Is soy really bad for you? Here’s what the science says

It’s now undeniable: the soaring interest in plant-based diets among the general population, science, and media has led to significant growth in the market for plant-based meat alternatives. Amidst this trend, soy remains at the heart of ongoing nutritional debates—is soy beneficial or harmful to our health?

There are two sides to this debate. One argues that soy is a healthful and crucial protein source for plant-based diets. The other warns about potential risks and advises moderation or even complete avoidance.

Why soy and its derivatives are worth consuming

First, soy is a legume; like its kin, it’s packed with protein and fiber. It boasts a low saturated fat and cholesterol profile. Still, it is rich in calcium, potassium, and magnesium—making it seem the perfect candidate for bone health and heart disease prevention.

But soy has a unique edge over other legumes: its amino acid spectrum is complete, offering a higher overall protein content. To give you an idea, here’s the nutritional breakdown of common soy-based foods per 100g:

• Soy milk: 40 Kcal, 4 g Protein, 23mg Calcium, 0.8 mg Iron
• Tofu: 144 kcal, 18 g Protein, 683 mg Calcium, 2.7 mg Iron
• Tempeh: 192 kcal, 20 g Protein, 111 mg Calcium, 2.7 mg Iron
• Miso: 198 kcal, 13 g Protein, 57 mg Calcium, 2.5 mg Iron
• Dehydrated soy mince: 375 Kcal, 52 g Protein, 270 mg Calcium, 6.9 mg Iron

Eating soy can easily fulfill protein and calcium requirements efficiently and affordably.

The soy controversy: where does it stem from?

Soy’s popularity boomed in the early 1990s, following recommendations to reduce cholesterol and the rise of “functional/health” foods. Soy was heralded as a miraculous cure for cancer, cardiovascular diseases, and obesity, noting low rates of chronic degenerative diseases and obesity in soy-heavy Asian diets.

However, correlation does not imply causation. Further research revealed soy’s isoflavones—compounds structurally and functionally similar to human estrogens—raising concerns about potential endocrine disruption and hormone-dependent tumor growth.

While some in vitro and animal studies seemed to support these concerns, they often involved unrealistically high isoflavone concentrations. Conversely, human studies continued to showcase soy’s protective benefits against heart disease and osteoporosis and its efficacy in managing menopause symptoms.

Clarification on phytoestrogens revealed they bind to estrogen receptors more weakly, offering benefits during menopause and for hormonal balance during reproductive years. Despite this, fears about soy’s hormonal impacts, particularly regarding male fertility and gynecomastia, gained traction, fueled by the popularity of low-carb and paleolithic diets.

What current science says about soy

Today, with over 400 published studies, the evidence strongly supports soy’s health benefits.

Soy and cancer

Research shows soy consumption and isoflavone intake protect against prostate, ovarian, stomach, and colorectal cancers, especially in postmenopausal women. Higher soy intake correlates with longer survival and lower recurrence rates for breast cancer survivors. The amount of phytoestrogens in just one cup of soy milk can reduce the risk of breast cancer recurrence by 25%. Improved survival was found in both women whose tumors responded to estrogen and those whose tumors did not.

Soy, fertility, and menopause

The positive effects of soy on fertility appear to be dose-dependent. Women undergoing in vitro fertilization who have been exposed to environmental BPA are more likely to get pregnant if they consume soy, as soy isoflavones help to neutralize the endocrine-disrupting effects of BPA.

On the contrary, consuming over 100 mg of soy isoflavones daily (the equivalent of 170 g of tempeh or 16 cups of soy milk) has been linked to reduced ovarian function, according to a review in the Journal of Nutrition. However, moderate soy consumption poses no problem.

It has also been seen that the protective effect against hot flashes is highly variable at the individual and population levels. The reason would be that only some individuals have a gut flora capable of creating a soy metabolite called equol, and only women who produce equol experience a significant reduction in hot flashes and night sweats (between 20% and 50% of North American and European women produce equol). Soy consumption in menopausal women is still recommended for maintaining bone health, both for its pro-estrogenic effect and its high calcium and magnesium content.

Soy and heart health

Heart disease is currently the number one killer of men and women and is on the rise. The WHO has reported that more than 75% of deaths from heart disease could be prevented with appropriate changes in diet and lifestyle. 

A 2020 European Journal of Clinical Nutrition study reported a significant inverse association between the intake of fermented soy products (miso, natto, tempeh) and the risk of heart disease. However, the same association was not found for men. 

What is certain is that partially or entirely replacing animal proteins with plant proteins, including soy, can drastically lower cholesterol intake, eliminating one of the main risk factors for cardiovascular diseases: excessive intake of saturated fats and cholesterol.

Soy and thyroid health

Concerns about soy and thyroid health persist, particularly regarding hypothyroidism. To date, most doctors continue to advise against the use of soy and derivatives in cases of hypothyroidism.

A 2006 review of 14 clinical trials demonstrated that soy consumption does not affect thyroid function in healthy individuals but that in hypothyroid individuals, it could affect the absorption of drugs administered to treat the condition. As a precaution, consuming soy and its derivatives away from the medication would be enough to eliminate this side effect.

Additional benefits of soy

Beyond the well-documented benefits, recent studies have linked soy to neuroprotection against Alzheimer’s, mood improvement, obesity management, metabolic syndrome improvements, blood pressure reduction, cardiac function enhancement, liver and kidney damage protection, and allergy and asthma symptom reduction.

How best to consume soy

The most benefits are seen with minimally processed soy forms like tempeh, tofu, miso, and edamame, which avoid the added sugars, unhealthy fats, salt, or preservatives found in highly processed products. 

Soy-based foods like meat substitutes, bars, yogurts, or protein powders usually contain only isolated soy proteins rather than whole legumes. This concentrates the presence of isoflavones, making it possible to reach the attention threshold for which negative effects begin to outweigh the positive ones. However, it also removes important nutritional attributes of soy, such as fibers, calcium, and iron.

How Much Soy is “Too Much”?

Defining an “optimal” soy portion is challenging, but research suggests that 2-5 servings daily are beneficial, with potential adverse effects at seven or more servings. A serving of soy can consist of a cup of milk, a yogurt, or 100 g of tofu or tempeh.

In conclusion, soy is just one of many foods that can be part of a healthy, balanced diet. Consuming in moderation and from unprocessed sources is neither a cure-all nor a significant health risk. Dispelling myths and biases against soy will enable consumers to enjoy its benefits with peace of mind.

 

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