Soy may be the most misunderstood food on the planet. Countless peer-reviewed studies have examined this humble legume and proven it to be a health-promoting food, and yet somehow it still gets a bad rap.
Here’s where the confusion started. Soybeans contain estrogen-like compounds called phytoestrogens, but because excess human estrogen is considered harmful, and some types of cancers use estrogen for fuel, several decades ago, many health influencers jumped to the conclusion that phytoestrogens were bad.
I was influenced by that claim and believed it for many years. But since then, numerous studies have shown the opposite, vindicating soy and phytoestrogens. Reading the research changed my mind.
Genistein: Soy’s anticancer superpower
Soybeans are protein-rich, high in fiber, and a great source of healthy fats and phytonutrients called isoflavones. The superstar of that group is genistein.
Genistein is a compound in soy that targets multiple cancer pathways. It inhibits cancer cell growth, disrupts survival signaling, and blocks angiogenesis (the process by which tumors increase their blood supply).
Genistein increases tumor-suppressor gene expression, including BRCA1, and targets cancer stem cells by interfering with the Hedgehog signaling pathway, which cancer cells rely on to survive and proliferate.
Genistein is a selective estrogen receptor modulator. It binds to one of two estrogen receptors, the beta receptor ( ER-β) rather than the alpha receptor (ER-α). That distinction is key. Human estrogen binds to both receptors equally, but soy isoflavones preferentially bind to the beta receptor, which slows down cell growth rather than stimulating it, and blocks the absorption of harmful excess estrogen and toxic chemical pollutants known as xenoestrogens.
Soy and breast cancer
The Shanghai Breast Cancer Survival Study, published in JAMA, followed more than 5,000 breast cancer survivors and found that women with the highest soy intake had a 29% lower risk of dying from the disease.
Other large studies link soy to reduced risk of recurrence and lower breast cancer incidence.
Whole soy foods are considered safe for women with hormone-sensitive cancers, and one expert consensus panel concluded that there is no evidence of harm from isoflavones in breast or uterine tissue.
One important note: These studies are on soy foods, such as tofu, tempeh, edamame, miso, soy milk, not soy isolates. Soy supplements and isolated soy protein concentrates are less studied, may not have the same benefits, and could be harmful. I recommend avoiding them.
Soy vs. hot flashes
Hot flashes affect roughly 80-85% of American women, but in Japan, only about 15% of women report experiencing them. Researchers suspect that diet plays a major role, specifically the intake of soy. Women who eat about 4 ounces of tofu per day are about half as likely to experience hot flashes compared to those who eat less.
Meta-analyses confirm that about two servings of soy per day reduce hot flash frequency by around 20% compared with placebo and hot flash severity by about 25%. Some Studies have found soy’s effectiveness comparable to hormone therapy for reducing hot flashes, muscle and joint pain, and vaginal dryness. HRT tends to work faster (about 3 weeks versus 3 months for soy to reach full effect), but soy has no harmful effects and reduces breast cancer risk. [Source]
Two randomized controlled trials found that combining a low-fat, plant-based diet with a daily half-cup of cooked whole soybeans reduced moderate-to-severe hot flashes by 84-88% in just 12 weeks. Nearly 60% of women in the plant-based group became completely free of moderate-to-severe hot flashes. This simple dietary addition can have a big impact on menopause symptoms.
If you want to go even deeper, Dr. Michael Greger at NutritionFacts.org has produced several excellent videos on soy and hot flashes.
Soy many benefits!
The benefits of soy for women go well beyond hot flashes. Vaginal dryness improves with regular isoflavone intake. Soy isoflavones have been shown to strengthen bone density after menopause.
Research links soy to sharper memory and reduced depression during menopause. Heart health benefits are also significant, including improved cholesterol, lower blood pressure, and reduced arterial stiffness.
A controlled study found that women consuming a low-fat, high-fiber diet had significant reductions in circulating estrogen levels, and Asian women following traditional low-fat, high-fiber diets have measurably lower blood estrogen both before and after menopause compared to Western women. This aligns with their lower rates of breast cancer and fewer menopausal symptoms overall.
Soy and thyroid health
Another persistent myth is that soy wrecks your thyroid. A systematic review and meta-analysis of 18 randomized controlled trials found that soy had no significant effect on thyroid hormones in people with normal thyroid function.
One nuance worth noting: if you take levothyroxine (synthetic thyroid hormone), don’t take it with a soy food meal, as soy may interfere with its absorption.
A note for the fellas
Soy does not feminize men, it does not cause man boobs, and it does not lower testosterone. A 2021 meta-analysis of 41 clinical studies found that soy had zero effect on testosterone or estrogen in men, regardless of dose or duration. A meta-analysis of over 1.4 million participants found that higher soy intake is associated with a reduced risk of prostate cancer, confirmed by multiple systematic reviews.
Why organic soy matters
Over 90% of conventional soy in the U.S. is genetically modified and sprayed with glyphosate and other highly toxic agrochemicals. Always choose organic soy products to reduce your exposure to harmful chemical residues and protect your health. You can read my article on the harms of glyphosate here.
How much soy should you eat?
Studies suggest that 2-3 servings per day of organic soy foods is optimal, especially for women. Here are examples of one serving of soy:
–Soy milk (1 cup, 7-10g protein): Stir into your morning oatmeal with berries and ground flaxseed, use as a base for smoothies, or just drink it straight.
–Edamame (1/2 cup, ~8g protein): Whole soy beans. Steam them from frozen, sprinkle with a pinch of sea salt and a squeeze of lemon.
–Tofu (3oz, 8-10g protein): Made from curdled soy milk. Scramble it with turmeric and veggies for breakfast, cube and roast it for salads, or add it to stir-fries. If you want some simple tofu recipes, I have five in my plant-based cookbook Beat Cancer Kitchen.
–Tempeh (1/2 cup, 15-20g protein): Fermented whole soybeans pressed together. Slice it thin, marinate it, and pan-fry it with olive oil. Crumble on a salad or tuck it into a wrap.
–Miso (1 tbsp): A fermented soybean paste. Stir into warm (not boiling) water with some greens and tofu, to make miso soup. You can also use it as a base for salad dressings. Fermented, probiotic, deeply savory, it is one of the most underrated health foods.
The bottom line
Soy is one of the most researched, yet most misunderstood foods in our culture. The evidence consistently links whole, organic soy foods to a lower risk of breast cancer, prostate cancer, cardiovascular disease, and menopausal symptoms, and legumes, including soy, are a cornerstone of the healthiest, longest-living populations in the world.
One expert consensus panel put it plainly: Soy isoflavones can be considered a first-line treatment for menopausal hot flashes and night sweats. That is not fringe nutrition advice; it’s scientists reviewing the totality of evidence and calling it what it is.
Don’t let outdated myths scare you away from this super-nutritious legume. Eating 1-2 servings of organic soy per day will do you good. Enjoy soy!
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