Soy and phytoestrogens in perimenopause

Soy and other phytoestrogens may ease hot flashes slightly for some women, but the effect is modest and inconsistent, and The Menopause Society does not recommend them as a treatment. Soy foods are safe for most women, including breast cancer survivors, and they are a good source of protein. If hot flashes are disrupting your life, proven treatments work far better.

What phytoestrogens are

Phytoestrogens are plant compounds with a structure similar to estrogen. The best studied are the isoflavones in soybeans, mainly genistein and daidzein, found in tofu, tempeh, edamame, soy milk and miso. Lignans, found in flaxseed and whole grains, are another group.

Isoflavones bind to estrogen receptors, and they bind more readily to one receptor type (estrogen receptor beta) than the other (alpha). The Menopause Society describes them as having both estrogen-like and estrogen-blocking properties. That mixed action is why they are not a substitute for hormone therapy.

What the evidence shows for hot flashes

Study	What it tested	What it found
Taku et al., 2012	Meta-analysis of 17 placebo-controlled trials of soy isoflavone supplements (median 54 mg a day) for 6 weeks to 12 months	Hot flash frequency down about 21% and severity down about 26% compared with placebo
Franco et al., 2016	Meta-analysis of 62 trials of plant-based therapies in 6,653 women	Phytoestrogens linked to about 1.3 fewer hot flashes a day; soy isoflavones alone about 0.8 fewer; no significant effect on night sweats
Barnard et al., 2023	84 postmenopausal women randomized to a low-fat vegan diet with 1/2 cup cooked soybeans daily, or no change, for 12 weeks	Moderate-to-severe hot flashes fell 88% vs 34% in the control group

The numbers look encouraging on paper, but each study has important limits.

The effects in meta-analyses are modest. About one fewer hot flash a day is noticeable for some women but small next to the effect of hormone therapy or the best nonhormonal medicines. Franco’s team also found that 74% of the trials had a high risk of bias in three or more areas of study quality.

The results vary widely. Taku’s meta-analysis found large differences between trials, and supplements with more genistein seemed to work better. The Menopause Society’s 2023 position statement noted that soy trials since 2015 used very different products and doses, were often small and short, and frequently relied on symptom checklists instead of hot flash diaries.

The diet trial changed many things at once. The 2023 trial was small and not blinded, and participants switched to a low-fat vegan diet as well as adding soybeans. It cannot show how much of the benefit came from soy itself.

After weighing all of this, The Menopause Society lists soy foods, soy extracts and equol as not recommended for hot flashes, on the grounds of mixed evidence. The National Center for Complementary and Integrative Health takes a similar line, saying soy may help reduce hot flashes but the effect may be small and results are not consistent.

Why soy may help some women and not others

When you eat soy, gut bacteria convert some daidzein into equol, a compound with stronger estrogen-like activity. According to The Menopause Society, only about 35% of North American women can make equol, and women who cannot may get little from soy. Equol supplements exist, but trials are small and the evidence is mixed. There is no commercially available test to tell you which group you are in.

Is soy safe?

Breast cancer. Concern that soy might fuel breast cancer has not been borne out for soy foods. A pooled analysis of 9,514 breast cancer survivors from two US cohorts and one Chinese cohort, followed for an average of 7.4 years, found that eating at least 10 mg of isoflavones a day was linked to a 25% lower risk of recurrence; the lower risk of death from breast cancer did not reach statistical significance. NCCIH notes that soy supplements have not been shown to reduce breast cancer risk, so the reassurance applies to food, not concentrated extracts.

Other effects. NCCIH reports that soy protein in place of other proteins may lower cholesterol slightly, and that soy isoflavones probably have a beneficial effect on bone density after menopause.

Side effects and interactions. Soy is one of the most common food allergens. Bloating, constipation, diarrhea and nausea can occur. Soy products high in tyramine, such as tofu and soy sauce, can interact dangerously with MAOI antidepressants. NCCIH notes that soy in amounts greater than those found in food may be unsafe during pregnancy, which is still possible in perimenopause.

How to try soy sensibly

If you want to see whether soy helps you:

• Start with food. Edamame, tofu, tempeh and soy milk are easy to add and supply protein. The 2023 trial used 1/2 cup (86 grams) of cooked soybeans a day.
• Give it time. The Menopause Society notes that meta-analyses suggest more than 13 weeks are needed to see half of the expected effect and more than 16 weeks for the full effect. Many studies stopped at 12 weeks.
• Track your symptoms. Keep a simple hot flash diary before and after so you can judge the change honestly.
• Be cautious with supplements. Isoflavone content and quality vary. Our guide on whether menopause supplements work explains what to look for.

Soy also counts toward your protein target, which matters more after 40. See how much protein women need after 40.

Who should be careful

• Women with a soy allergy.
• Anyone taking an MAOI antidepressant.
• Women who could become pregnant, if considering concentrated supplements.
• Breast cancer survivors taking supplements rather than food, who should check with their oncology team first.
• Women taking thyroid medication, who should ask a pharmacist about spacing doses from soy products.

When to see a clinician

If hot flashes are frequent, wake you at night or affect your work or mood, talk to a clinician rather than relying on soy. Hormone therapy is the most effective treatment, and several nonhormonal medicines and therapies have strong evidence. Our guides to hot flashes and nonhormonal treatments for hot flashes compare the options. You can also find a menopause-informed clinician.