Soy Good or Soy Bad?

Posted in Controversy

Soy Good or Soy Bad?

Thank you to our nutrition student (now RD), Shandelle Shroeder, for summarizing a difficult topic!

Infertility? Thyroid disorder? Breast cancer?!  The media is plagued with mixed messages about soy.  This article will address those concerns and help sort out the myth from the fact so you can make an informed decision.   

What is Soy?

Soy beans are a legume and considered a Meat and Alternative food source.  They are a high-quality, plant-based source of protein and a good source of minerals such as calcium, iron, and zinc.  Soy contains phytoestrogenic compounds, known as isoflavones, which are antioxidants that may have health benefits.  The isoflavone content varies between different types of food, for example, a serving of edamame (green soybeans) has a higher amount of isoflavones than further processed soy products such as soy sauce.   

What foods are Made from Soy?

Soy-based foods include soybeans (yellow, black, and brown varieties), edamame (green soybeans), tofu (soybean curd), tempeh (fermented soybean cake with a smoky flavour), soy protein products (vegetarian meat replacements), isolated soy protein (protein supplement), soy beverages (ground-up soybeans made into a milk alternative), natto (sticky fermented soybeans with cheese-like flavour), miso (fermented soybean paste), and soy sauce (fermented soy bean).  Note: soy sauce is high in sodium and doesn’t offer the same health benefits as other soy products.

Soy protein isolate is a common additive in many foods.  You will find it on the ingredient lists of muscle gain fitness products, power bars or granola bars, baked goods, breakfast cereals, soups, sauces, and bottled fruit drinks. 

Why the Hype?

Many of the concerns around soy are because the isoflavones are phytoestogenic compounds, meaning they mimic the effects of estrogen.  This has fueled the public’s belief that eating soy products will lead to the development of feminine qualities or infertility in men.  The research does not support these claims. 

Soy consumption has been evaluated for its effects on levels of follicle stimulating hormone (FSH - a measure of infertility in men), testosterone (high levels are a risk factor for prostate cancer; low levels are associated with infertility), and other reproductive measures.  Study results indicate eating soy foods does not negatively affect fertility in men.

The evidence around the safety of soy for breast cancer survivors continues to be controversial.  In vitro (in test tube) and in vivo (in body) experimental studies have shown an increase in cell tumor growth - a theoretical risk of soy consumption in women with breast cancer.  However, this relationship has yet to be demonstrated with proper clinical trials.  More recent research indicates it is not necessary to avoid soy isoflavones in women who have (or previously had) estrogen-sensitive breast cancer, and the Canadian Cancer Society suggests that consuming 3 servings of soy-based foods daily, is safe.  One serving is equivalent to: 1 cup soy milk, ½ cup soybeans, or ¾ cup (150g) tofu. 

An area requiring more research is the relationship between high isoflavone exposure in infants consuming soy formula and risk of long-term adverse health effects such as altered reproductive function, immune function, gastrointestinal effects, and cancer.  Current guidelines recommend the use of soy formulas only when clinically necessary, specifically for a genetic disorder called galactosemia.

Another area of concern is the effect of soy isoflavones on thyroid function.  Ultimately, a moderate intake (ie. 3 servings daily) of soy food has no impact on individuals with normal thyroid function.  The concern relates to high doses of soy isoflavone from supplementation, which may perpetuate clinical hypothyroidism in individuals with pre-existing sub-clinical hypothyroidism.  Individuals taking thyroid replacement hormones for hypothyroidism should be mindful of intake of soy foods because of a potential drug-nutrient interaction where soy may decrease medication absorption.  Talk to your doctor to reassess thyroid function if your intake of soy foods changes substantially. 


The isoflavones in many soy products are capable of antioxidant and anti-inflammatory properties that may help reduce cancer growth, insulin resistance, blood pressure, and inflammatory markers.

Heart health and breast cancer are two areas of focus where soy has demonstrated a beneficial role.  Part of the heart health benefits is associated with replacing high-fat animal and dairy products with soy foods.  At least two daily servings of soy-based foods are needed to see a decrease in LDL (lousy) cholesterol.  Soy foods (NOT soy supplements) may help prevent breast cancer.  Several studies evaluating soy intake have found that eating soy lowered the risk of breast cancer recurrence, but the specific amount to consume is not known – more research is still needed to better understand.  Supplements are not recommended because they are not well regulated and concentrated amounts of supplemental soy isoflavones may increase cancer risks with excessive consumption.    

There is also interest in soy foods helping to reduce menopause symptoms such as hot flashes.  Some trials have observed improvements in frequency and severity of hot flashes with 50-150mg of soy isoflavone but the evidence for these claims are quite poor.  Additional benefit to conumption of soy isoflavones from whole foods may be a second-line preventative therapy for osteoporosis in postmenopausal women.

The Bottom Line?

Two to three servings of traditional soy foods daily can safely be included in your diet and may have health benefits.  The typical amount of soy consumed in the western diet is usually below these recommendations.  Soybeans are a high-quality, plant-based protein that also add fibre and variety to your meals. 

For women with a history of breast cancer, it is safe to have 2-3 servings of soy foods daily, but you should discuss with your doctor any changes in intake of soy foods. Avoid soy supplements.

Incorporating Soy into Your Meals:

Tofu – firm or extra firm varieties are great in a stir-fry or soup as a meat substitute.  Soft or silken tofu can be used in sauces, dips, creamy desserts, shakes, soups, or scrambled “eggs”.  Check out Food to Fit’s recipe for Sesame Ginger Power Stir-Fry

Soy veggie ground - can be a direct substitute for the ground meat you usually use in chili, tacos, or spaghetti sauce.

Soybeans – make a great addition to salads, soups, or pasta dishes.  Soybeans can also be roasted for a crunchy snack on their own!

Edamame – steam on their own as a side dish or add to salads or soups.

Soy beverage – compared to other plant-based milk (almond, coconut, rice, etc.), soy is higher in protein and can be used as an alternative to cow’s milk.  However, make sure you are selecting a fortified soy beverage – this ensures levels of vitamin A, D, B12, calcium, and zinc are the same levels as cow’s milk.

Miso – great sauce for Asian-inspired cuisine flavours.



Canadian Cancer Society – Eating Well After Breast Cancer:

Dietitians of Canada: and

Eating Well with Canada’s Food Guide:

PEN Knowledge Pathway for Soy:

Bolca, S., Bracke, M., & Depypere, H. (2012). Soy consumption during menopause. Facts, Views & Vision in ObGyn, 4(1), 30–37.

Lou, D., Li, Y., Yan, G., Bu, J., & Wang, H. (2016). Soy Consumption with Risk of Coronary Heart Disease and Stroke: A Meta-Analysis of Observational Studies. Neuroepidemiology,46(4), 242-252.

Zhang, C., Li, J., & Zhou, P. (2011). Soy consumption and breast cancer risk: four recent meta-analyses. Breast Cancer Research and Treatment,127(2), 573-576.


Submitted by Shandelle Shroeder, Nutrition Student