Did you know that soy is one of the most well-researched foods with over 10,000 published peer reviewed articles? Yet, despite all that research you will still hear a lot of conflicting information about whether soy is good for you or not. As a plant-based eater and dietitian who regularly includes soy in their diet, I wanted to explain why you don’t need to fear soy!
Why is Soy Good for You?
Let’s start by talking about some of the reasons you would want to include soy foods into your diet! Of course, when we talk about the health benefits associated with soy, we are referring to minimally processed soy foods like tempeh, tofu, soy milk, miso, and natto — NOT soy isolates or other heavily processed forms of soy.
- Soybeans are a complete protein, which means they provide an adequate amount of all the essential amino acids. They also have a high protein digestibility score so they are considered a high quality source of protein.
- Unprocessed soy like edamame, is a good source of fiber, which is a nutrient most Americans are lacking in their diet.
- Soy foods contain many phytonutrients which may have health benefits such as:
- Isoflavones: the compound in soy also known as “phyto-estrogen” that is the focus of the majority of research.
- Saponins: studies suggest these may help lower cholesterol.
- Phytates: these compounds get a lot of bad press, but they are also an antioxidant!
- Phenolic acids: these are an antioxidant that is abundant in many fruits, vegetables, and legumes.
- Soy foods are a good source of polyunsaturated fat, including Omega-3 fatty acid.
- Soy foods like tofu are a good source of calcium and iron. Tempeh is a good source of potassium, magnesium, calcium, iron, and vitamin B6.
- Make your gut happy by including fermented soy foods in your diet like tempeh and miso which contain probiotics.
Soy and Isoflavones
Soy is rich in a group of phytochemicals called, isoflavones. Isoflavones are also known as phytoestrogens or “plant estrogens” because they have a similar chemical structure to the hormone estrogen and can attach to estrogen receptors in cells but they are much, much weaker than estrogen. There are three primary isoflavones found in soy foods: genistin, daidzin, and glycitin. Although soy has the highest concentration of isoflavones, other foods like berries, red wine, grains, nuts, and other legumes also contain small amounts of isoflavones.
Soy and Breast Cancer
Common Concern: Does soy cause breast cancer? Since the isoflavones (ie. phytoestrogens) in soy have a weak interaction with the estrogen receptors in our cells, a lot of past research has focused on soy and breast cancer, especially estrogen receptor positive breast cancer. Large observational research studies have found that soy intake does not increase risk of breast cancer (1, 2). Breast cancer risk is lowest in Asian women with the highest intake of soy (2). Soy intake did not increase breast cancer risk for women in Western populations (2, 3). In a large analysis that looked at soy intake and breast cancer recurrence, women (both of Asian and Western descent) who consumed at least 10 mg isoflavones/day (~1 serving of soy) had a 25% reduced risk of breast cancer recurrence (4). As far as isoflavone supplements, there isn’t enough research to say they can help lower risk of breast cancer and I would recommend just consuming whole soy foods rather than a supplement (5).
Soy and Thyroid Health
Common Concern: Is soy bad for my thyroid? Healthy individuals do not have to fear that eating soy will have any negative impact on their thyroid. Those who take medications to control hypothyroidism, should talk with their healthcare provider about their soy intake so that their medications can be adjusted appropriately (3). Everyone who does regularly eat soy should also makes sure they get enough iodine in their diet either through use of iodized salt or a supplement.
Soy Intake for Men
Common Concern: Does eating soy lower testosterone? Does eating soy make you feminine? A popular rumor about soy is that it will cause feminization in men. However, there is no clinical research to back this up. In studies where men take isoflavone supplements, it does not effect their testosterone levels, cause “man boobs”, or lead to erectile dysfunction (6). Men who consumed more soy foods had significantly lower risk of prostate cancer in observational studies with both Asian and Western men (7, 8). One interesting thing to know is that unfermented soy products (tofu, soy milk) were found to be more beneficial than fermented soy foods (miso, tempeh) in these studies.
Soy and Cardiovascular Health
Common Concern: Is soy good for my heart? Previous studies have noted that consuming soy foods can help lower cholesterol (3). Large Asian-population based studies have found that intake of soy foods likely decreases the risk of coronary heart disease and stroke in women (5). A lot of the research doesn’t point to a strong connection between soy foods specifically and heart health, but rather using soy foods as a replacement for foods high in saturated fat like red meat may be beneficial (9).
Soy foods like tofu, tempeh, and soy milk may be beneficial to cardiovascular and overall health because they are high in polyunsaturated fats, fiber, vitamins, and minerals but low in saturated fat. In fact, randomized control trials have shown that replacing foods high in saturated fat (red meat, cheese, dairy) with foods high in polyunsaturated fat (soy, walnuts, flax) can reduce the risk of cardiovascular disease by up to 30%! (10)
Soy and Mineral Absorption
Common Concern: Do the phytates in soy cause nutrient deficiencies? Some people may be concerned that the phytates in soy decrease the absorption of minerals like calcium, iron, magnesium, and zinc. However, the impact of phytates on nutrient absorption is minor and would only become a concern if someone already had a nutrient poor diet and was consuming a lot of uncooked or unrefined vegetables and legumes (11). Soy isn’t the only plant-based food that contains phytates. Beans, grains, nuts, and seeds all contain phytates but the health benefits of regularly eating these foods far outweighs the minimal impact phytates may have on mineral absorption.
Soy and the Environment
Common Concerns: Are soy foods GMO? Soybeans are used for animal feed and this type is typically genetically modified to reduce the need for pesticides or herbicides. When purchasing soy products in the grocery store, just look for organic or “made from non-GMO soybeans” on the label.
Common Concerns: Does soy production contribute to the deforestation in places like Brazil and Argentina? The soy crops grown after deforestation are for feed for livestock, not for use in soy products sold in the grocery store (12).
Bottom Line: In most research studies to date, consuming about 2 servings of soy foods like tofu, tempeh, soy milk, and edamame daily as part of a healthy diet can provide health benefits (3). Compared to whole soy foods, soy isoflavone supplements have not been shown to be as beneficial. Also, the quality of these supplements is not regulated so the amount of isoflavones they contain is not standardized so stick with whole soy foods.