When choosing foods for a balanced diet, some healthy choices are easier to identify than others, such as fruits and veggies rather than deep fried foods. But some foods, like soy, fall somewhere in the middle.
The effects of soy on breast cancer patients came into question when early studies in rodents found that isoflavones, a compound that’s found in many foods, was especially concentrated in soy. Isoflavones were found to increase the growth of certain breast cancer cells. However, follow-up studies in humans showed that rodents metabolize soy differently and that the effects of soy may be beneficial to humans.
While whole soy foods aren’t harmful and can be helpful, there is not enough evidence to make a broad recommendation.
Studies of Asian populations with lifelong moderate soy consumption show it could help prevent breast cancer, possibly from soy consumption during childhood and adolescence. For those with a breast cancer diagnosis, there is limited evidence of greater survival and decreased recurrence with moderate intake of soy. Moderate intake is one to two servings of whole soy foods (for example, 1/3 cup tofu, 1 cup soy milk, 1/2 cup edamame and 1/4 cup soy nuts). Whole soy foods are a good source of plant-based protein and dietary fiber. They can also improve your gut microbiome.
So, how much soy is OK? If you’ve had cancer and soy was a part of your diet before, it’s fine to continue eating in moderation, up to three servings a day. But if soy is new to your diet, stick to the guidelines of one to two servings a day, where a serving contains 7g of protein and 25mg of isoflavones.
Isoflavone content isn’t typically given in nutrition facts and can vary based on preparation and brand. The U.S. Department of Agriculture has a partial list of isoflavone content of common foods. One cup of soy milk can be as low as 6mg. Three ounces of uncooked tempeh can be as high as 51.5mg, whereas 3 ounces of soft tofu is about 19mg.
People may respond differently to soy based on their gut bacteria and genetics. There is a considerable amount of scientific research and how soy affects the body – but we’re constantly learning more.
Eat whole soy foods with confidence as part of a healthy diet, but avoid soy supplements, isoflavone-enriched powders and soy protein powders due to the extremely high isoflavone content.
A plant-based diet has anti-inflammatory properties that are a keystone for cancer prevention and recurrence. Having tofu instead of meat is a positive switch that can reduce your overall cancer risk, especially if you’re replacing red meat.
According to the American Institute for Cancer Research and World Cancer Research, certain foods can increase or decrease your cancer risk.
Foods with strong evidence to decrease cancer risk:
• Whole grains.
• Colorful vegetables.
• Whole foods (not supplements).
Foods with strong evidence to increase cancer risk:
• Processed meat (ham, salami, bacon).
• Red meat (beef, lamb, pork).
Abigail Galbraith is a dietitian for the Samaritan Cancer Program. To learn more, visit samhealth.org/Cancer.