Soy Protein and Health
Soy protein is both praised and loathed for its supposed health benefits and risks. Some claim it protects against various cancers while others say it upends hormonal balance. Is soy a superfood or does it create more issues than it solves?
Soy, Isoflavones, and Health
Soy is special in the nutrition world because it contains a high concentration of isoflavones; a plant estrogen also known as a phytoestrogen. This compound is similar to human estrogen but is much weaker. Isoflavones have the ability to bind to estrogen receptors in the body. They cause both estrogenic and anti-estrogenic effects1.
There’s a large amount of research on soy’s effects on the body. The confusion on soy’s value is due, at least in part, to the contradictory nature of the literature. Isoflavones affect various ethnic groups in different ways and can even impact individuals based on existing hormone levels. Another variable is the type of soy being studied: whole soy products (tofu, soy beans), processed soy (protein powders, veggie burgers), or fermented vs unfermented soy1.
Soy and Prostate Cancer
Studies on prostate cancer show that both incidence and mortality is highest in Western countries and lowest in Asian ones. Among Japanese and Chinese men who move to the West, cancer risk only increases in those who adopt a Western diet. Those who keep their traditional diets do not see an increased risk. This indicates a dietary risk factor of prostate cancer. Isoflavone intake is estimated to be 25-50 mg/day in Japan, compared to less than 1 mg/day in Europe2.
A 2013 meta analysis made a few conclusions2:
- Prostate specific antigen (PSA) is one way to measure prostate health. Generally, increased levels are a cause for concern3. This analysis did not find that soy supplementation could lower PSA levels.
- Soy supplementation did not alter hormone levels.
- There is a potential role for soy in the reduction of prostate cancer risk.
A separate 2005 meta-analysis on the same topic concluded that the intake of soy food decreased prostate cancer risk 30%. Researchers found soy protein provided 4 times more protection against prostate cancer than any other dietary factor.
One study found that Japanese men with higher serum levels of isoflavones had a lower risk of prostate cancer than those with lower isoflavone levels. Researchers also found that soy significantly decreased PSA levels4. A 2009 meta analysis had similar findings5.
Soy and Heart Disease
Soy first became “heart healthy” after a 1995 meta analysis of 38 studies found it could lower total and LDL cholesterol and triglyceride levels6. Beginning in 1999, the FDA approved a heart healthy food label for soy products, and in 2000, the American Heart Association recommended including soy in a balanced diet7. Since then, some studies have cast doubt on whether soy is as heart healthy as we once thought.
A 2006 American Heart Association statement released the following about soy and heart health6:
- Individuals getting more than half of their daily protein intake from soy protein could lower LDL cholesterol by a few percentage points. This decrease was seen in those replacing dairy and other animal proteins with soy.
- There were no beneficial changes to HDL cholesterol, triglyceride levels, or blood pressure.
- Soy protein may not be as uniquely important as previously thought.
- Soy foods remain a healthy choice due to their high polyunsaturated fat, fiber, vitamin, and mineral content combined with low saturated fat levels.
Soy and Breast Cancer
Phytoestrogens don’t always act in the same way as estrogen; sometimes they exhibit anti-estrogenic behavior by blocking the effects of estrogen. In breast tissue, soy may have the ability to reduce the risk of breast cancer1.
A 2010 meta-analysis of 18 studies looked into the effects of soy products on breast cancer risk. Researchers came to the following conclusions8:
- There is a significant association between increased soy isoflavone intake and reduced breast cancer incidence and risk of recurrence.
- This risk reduction was seen in Asian populations, but not Western ones.
- Researchers believe Asian populations saw a bigger decrease in risk because their intake of soy isoflavones is much higher. Even the lowest intakes in Asian populations is far higher than the highest intakes in Western populations.
- Menopausal status was important in seeing the benefits of soy. There was little to no cancer risk reduction in pre-menopausal women.
The Bottom Line – Are Soy Foods Healthy?
According the the meta-analyses above, there is ample evidence soy has the ability to improve overall health. Though some findings from the 90s, specifically dealing with soy’s ability to improve heart health, may have been overblown, there is still value in replacing foods such as fatty meats and dairy, with soy products.
- Straight Talk About Soy. (2018, August 07). Retrieved from https://www.hsph.harvard.edu/nutritionsource/soy/
- Die, D., Bone, K. M., Williams, S. G., & Pirotta, M. V. (2013). Soy and soy isoflavones in prostate cancer: A systematic review and meta‐analysis of randomized controlled trials. BJU International,119-130. doi:10.1111/bju.12435
- Prostate-Specific Antigen (PSA) Test. (2017, October 04). Retrieved from https://www.cancer.gov/types/prostate/psa-fact-sheet
- Yan, L., & Spitznagel, E. L. (2005). Meta‐analysis of soy food and risk of prostate cancer in men. International Journal of Cancer, 117, 667-669. doi:10.1002/ijc.21266
- Yan, L., & Spitznagel, E. L. (2009). Soy consumption and prostate cancer risk in men: A revisit of a meta-analysis. The American Journal of Clinical Nutrition, 89(4), 1155-1163. doi:10.3945/ajcn.2008.27029
- Sacks, F. M., Lichtenstein, A., & Van Horn, L. (2006, August 01). Soy Protein, Isoflavones, and Cardiovascular Health. Retrieved from https://www.ahajournals.org/doi/10.1161/01.ATV.0000227471.00284.ef
- Sacks, F. (2006, January 17). Soy Protein, Isoflavones, and Cardiovascular Health. Retrieved from https://doi.org/10.1161/CIRCULATIONAHA.106.171052
- Dong, J., & Qin, L. (2010). Soy isoflavones consumption and risk of breast cancer incidence or recurrence: A meta-analysis of prospective studies. Breast Cancer Research and Treatment, 125(2), 315-323. doi:10.1007/s10549-010-1270-8