S-equol vs R-equol: which form has human evidence, and what the menopause trials actually show

Equol is not something you eat. It is made in the gut, when certain bacteria convert daidzein, an isoflavone in soy, into a compound that binds oestrogen receptors more strongly than daidzein itself. Not everyone carries those bacteria, which is why equol supplements exist.

Labels now name S-equol or R-equol, mirror-image versions of one molecule. Only S-equol is made by gut bacteria, and only S-equol has clinical trials behind it.

The short answer: if you are comparing equol products, S-equol is the form with human evidence. R-equol, and the racemic mixes that contain it, have been given to people only in small absorption studies, with no published trials of symptoms or health outcomes. The S-equol evidence itself is modest: mostly small, short trials in Japanese women, often run or supplied by the company that makes the main ingredient, with the clearest signal for hot flush frequency in women who cannot make equol themselves.

What equol is, and who makes it

Equol was first isolated from horse urine in 1932 and identified in human urine about 50 years later as a metabolite of the soy isoflavones daidzin and daidzein, produced by intestinal bacteria in some adults but not others. People who make it are called equol producers. In a dietary study of 159 healthy adults, producers made up 30.3% of the US participants and 28.6% of the Australian participants, and producer status stayed the same over two years in all non-producers and in 80% of producers. A 2015 review put it at about 50% of Asian adults and 25% of non-Asian adults, though samples vary: 34.3% of the women in one Japanese trial were producers.

Non-producers cannot easily become producers: a 2026 systematic review of 12 randomised trials (730 participants) of soy, probiotics, fermented soy, seaweed and wheat bran found none that reliably converted them.

S-equol and R-equol: same formula, different shape

Equol has a chiral centre, so it has two enantiomers. In 2005, Kenneth Setchell's group showed that human gut bacteria make S-(-)-equol exclusively. In the same paper, S-equol bound oestrogen receptor beta (ERβ) with high affinity, while R-equol was relatively inactive at that receptor. An earlier laboratory study reached a similar conclusion with different numbers: S-equol bound ERβ about 13 times more strongly than ERα, whereas R-equol bound more weakly overall and leaned towards ERα. Test-tube binding is not a health outcome, but it is why researchers treat the two forms as different ingredients.

Both forms are absorbed. In a crossover study of 12 healthy adults, both enantiomers reached high blood levels with a half-life of seven to eight hours, and R-equol had higher bioavailability than S-equol. The racemic mixture was absorbed more slowly and peaked lower than either pure form, so results from one cannot be carried over to another.

Otsuka's SE5-OH is a fermented soy germ powder in which the bacterium Lactococcus 20-92 converts daidzein to S-equol. Chemical synthesis can produce pure S-equol, pure R-equol or a racemic mix. A label that says only "equol" does not tell you which.

Does R-equol have any human evidence?

Very little. The human R-equol data are the 2005 absorption study (three adults) and the 2009 crossover study (12 adults). No randomised trial of R-equol or racemic equol on menopausal symptoms, bone, skin, blood vessels or cognition turned up in the trial literature cited here. The rest is animal and laboratory work, and it does not point one way. In ovariectomised mice, S-equol protected bone mineral density while R-equol did not. In rats exposed through the diet around birth, both enantiomers caused overgrowth of uterine tissue; with R-equol the effect faded after exposure stopped, but with S-equol it lasted longer. A 2017 review of skin gene-expression work by an author holding equol patents reported more activity for R-equol and racemic equol than for S-equol on laboratory markers. None of that shows what R-equol does in a person.

S-equol and menopausal symptoms

• Aso 2012, Journal of Women's Health: 160 postmenopausal Japanese women who were equol non-producers and had at least one hot flush a day took 10 mg of S-equol or placebo daily for 12 weeks; 126 finished. Hot flush frequency fell by 1.9 a day (58.7%) on S-equol against 1.0 a day (34.5%) on placebo (p = 0.009). Severity and neck or shoulder stiffness also fell more than on placebo, with no serious adverse effects.
• Ishiwata 2009, Menopause: 134 Japanese women aged 40 to 59 took placebo, 10 mg of equol once a day or 10 mg three times a day for 12 weeks. Compared with placebo, the three-times-daily group had lower depression scores and better scores for tension-anxiety, depression-dejection, fatigue and vigour on a mood questionnaire. One woman in that group had a systemic rash.
• Jenks 2012, Journal of Women's Health: 102 postmenopausal women with at least five hot flushes a day took 10, 20 or 40 mg of S-equol or soy isoflavones for eight weeks. There was no placebo group. Reductions at week eight were similar across groups; over the eight weeks, 40 mg cut hot flush frequency more than isoflavones (p = 0.021). The first author worked for Pharmavite, a subsidiary of Otsuka.
• Daily 2019, Journal of Medicinal Food: a meta-analysis of five randomised trials (728 women) of equol, or soy in equol producers, found a significant overall reduction in hot flush scores, though two of the five found no significant benefit. The first author worked for a supplement manufacturer.
• Palacios 2026, BMC Women's Health: an open-label study in Spain gave 10 mg of S-equol daily for 12 weeks to 159 women, aged 52.5 on average. Menopause Rating Scale scores improved by 11.6 points, but with no placebo group that cannot be separated from placebo response. Two of its four authors work for Nutrition & Santé, Otsuka's European partner on the ingredient.

A 2015 review co-written by Setchell, who holds intellectual property on equol enantiomers, said the trials were limited in scope and mainly in Japanese women, the only US hot flush trial had no placebo, and further studies were needed. The newest study is open-label and industry-linked, so that still holds.

Skin, bone, metabolism and blood vessels

• Skin: in a 12-week trial in 101 equol-non-producing postmenopausal Japanese women, 10 mg and 30 mg a day reduced crow's-feet wrinkle area against placebo, and 30 mg reduced wrinkle depth. Hydration, water loss and elasticity did not differ. The first author worked at Otsuka's research institute.
• Bone: a one-year pilot trial in 93 non-producing postmenopausal Japanese women found that 10 mg a day lowered urinary deoxypyridinoline, a marker of bone breakdown, by 23.94% against 2.87% on placebo (p = 0.020), and prevented the fall in whole-body bone density seen without it. Sex and thyroid hormone levels did not change. That is not evidence of fewer fractures.
• Blood vessels, acute: in a UK trial in men at moderate cardiovascular risk, a single 40 mg dose of S-equol in non-producers had no effect on arterial stiffness or other vascular measures two hours later, even though blood equol reached 3.2 µmol/L. Soy did improve arterial stiffness at 24 hours in producers.

Cognition: a pilot, and one large trial still to report

A two-week single-arm pilot gave 10 mg of S-equol twice a day to 15 women with Alzheimer's disease and measured a platelet mitochondrial enzyme; 11 of 15 met the study's definition of a positive response (p < 0.06). That is an enzyme signal, not a memory result. The ACE trial in the US has randomised 369 adults aged 65 to 85 without dementia to 10 mg of equol or placebo daily for 24 months, with arterial stiffness as the primary outcome and white-matter lesions and cognition as secondary outcomes. Results have not been reported; it will be the first large test of whether equol changes vascular or cognitive ageing.

Safety

In two placebo-controlled studies of synthetic S-equol, single doses up to 320 mg and 14 days of up to 160 mg twice a day caused no significant drug-related adverse events. The menopause and skin trials above reported no serious adverse effects. In the skin trial, hormone tests, endometrial thickness and mammograms showed no abnormal results after 12 weeks, and in the one-year bone trial sex and thyroid hormone levels did not differ from placebo. Reassuring, but short and small.

Equol acts on oestrogen receptors, so hormone-sensitive conditions are the obvious question. A 2025 study in 24 healthy postmenopausal women found that a 10 mg S-equol supplement taken for a week did not meaningfully change blood levels of anastrozole, letrozole, exemestane or tamoxifen. Two of its authors were Otsuka employees. That says nothing about safety for someone with breast cancer, endometriosis or fibroids. The European application for SE5-OH proposes use by healthy adults only, excluding pregnant and breastfeeding women, with soy labelled as an allergen.

How to read an equol label

Three questions sort most products. Does the label name S-equol, or just equol? How much S-equol is in a daily dose? Most trials used 10 mg a day, sometimes as two 5 mg doses. Is it from fermented soy germ, which contains soy, or synthetic? Most menopause trials used the fermented ingredient. None of this evidence shows that equol treats any disease.

UAE rules

In Dubai, Dubai Municipality's health-supplement guideline says no health supplement may be manufactured, imported, exported, advertised, sold or distributed unless it is registered in the Montaji system, and shoppers can scan a barcode in the Montaji app to check. The same guideline says health supplements must not be intended to treat, diagnose, prevent or cure any disease. Federally, the UAE government portal says the Emirates Drug Establishment will be responsible for medical products, including food supplements, across the UAE and its free zones. A Montaji record is a safety registration, not evidence that equol works.

FAQ

How do I know if I make equol?

Studies test urine or blood after eating soy. Roughly a quarter to a third of Western adults are producers, against about half of Asian adults. Several trials enrolled only non-producers.

Is S-equol safe with a history of breast cancer?

There are no outcome trials in people with breast cancer. A small drug-level study found no meaningful change in tamoxifen or aromatase-inhibitor levels in healthy women, which is not the same as showing safety. Anyone with a hormone-sensitive condition should speak to their specialist first.

How do I check an equol supplement in Dubai?

Scan the barcode in Dubai Municipality's Montaji app. Registration shows the product has been through the municipal safety process; it does not show that it works.