Red Clover (Trifolium pratense L.): A Comprehensive Reference
1. Identity and Botanical Classification
Botanical name: Trifolium pratense L. Family: Fabaceae (Leguminosae). Trifolium means "three leaves" in Latin, while pratense means "grown in meadows." The plant is a perennial member of the legume family, closely related to beans and peas.
Red clover is also known by the common names cow clover, wild clover, purple clover, beebread, cow grass, and meadow clover. As it propagated outward from its native habitat in Europe, North Africa, and central Asia, red clover was adopted by Russian herbalists, practitioners of traditional Chinese medicine, and indigenous North American healers.
Red clover dietary supplements are currently used to treat menopausal symptoms because of their high content of the mildly estrogenic isoflavones daidzein, genistein, formononetin, and biochanin A. The importance of Trifolium pratense L. as a dietary supplement and its use in traditional medicine has prompted the preparation of thorough metabolite profiles.
Plant Description
Crowned with a soft, brush-like, deeply pink blossom, the red clover plant is as pretty as it is persistent. Though it likes sunny, well-drained soils best, it can grow in many kinds of soil conditions and climates. It is well known as one of the best rotation crops for fixing nitrogen into farmland.
Common Forms and Preparations
Red clover is available in the form of teas, tinctures, tablets, and capsules, as an ointment for topical application, and as extracts standardized to specific isoflavones. The flowering tops of Trifolium pratense L. are used in ethnic Western and Traditional Chinese medicine, in a variety of preparations, including infusions, decoctions, and tinctures.
Two heavily promoted extracts include Promensil, for the treatment of menopausal symptoms, and Trinovin, for prostate health. There are also commercial extracts marketed for menopause-related health concerns such as bone loss and dyslipidemia (Rimostil®).
Total aboveground parts of red clover generally contain more isoflavones than flower heads alone, and would therefore be the optimal plant part to use in making isoflavone botanical dietary supplements.
2. Traditional and Historical Use
European Traditions
European herbalists documented its use for respiratory conditions, wound healing, and women's reproductive wellness as early as the 14th century. In folk tradition, red clover was associated with the Christian doctrine of the Trinity because of its threefold leaflets; in England it was worn as a magic charm to protect against evil. Greeks and Romans associated it with the triad goddesses, while the clover reminded Celtic priests of their three-lobed symbol of the sun.
King's American Dispensatory (1898) recorded its use as an alterative and remedy for whooping cough and other spasmodic coughs. It described red clover as "an excellent alterative" with noted usefulness in pertussis and other spasmodic coughs, including those of measles, bronchitis, laryngitis, and phthisis.
North American Indigenous Use
The herb's value as a medicinal remedy became more widely known after it made its way to North America, where Native American herbalists found numerous medicinal uses for this common wayside plant. Indigenous peoples of North America used red clover as a spring tonic and blood purifier. Red clover was used as a cancer treatment; the blossoms, combined with other herbs, became commercially popular in the United States in the 1930s, and numerous so-called "Trifolium Compounds" were marketed as blood purifiers, or alteratives, to help clear the body of metabolic toxins.
Traditional Chinese Medicine and Russian Folk Medicine
The flowering tops of Trifolium pratense L. are used in Traditional Chinese medicine for a variety of purposes. Traditional Chinese medicine recognized red clover for clearing heat and supporting skin health. Russian folk healers and traditional Chinese physicians have long used red clover for asthma, bronchitis, and to clear the lungs.
Traditional Preparations by Purpose
- Respiratory complaints: Red clover was highly regarded as a cleansing medicinal herb, especially when chronic disease had set into the lungs or the skin; Russian folk healers and traditional Chinese physicians used it for asthma, bronchitis, and to clear the lungs.
- Skin conditions: Red clover blossoms are traditionally used to support skin health; early herbal practitioners found its benefits best experienced when harvested and dried then consumed as a tea or herbal tincture.
- Alterative / blood purifying: Part of a classification of herbs known as alteratives, red clover traditionally supports the detoxification functions of the liver and lymphatic systems.
- Women's health and reproductive support: Red clover has been used traditionally across European and North American herbal systems for women's health, gentle detoxification, and respiratory support.
3. Key Constituents and Active Compounds
Isoflavones: The Principal Bioactives
Thirty-one isoflavones in the leaves and stems, 25 in the roots, and 26 in flowers of red clover (Trifolium pratense) have been identified, with the highest concentrations of formononetin and biochanin A — the two major red clover isoflavone aglycones — found in the leaves.
Biochanin A and formononetin are the 4′-O-methylated plant precursors to genistein and daidzein, respectively, and are the predominant isoflavones in alfalfa, chickpea, and red clover. Red clover has been shown to contain substantial amounts of isoflavonoids, especially biochanin A and formononetin, along with their respective glucosides, that constitute one of the major sources of isoflavonoids in western dietary supplements.
In isoflavonoid-enriched clinical red clover extracts (RCE), isoflavonoid content can be much higher, reaching levels such as 14.5% w/w of biochanin A and 14.3% of formononetin, in addition to 0.23% of daidzein and 0.41% of genistein.
The four principal isoflavones in red clover are:
- Biochanin A — the most abundant in most extracts; a precursor to genistein upon demethylation in the body.
- Formononetin — the second most abundant; a precursor to daidzein, which may be further metabolized to equol in certain individuals.
- Genistein — present in smaller quantities as an aglycone in the plant.
- Daidzein — present in smaller quantities; can be metabolized to equol by gut microbiota in some individuals.
A total of 28 phenolic compounds have been tentatively identified in red clover roots. The most abundant phenolics in pot-grown roots were formononetin glycoside malonate (1.51–4.26 mg/g), formononetin (2.21–3.57 mg/g), and biochanin A (1.73–2.17 mg/g).
The predominant isoflavones accumulated in red clover tissues are glycosides and glycosyl-malonate conjugates of the aglycones formononetin, calycosin, pratensein, genistein, and biochanin A. In addition to these, two isoflavonoid glycosides, 17 isoflavonoid aglycones, and related compounds have been identified and quantified; three tricin derivatives (B-ring methoxylated flavonoid aglycones) have been identified from red clover for the first time.
Metabolic Conversion and Bioavailability
In vivo, formononetin is metabolized to daidzein, which may be metabolized to equol among equol producers. Aglycone-rich isoflavone supplements are thought to be more bioavailable than glycoside-rich products. Analysis of red clover isoflavones in serum shows primarily conjugated metabolites that explain, at least in part, the red clover pharmacokinetic safety profile.
4. Mechanisms of Action
Estrogenic and Selective Estrogen Receptor Modulator (SERM) Activity
Red clover isoflavones, with structural similarities to endogenous 17β-estradiol, reveal their biological effects via activating estrogen receptors (ER), with a higher affinity to ER-β in comparison to ER-α. Isoflavones resemble 17-beta-estradiol in structure and as such are able to bind the estrogen receptor (ER) in vitro, behaving much as a natural selective estrogen receptor modulator (SERM).
Soy and red clover isoflavones have been found to exert partial ER agonist and antagonist activity depending on local estrogen concentrations, with antagonist properties in concentrations similar to premenopausal levels, and agonist properties with postmenopausal levels.
Although less potent than conventional estrogenic compounds, their selective beta-estrogenic receptor-binding properties allow beneficial effects on specific organs or systems.
Aromatase and Enzyme Modulation
The red clover isoflavone biochanin A has been shown to inhibit aromatase activity and expression (Wang et al., published in the British Journal of Nutrition, 2008). Isoflavones, which are structurally similar to 17-β-estradiol, reveal estrogenic activity both by affecting estrogen receptors and aromatase activity, and can also affect 5-α-reductase activity, which is responsible for the transformation of testosterone to dihydrotestosterone.
Non-Hormonal Mechanisms
In addition to estrogen receptor activation, a number of non-hormonal effects have been reported for isoflavones, including tyrosine kinase inhibition and antioxidant activity. Isoflavone-enriched extracts have shown neuroprotective effects in human cortical neurons and reduced skin aging in mice by increasing collagen content.
5. Scientific Evidence by Area of Use
5.1 Menopausal Vasomotor Symptoms (Hot Flushes, Night Sweats)
This is the most heavily researched area of red clover supplementation. Studies on the effects of red clover on hot flashes associated with menopause have had inconsistent results, and some of the studies had a high risk of bias.
Systematic Reviews and Meta-Analyses:
A meta-analysis of eight trials (ten comparisons) demonstrated a statistically significant reduction in the daily incidence of hot flushes in women receiving red clover compared to those receiving placebo: weighted mean difference (WMD) −1.73 hot flushes per day, 95% CI −3.28 to −0.18; p = 0.0292. The analysis showed substantive differences particularly in comparisons of postmenopausal women with ≥5 hot flushes per day, when the follow-up period was 12 weeks, with an isoflavone dose of ≥80 mg/day, and when the formulations contained a higher proportion of biochanin A.
There is evidence for a statistical and clinically significant benefit for using a specific standardised extract of red clover isoflavones (Promensil) at 80 mg/day for treating hot flushes in menopausal women across the studies included in that meta-analysis. The preparation was safe over the short-term duration of the studies (3 months).
Despite these positive findings, earlier systematic reviews were more cautious. Conclusions from systematic reviews have been conflicting: one suggested marginal benefit, whereas another found no evidence of effectiveness. One review applying FDA and EMEA well-designed trial criteria found that none of the five well-designed red clover trials demonstrated a significant or clinically meaningful benefit for the relief of hot flashes.
Studies on the effects of red clover on menopause symptoms, such as hot flashes, and on blood levels of cholesterol and other lipids have had inconsistent results. A 2016 systematic review and meta-analysis found that red clover significantly improved vaginal dryness and vaginal atrophy, but showed less therapeutic effect on psychology status, sexual problems, and sleeping disorders. That review concluded that red clover consumption may decrease frequency of hot flashes, especially in women with severe hot flashes (5 or more per day).
Specific Clinical Trials:
A parallel, double-blind, randomized controlled trial enrolled 62 perimenopausal women aged 40–65 reporting ≥5 hot flushes per day and follicle stimulating hormone ≥35 IU/L. Participants received either twice-daily treatment with a bioavailable red clover extract providing 34 mg/day of isoflavones with probiotics, or a masked placebo formulation for 12 weeks. A significant decrease in 24-hour hot flush frequency (P < 0.01) and hot flush intensity (P < 0.05) was found when comparing change from baseline of the red clover extract group (−4.3 HF/24 hr) with placebo (+0.79 HF/24 hr). Self-reported hot flush frequency was also significantly reduced in the red clover extract group compared to placebo.
In one trial, red clover isoflavone (80 mg/day) significantly relieved hot flashes, mood, pain, and cognitive symptoms. The only trial in one systematic review that significantly reduced cognitive symptoms was red clover at 80 mg.
Evidence strength: Red clover has been studied in people, but the research hasn't shown conclusive evidence of benefits for any health condition. Some of the research on red clover for hot flashes associated with menopause was not high in quality, findings on cholesterol levels were conflicting, and only a few studies have been done on bone density. Studies on the effects of red clover on hot flashes associated with menopause have had inconsistent results, and some of the studies had a high risk of bias. Overall, the evidence leans toward a modest benefit, particularly at higher doses (≥80 mg/day total isoflavones) and in women with more frequent symptoms, but cannot be considered conclusive.
5.2 Cardiovascular Health
The possibility that the heightened cardiovascular risk associated with the menopause can be reduced by increasing dietary isoflavone intake was tested in 17 women by measuring arterial compliance — an index of the elasticity of large arteries such as the thoracic aorta. An initial 3–4 week run-in period and a 5-week placebo period were followed by two 5-week periods of active treatment with 40 mg and then 80 mg isoflavones derived from red clover.
Arterial compliance rose by 23% relative to that during the placebo period with the 80-mg isoflavone dose and slightly less with the 40-mg dose (placebo: 0.197 ± 0.015; 40 mg: 0.237 ± 0.007; 80 mg: 0.244 ± 0.014). Plasma lipids were not significantly affected.
A meta-analysis of a standardized extract suggests potential reductions in total cholesterol in peri- and postmenopausal women, but studies were heterogeneous. Red clover may have a beneficial effect on cholesterol levels in postmenopausal women, but more research is needed before definite conclusions can be reached.
Evidence strength: The Nestel et al. arterial compliance study (1999) provided notable positive results for vascular elasticity, but involved only 17 women in a crossover design. Clinical evidence is presently lacking to support the efficacy of semipurified red clover isoflavone extracts for reduction of low-density lipoprotein levels in the blood. Cardiovascular evidence overall is preliminary; the arterial compliance finding has not yet been replicated in large-scale trials.
5.3 Bone Mineral Density
Red clover isoflavones have been reported in some studies to reduce bone loss, in addition to improving arterial compliance. In a trial involving osteopenic postmenopausal women, a red clover extract rich in isoflavones, aglycones, and probiotics attenuated bone mineral density loss caused by estrogen deficiency and improved bone turnover.
Only a small number of studies have been done on the effects of red clover on bone density in postmenopausal women, and the results have been inconsistent.
Evidence strength: Preliminary. The number of human clinical trials addressing bone endpoints is small, and findings are not uniform. The available evidence comes largely from short-duration trials in postmenopausal women.
5.4 Skin and Mucosal Health
Isoflavone-enriched red clover extracts have shown neuroprotective effects in human cortical neurons and reduced skin aging in mice by increasing collagen content. Although less potent than conventional estrogenic compounds, their selective beta-estrogenic receptor-binding properties allow beneficial effects on specific organs or systems. Interest in red clover isoflavones has equally grown among physicians and women with reports evidencing positive effects over menopausal symptoms, vaginal health, serum lipids, and a promising safety profile.
A 2016 systematic review and meta-analysis found that red clover significantly improved vaginal dryness and vaginal atrophy.
Evidence strength: The preclinical evidence (animal and in vitro) for collagen synthesis and skin effects is consistent. Clinical trial data for skin and vaginal mucosal outcomes are limited in volume but directionally positive in the context of the menopausal transition.
5.5 Breast Cancer — Safety and Efficacy Considerations
Soy and red clover isoflavones are controversial due to purported estrogenic activity and possible effects on breast cancer. Systematic reviews have examined soy and red clover for efficacy in improving menopausal symptoms in women with breast cancer, and for potential impact on risk of breast cancer incidence or recurrence.
Preclinical studies suggest red clover extract may act as an estrogen agonist and stimulate proliferation of ER-positive breast cancer cells. However, the isoflavone biochanin A inhibited aromatase activity and expression and may therefore confer a protective effect.
In one in vitro study, isoflavones and plant extracts significantly reduced the proliferation activity of treated cancer cell lines; growth promotion was not observed, but apoptosis or necrosis induction was, as was cell cycle arrest, with genistein as the most potent isoflavone. Isoflavones and plant extracts from soy and red clover do not promote the growth of human cancer cells but induce decreased cell proliferation, increased apoptosis, and cell cycle arrest in vitro.
A study investigating the safety of red clover in women with a family history of breast cancer found no changes in breast density or thickness of the uterine lining over a three-year period, which is somewhat reassuring. However, the study was much too short to determine red clover's long-term effect on cancer risk.
Evidence strength: In vitro and animal data are mixed and mechanistically complex. In summary, red clover appears useful in alleviating menopausal symptoms, but there are no data to support its anticancer effects. Clinical extrapolation from in vitro findings is not warranted.
5.6 Prostate Health
Some early research suggests that red clover supplements might improve some symptoms of an enlarged prostate and may reduce nighttime urination. However, red clover does not appear to affect urine flow rate, PSA values, or prostate size.
Of concern are findings that red clover inhibits the growth of normal prostate cells and increases resistance of prostate cancer cells to high-dose radiation in vitro. Recent findings from a preclinical study suggest that red clover may increase resistance of prostate cancer cells to high-dose radiation while inhibiting the growth of normal prostate cells. Until definitive data are available, patients should avoid use of red clover during radiotherapy for prostate cancer.
Evidence strength: Human data for prostate-related endpoints are very limited. In vitro findings raise concerns about use during radiation therapy; however, clinical evidence is insufficient to draw firm conclusions.
6. Body Systems and Health Areas Associated with Red Clover
- Endocrine / hormonal: Phytoestrogenic modulation of estrogen receptors; effects on menopausal vasomotor symptoms and hormonal balance.
- Cardiovascular: Improvement of arterial compliance; modest lipid-profile effects in some studies.
- Skeletal: Potential attenuation of postmenopausal bone mineral density loss.
- Skin and mucosa: Collagen synthesis; vaginal atrophy; mucosal health in menopause.
- Reproductive / gynecological: Red clover has been reported to be helpful for treatment of menopausal symptoms, premenstrual syndrome, and mastalgia.
- Respiratory: Traditional use as an expectorant for bronchitis and cough; not substantiated by modern clinical trials.
- Neurological: Isoflavone-enriched red clover extracts demonstrated neuroprotective effects in human cortical neurons. This finding is preclinical only.
7. Dosage Forms and Doses Reported in Studies
Red clover is available in the form of teas, tinctures, tablets, and capsules, as an ointment for topical application, and as extracts standardized to specific isoflavones.
Most of the clinical trial evidence centers on standardized isoflavone extracts. Key doses documented in the primary literature and systematic reviews include:
- 40 mg and 80 mg of red clover-derived isoflavones (containing genistein, daidzein, biochanin, and formononetin) were used in 5-week crossover treatment periods in the Nestel et al. arterial compliance trial.
- 34 mg/day of isoflavones (as a bioavailable red clover extract with probiotics) was used in the 12-week double-blind RCT of 62 perimenopausal women.
- 80 mg/day of red clover isoflavones was the dose used in the trial that significantly relieved hot flashes, mood, pain, and cognitive symptoms.
- One double-blind study of postmenopausal women found that red clover isoflavones at a dose of 80 mg daily for 90 days resulted in increased levels of testosterone.
- The meta-analyses suggest that a dose of ≥80 mg/day and a follow-up period of 12 weeks produced the most consistently positive results for vasomotor symptoms.
Regarding commercial standardized preparations, the most widely studied extract (Promensil) is formulated at 40 mg or 80 mg of four isoflavones — genistein, daidzein, formononetin, and biochanin A — sourced from red clover (Trifolium pratense). Differences in outcomes between products may be due partly to the low quality of certain preparations, or in part to the amount of aglycones within the extract in comparison to glycosylated isoflavones; aglycone-rich isoflavone supplements are thought to be more bioavailable than glycoside-rich products.
8. Safety Considerations and Drug Interactions
General Safety Profile
There is evidence for a statistical and clinically significant benefit for using a specific standardised extract of red clover isoflavones at 80 mg/day for treating hot flushes in menopausal women. The preparation was safe over the short-term duration of the studies (3 months). Red clover extract was well tolerated. Results suggest that moderate doses of RCE were more effective and superior to placebo in reducing physiological and self-reported vasomotor symptoms.
Anticoagulant and Antiplatelet Interactions
Red clover may increase the effects of anticoagulants and antiplatelet drugs. Subarachnoid hemorrhage was reported in a 53-year-old woman following use of an herbal supplement that contained red clover, dong quai, and Siberian ginseng, which she took for hot flashes associated with perimenopause. Red clover was also associated with toxic effects when used by a patient receiving methotrexate injections for severe psoriasis.
Cytochrome P450 Interactions
Red clover inhibits CYP1A2, 2C8, 2C9, 2C19, 2D6, and 3A4, and may interact with substances metabolized by these enzymes. However, a dedicated clinical pharmacokinetic study found reassuring results: analysis of red clover isoflavones in serum showed primarily conjugated metabolites that explain, at least in part, the red clover pharmacokinetic safety profile.
Research investigating the effect of red clover on cytochrome P450 enzyme mRNA levels in rats found that red clover's effect on cytochrome P450 expression was also investigated when co-administered with tamoxifen and raloxifene. Exposure to red clover resulted in significant downregulation of most cytochrome P450 isoform mRNA. Clinical significance of these findings in humans requires further investigation.
Hormone-Sensitive Conditions
Patients with breast cancer should avoid red clover because it may stimulate proliferation of estrogen receptor-positive breast cancer cells in vitro. However, biochanin A, a red clover isoflavone, inhibits aromatase activity and expression, and may confer a protective effect. The clinical net effect in cancer patients has not been adequately characterized.
One study found that red clover isoflavones reduced the thickness of the uterine lining, a finding that suggests a low possibility of increasing the risk of endometrial cancer. A statistically significant decrease in endometrial thickness in postmenopausal women following consumption of red clover was also observed in one systematic review, based on one trial.
Prostate Cancer and Radiotherapy
Preclinical findings suggest that red clover may increase resistance of prostate cancer cells to high-dose radiation while inhibiting the growth of normal prostate cells. Until definitive data are available, patients should avoid use of red clover during radiotherapy for prostate cancer.
Pregnancy and Lactation
Red clover has phytoestrogens which can act like human estrogen. There isn't enough reliable information on the safety of red clover use during pregnancy and breastfeeding. Because of its blood-thinning and estrogen-like constituents, red clover should not be used by pregnant or nursing women or by women who have had breast or uterine cancer.
Pediatric and Hepatic Populations
Safety in young children and in those with severe liver or kidney disease has also not been established.
Herb-Drug Interactions
Based on their constituents, red clover extracts may conceivably interfere with hormone treatments and anticoagulant drugs. Red clover may slow down the breakdown of drugs that are metabolized by the liver and increase their effects.
References
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