrogen receptors in the body – ERα and ERβ. Oestrogen binds to both these receptors while isoflavones bind only to ERβ. This creates an important difference between the effects of oestrogen and isoflavones as ERα and ERβ have different and sometimes opposite effects when activated.
The effect of phytoestrogens has been poorly understood in the past and the source of previous concerns with soy intake and health. However, current research now shows soy and phytoestrogens offer many protective health benefits.
This nutritional profile of soy makes it a valuable addition to the diet, and research has demonstrated the health benefits of soy foods. Outlined below is what the current research findings are in relation to eating soy.
Cholesterol and heart disease
Eating soy foods has been shown to reduce LDL (bad) cholesterol levels and therefore reduce the risk of cardiovascular disease. Many reviews have been conducted analysing the results of research studies involving thousands of participants. The evidence indicates that soy protein lowers LDL cholesterol by 4-6%. This is a statistically significant reduction that leads to a reduced risk of heart disease.
Other cardiac health benefits of consuming soy are being researched with some possible protective effects on blood pressure and coronary heart disease risk.
Several countries have approved the use of health claims in relation to soy protein lowering cholesterol levels. In order to reduce cholesterol it is recommended to consume 25g of soy protein per day. This can be achieved by eating 2-3 serves of soy each day. See below for the protein content of soy foods.
Research indicates that soy is protective against breast cancer, particularly when consumed as a child or teenager. The risk of developing breast cancer was shown to be reduced by 25-60% for those having a higher intake of soy in early life.
The earlier concern held regarding soy, particularly isoflavones, stimulating breast cancer cell growth, originated from experiments conducted in animals. This finding has since been further investigated and is now discredited. Current research confirms that soy and isoflavones do not have harmful effects on breast tissue or reproductive hormones in women.
Significant benefit is also shown for women who have breast cancer. A study looking at more than 11,000 women with breast cancer identified a significant reduction in breast cancer recurrence (the cancer returning) and deaths among those who ate soy after their diagnosis of breast cancer. Additionally, the World Cancer Research Fund International states there is some evidence that eating foods containing soy is linked to better survival after breast cancer.
Soy infant formula
Concerns around potential harmful effects on babies consuming soy infant formulas have not been confirmed in research. The exception is infants with congenital hypothyroidism who will require monitoring of thyroxine levels if consuming soy formula, in order to avoid an abnormal thyroid function.
Adults fed soy formula as babies compared with those fed cow’s milk formula have been shown to have no significant differences in adulthood. Regarding high phytoestrogen contents of soy infant formula, almost all (~94%) of the phytoestrogens are in a biologically inactive form.
Soy formulas have been available for decades and widely consumed, with no adverse effects noted on growth or development.
Many studies on men have confirmed there is no impact on testosterone levels from soy or isoflavones, and no impact on oestrogen levels from isoflavones even at high doses (up to 150mg/day).
Neither soy nor isoflavones have been shown to have negative effects on reproductive hormones in women.
With respect to children, while there is only limited research to date, it also suggests there is no effect on hormone levels in children from soy or isoflavones.
Of the limited research into soy and fertility, results show either no impact or positive results on reproduction. One study of women undergoing assisted reproductive technology showed isoflavone intake to have a positive association with birth rates. Studies on sperm concentration and quality show no effect from isoflavones.
Women consuming soy in their diet have been shown to experience less menopausal symptoms, particularly hot flushes. This is attributed to the phytoestrogen content of soy. Many studies have been conducted on the impact of hot flushes, with consumption of phytoestrogens from soy resulting in a 20% reduction in the frequency of hot flushes and a 26% reduction in severity.
The greatest benefit was seen with the higher intakes, about 40mg isoflavones per day, and from a whole soy food source. See below for the isoflavone content of soy foods.
Other health effects
Studies indicate that soy intake may reduce the risk of endometrial cancer, while there are possible benefits for depression and kidney disease though further research is needed.
GM (Genetically Modified) soy
Foods produced from genetically modified organisms are called GM foods. A large amount of GM soy is used in US food products though this is not the case in Australia. Australian labelling laws stipulate it is mandatory to list GM ingredients on the packaging if they make up more than 0.1% of the product. If you prefer to avoid GM foods, check the product label for GM ingredients.
How much to eat?
Health benefits can be obtained from consuming 2-4 serves of soy per day. Whole soy foods or minimally processed soy foods are recommended rather than highly processed soy products or soy supplements.
|Soy food||Protein (mg)||Isoflavones (mg)*|
|Soy milk (1 cup/250ml)||8||2-27|
|Tofu, firm (100g)||15||23-35|
|Tofu, soft (100g)||8||18-30|
|Soy & linseed bread (1 slice)||6||6|
*Isoflavone content can vary greatly, and is affected by such factors as soil temperature and moisture content.
So what’s the bottom line? Yes, you can safely include soy as part of a balanced diet. This will provide you with a range of beneficial nutrients such as protein, omega-3 fats, fibre, and phytoestrogens. You may also achieve positive health impacts such as lowered cholesterol, relief from menopausal hot flushes, or a reduced risk of breast cancer.
So try adding soy foods to your diet. There are many types to choose from and lots of delicious ways to include them in your meals.
Messina, M. Soy and health update: Evaluation of the clinical and epidemiological literature. Nutrients. 2016. 8 (12), 754.
World Cancer Research Fund International. Continuous Update Project: diet, nutrition, physical activity and breast cancer survivors (2018). https://www.wcrf.org/dietandcancer/breast-cancer-survivors (accessed 15th October, 2018)
Canadian Paediatric Society. Practice point: Concerns for the use of soy-based formulas in infant nutrition (2016). https://www.cps.ca/en/documents/position/use-soy-based-formulas#ref7
Food Standards Australia New Zealand (FSANZ). General information about GM foods. http://www.foodstandards.gov.au/consumer/gmfood/gmoverview/Pages/default.aspx (accessed 19th October, 2018)
United States Department of Agriculture. National Agricultural Library. USDA database for the isoflavone content of selected foods, release 2.0 (2015). https://data.nal.usda.gov/dataset/usda-database-isoflavone-content-selected-foods-release-20/resource/5cdf071c-4cde-4df8-87d6 (accessed 19th October, 2018)
Rizzo, G; Baroni, L. Soy, soy foods and their role in vegetarian diets. Nutrients. 2018. 10 (1), 43.