Plant estrogens sound like the perfect middle ground: something from food that might ease hot flashes without a prescription. Soy and flaxseed are the two you'll hear about most, and both come with digestive effects that wellness headlines usually skip.
Here's how they compare on the evidence for menopause symptoms, what they do in your gut, and who might reasonably give them a try.
What phytoestrogens are
Phytoestrogens are plant compounds that can act a little like estrogen in the body. Soybeans contain isoflavones, such as genistein and daidzein. Flaxseed is a rich source of a different group, called lignans.
Your gut bacteria are part of the story. Intestinal bacteria can convert daidzein from soy into a compound called equol, but only some adults make it. Equol has a selective affinity for one type of estrogen receptor, and researchers have proposed that soy's benefits may be greater in these "equol producers" (Setchell and Clerici, 2010). That may help explain why soy seems to help some women more than others.
Soy: a modest effect on hot flashes
A meta-analysis of 17 randomized trials found that soy isoflavone supplements, at a median dose of 54 mg a day taken for 6 weeks to 12 months, reduced hot flash frequency by 20.6% and severity by 26.2% compared with placebo. Supplements higher in genistein appeared to be more than twice as effective at reducing frequency as lower-genistein products (Taku et al., 2012).
A broader meta-analysis published in JAMA looked at 62 studies with 6,653 women. Phytoestrogens overall were linked to about 1.3 fewer hot flashes a day, and soy isoflavones specifically to about 0.8 fewer, with no significant effect on night sweats. The authors cautioned that 74% of the trials had a high risk of bias in three or more areas (Franco et al., 2016).
Put simply, soy may take the edge off hot flashes for some women. The effect is modest, and the quality of the research is mixed.
Soy and your gut
Whole soybeans and soy flour contain raffinose and stachyose, carbohydrates your small intestine can't digest. Gut bacteria ferment them instead, and that produces gas. In a double-blind crossover study, healthy volunteers who ate 80 grams of conventional soy flour passed gas about 7.5 times in 12 hours, compared with 3.2 times after a rice meal. Flour made from soybeans naturally low in these carbohydrates brought it down to 3.9 times. Other abdominal symptoms didn't differ significantly (Suarez et al., 1999).
Eighty grams of soy flour is a large amount, so everyday portions will usually have a smaller effect. Isoflavone supplements are concentrated extracts rather than whole beans, so they may carry fewer of these gas-producing carbohydrates, though how well people tolerate them still varies.
Flaxseed: disappointing for hot flashes, and high in fiber
Early, preliminary data raised hopes for flaxseed. Then a larger phase III randomized trial put it to the test. In that trial, 188 postmenopausal women ate either a flaxseed bar providing 410 mg of lignans or a placebo bar every day for six weeks. Hot flash scores fell by 4.9 in the flaxseed group and by 3.5 in the placebo group, a difference that wasn't statistically significant (Pruthi et al., 2012).
Digestive effects showed up in both groups. Women eating either bar reported abdominal distension, gas, diarrhea and nausea, which the researchers thought was probably due to the fiber in the bars (Pruthi et al., 2012). If you add flaxseed for other reasons, start with a small amount and build up gradually.
Side by side
Soy foods, such as tofu, edamame and soy milk. Hot flashes: soy isoflavones from foods and supplements were linked to a modest reduction in one large analysis. Gut: whole-bean products can increase gas. Might suit you if: you already enjoy soy and prefer a food-first approach.
Soy isoflavone supplements. Hot flashes: pooled trials showed modest reductions in frequency and severity. Gut: concentrated extracts may cause less gas than whole soy, but products vary. Might suit you if: you want a measured daily dose and your doctor agrees.
Flaxseed. Hot flashes: no significant benefit over placebo in the largest trial. Gut: fiber-rich, and digestive side effects were common in that trial. Might suit you if: you want it as a food for other reasons, not as a hot flash remedy.
Before you try either
Talk to your doctor first if you've had breast cancer or another hormone-sensitive condition, or if you take medications, including hormone therapy.
Give it at least six weeks, the shortest duration used in the trials above, and track hot flashes and digestion while you do.
Change one thing at a time so you can tell what's helping and what's causing gas or bloating.
Don't settle for bothersome hot flashes. If a food-based approach isn't enough, ask your doctor about other treatment options.
Setchell KD, Clerici C. Equol: history, chemistry, and formation. J Nutr. 2010;140(7):1355S-62S. PMID 20519412
Taku K, Melby MK, Kronenberg F, Kurzer MS, Messina M. Extracted or synthesized soybean isoflavones reduce menopausal hot flash frequency and severity: systematic review and meta-analysis of randomized controlled trials. Menopause. 2012;19(7):776-90. PMID 22433977
Franco OH, Chowdhury R, Troup J, et al. Use of plant-based therapies and menopausal symptoms: a systematic review and meta-analysis. JAMA. 2016;315(23):2554-63. PMID 27327802
Suarez FL, Springfield J, Furne JK, Lohrmann TT, Kerr PS, Levitt MD. Gas production in human ingesting a soybean flour derived from beans naturally low in oligosaccharides. Am J Clin Nutr. 1999;69(1):135-9. PMID 9925135
Pruthi S, Qin R, Terstreip SA, et al. A phase III, randomized, placebo-controlled, double-blind trial of flaxseed for the treatment of hot flashes: North Central Cancer Treatment Group N08C7. Menopause. 2012;19(1):48-53. PMID 21900849
This article is for general information and isn't medical advice. Check with your healthcare provider before taking soy isoflavone or flaxseed supplements, especially if you have a history of breast cancer or take hormone therapy or other medications.
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