The Truth About Soy from a Registered Dietitian

One common question I receive as a dietitian is: what are the health impacts of soy? Is it harmful? The short answer is no – for most people. Recent studies show that including soy in the diet can be helpful and provide many health benefits, especially when replacing other animal protein options like red meats. Here I will discuss where the controversy came from about soy and all the ways that it can actually benefit your health.

Background on soy

Soy and soy products are made from soybeans, a type of legume. Soy is high in protein, and is one of the few complete plant proteins, meaning that it contains all essential amino acids needed for our bodies to function optimally. Soy is relatively well balanced in that it also contains fat and carbohydrates as well. Soy contains isoflavones, or plant-compounds that are structurally similar to the human hormone estrogen. Because of this, isoflavones are often also called phytoestrogens, literally meaning “plant-estrogens”. This is where much of the controversy around soy comes into play. Soybeans are also high in fiber, iron, B-vitamins, vitamin C, manganese, phosphorus, and contain vitamin K, magnesium, zinc, calcium, and potassium as well.

Why was soy said to be harmful?

Because soy contains a high amount of isoflavones (or plant-estrogens), it can bind to estrogen receptors in the body and causes what is referred to as “estrogenic activity”, which basically means it mimics the impacts of estrogen. This caused fears that soy could contribute to hormone-dependent cancers, such as breast cancer, as well as potentially causing thyroid issues. However, some of this original research was performed in animal models, and with more research in humans coming out about soy, there is sufficient evidence that soy has many positive impacts on risk of cancer, heart health, and much more (1,2).

What Health Impacts Does Soy Have?

Soy and Cancer

Soy is often discussed in terms of risk of breast cancer. However, there is substantial evidence that soy may have a preventative effect against breast cancer. Studies range from showing that people who eat more soy have a 22% to 64% lower risk of developing breast cancer1. Even in people who have had breast cancer, soy intake after breast cancer diagnosis was found to be associated with a 25% reduction in cancer recurrence (3). Soy isoflavones, despite being structurally like estrogen, may in fact portray some anti-estrogenic impacts – the opposite of what researchers originally thought. Since many forms of breast cancer are hormone-receptor positive, these anti-estrogenic isoflavones are posited to therefore have anti-cancer properties in these cases (4). Genistein, one of the soy isoflavones, was found to be able to inhibit many cancer pathways, including angiogenesis, which is used by cancers to divert blood flow to the cancer cells so that they can keep growing (5). Without this capability, it is more difficult for cancers to grow and spread. Overall, research strongly suggests that soy consumption is most associated with a reduced risk of hormone-dependent cancers such as breast and prostate cancer (5,6,7).

Soy and Thyroid Conditions

Soy is next most often mentioned in the context of thyroid health. Soy does seem to inhibit the utilization of iodine in the thyroid. This is important because iodine is essential for the production of thyroid hormone. However, there is mixed evidence about whether soy consumption still impacts thyroid function (8). It seems that soy may inhibit thyroid function in people who are iodine deficient but does not seem to pose a risk for people with no thyroid issues (9). It is recommended that people with hypothyroidism should focus on getting adequate iodine from their diet, but that they may not need to limit soy foods (10). The Physicians Committee for Responsible Medicine also states that they believe there is no adverse effect of soy on thyroid function2. Since evidence is not strong on this subject, it seems that more research needs to be done before drawing a firm conclusion.

Soy and Heart Disease

The relationship between soy protein consumption and cholesterol reduction is well established. The FDA even allows health claims that 25 grams a day of soy protein may reduce the risk of heart disease (11). In addition, components of soy outside of the protein itself can also have beneficial impacts on cardiovascular disease risk, such as high blood pressure, inflammation, and more (12). Studies suggest that a metabolite of specific soy isoflavone, called daidzein, may also improve arterial stiffness, therefore preventing coronary heart disease as well as cognitive impairment (13).

Soy and Insulin Resistance

Soy consumption has been found to improve insulin resistance and plays a role in the prevention and management of type 2 diabetes. Soybeans are high in fiber and have a low glycemic index, making them ideal for maintaining a steady blood sugar level (14). Research studies performed in rodent models show soy consumption contributing to lower blood sugar levels, increase insulin secretion, and lower fasting blood sugar levels (15). In humans, soy consumption was found to be associated with improved fasting glucose, serum insulin, and insulin resistance in women with gestational diabetes (16) as well as improved insulin resistance in postmenopausal women (17).

Soy and Menopause

When thinking of menopause, many think of the hot flashes often associated with this stage. There is some evidence that soy consumption may also offer some relief of hot flashes (18,19,20), however, other research showed no significant relief of hot flashes (21). More research needs to be done to know the true impact of soy on hot flash occurrence and frequency. Postmenopausal women are also at higher risk for osteoporosis and high cholesterol (22,23). Soy, however, has been found to reduce the risk of osteoporosis in postmenopausal women (24,25). This same effect was not found for men (25). As already discussed above, soy consumption can be beneficial for cholesterol as well.

Soy and PCOS

PCOS is associated with a higher risk of high cholesterol, high blood pressure, and insulin resistance. Because of soy’s role in reducing many of these same risks, soy consumption can mitigate some of the nutritional risks associated with PCOS (26). Genistein, the same isoflavone mentioned above, is thought to reduce complications of PCOS by not only decreasing insulin resistance, but also by regulating reproductive hormones (27).

In conclusion, soy offers a wide array of health benefits and can protect against many conditions and diseases. If you have any thyroid issues, discuss with your primary care provider before incorporating soy. For everyone else, soy is a safe way to promote health and reduce chronic disease risk. Schedule an appointment with a dietitian to discuss ways to incorporate soy into a balanced diet by calling our nutrition coordinators at 919.237.1336, or visit avancecare.com.

References:

1. Harvard T. H. Chan School of Public Health. Straight Talk about Soy. 2023. Retrieved from https://www.hsph.harvard.edu/nutritionsource/soy/
2. Physicians Committee for Responsible Medicine. New Research Disputes the Biggest Soy Myths. 2021. Retrieved from https://www.pcrm.org/news/health-nutrition/new-research-disputes-biggest-soy-myths
3. Nechuta SJ, Caan BJ, Chen WY, Lu W, Chen Z, Kwan ML, Flatt SW, Zheng Y, Zheng W, Pierce JP, Shu XO. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. The American journal of clinical nutrition. 2012 Jul 1;96(1):123-32.
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