The Truth About Soy

Should I Eat Soy?

Confusion surrounds soy and soy products. Are they healthful or harmful? Let’s examine the myths and facts surrounding soy.

Not a Magic Pill or a Poison; Just a Bean

Compared to other beans, soybeans are unique, possessing a different macronutrient composition. They are higher in protein and fat and lower in carbohydrate than other beans. Soybeans come in a variety of different colors and shapes. The ones we may be most accustomed to seeing are the green soybeans, called edamame. But there are yellow soybeans that are used for making soy milk, tofu and tempeh, and there are black soybeans which are usually sold canned. Additionally, there is natto, a whole fermented soybean not commonly eaten in the United States, and miso and soy sauce which are high in salt and thus not healthful.

Soy and Cancer

A lot of the controversy surrounding soy is related to its effect on hormones. The fear that consuming soy would elevate breast cancer risk is a persistent, yet unfounded myth. Soybeans are rich in isoflavones, a type of phytoestrogen, a plant substance that is chemically similar to estrogen. Since estrogen exposure had been known for many years to negatively influence breast cancer risk, soy was thought by some to be potentially dangerous. In Asian countries where soy is a staple food, rates of breast cancer have traditionally been much lower than those in the U.S. There is no controversy in the scientific literature; higher intake of soy is consistently associated with a lower risk of breast cancer and breast cancer recurrence. The effect is often found to be stronger in Asian countries, where soy intake is higher.¹ Soy also appears to be most strongly protective when eaten during childhood and adolescence.² A 2009 meta-analysis of studies on soy and prostate cancer found that higher soy intake was associated with a 26 percent reduction in risk.³ Soy foods are not only associated with decreased risk of hormonal cancers, but also lung, stomach, and colorectal cancers.⁴

Is All Soy the Same?

Avoid processed soy foods, like nutrient-depleted isolated soy in protein powders and genetically modified soy (its phytochemical value and environmental impact remain questionable). The soy products that are problematic are those that contain unnaturally concentrated soy protein, such as protein powders and meat substitutes — highly processed foods that retain little or none of the beneficial nutrients contained in whole soybeans. Soybeans should be eaten in their minimally processed forms.

Minimally Processed Soy Foods:

• Whole soybeans or edamame
  • Tofu
  • Tempeh
  • Unsweetened soymilk

As little as 10 mg of soy isoflavones consumed per day has a protective effect with regard to breast cancer – this equates to approximately one ounce of one of these soy foods. In Asian countries, the average daily intake of soy isoflavones is 25-50 mg, and is consumed via tofu, soybeans, soy milk, miso, and natto. In the U.S., the average soy isoflavone intake is less than one mg, and most of this amount is consumed via soy-based additives or isolated soy protein in processed foods.

The Bottom Line

Soy products can be useful in moving toward a plant-centered diet with less saturated fat, less animal protein, more plant protein and more fruits and vegetables. Soybeans are rich in protein and the amino acid content makes them a complete vegetable protein. However, one should not eat lots of soy products to the exclusion of other valuable foods. Variety is crucial for obtaining diversity in protective phytochemicals, and a variety of beans are health promoting, along with many other foods. So use good judgment. There should be a variety of beans in the diet, not just soybeans. Eat a variety of whole, natural plant foods, including other beans such as black beans, chickpeas, and lentils. And occasionally, enjoy some edamame, tofu and tempeh as well.

References

¹Trock BJ, Hilakivi-Clarke L, Clarke R: Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst 2006;98:459-471.

Wu AH, Yu MC, Tseng CC, et al: Epidemiology of soy exposures and breast cancer risk. Br J Cancer 2008;98:9-14.

Dong JY, Qin LQ: Soy isoflavones consumption and risk of breast cancer incidence or recurrence: a meta-analysis of prospective studies. Breast Cancer Res Treat 2011;125:315-323.

²Korde LA, Wu AH, Fears T, et al: Childhood soy intake and breast cancer risk in Asian American women. Cancer Epidemiol Biomarkers Prev 2009;18:1050-1059.

Lee SA, Shu XO, Li H, et al: Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women's Health Study. Am J Clin Nutr 2009;89:1920-1926.

³Hwang YW, Kim SY, Jee SH, et al: Soy food consumption and risk of prostate cancer: a meta-analysis of observational studies. Nutr Cancer 2009;61:598-606.

⁴Yang WS, Va P, Wong MY, et al: Soy intake is associated with lower lung cancer risk: results from a meta-analysis of epidemiologic studies. Am J Clin Nutr 2011;94:1575-1583.

Kim J, Kang M, Lee JS, et al: Fermented and non-fermented soy food consumption and gastric cancer in Japanese and Korean populations: a meta-analysis of observational studies. Cancer Sci 2011;102:231-244.

Yan L, Spitznagel EL, Bosland MC: Soy consumption and colorectal cancer risk in humans: a meta-analysis. Cancer Epidemiol Biomarkers Prev 2010;19:148-158.