Scientific Name(s): Trifolium pratense L.
Common Name(s): Cow clover, Creeping clover, Flos Trifolii, Meadow clover, Purple clover, Trefoil
This perennial herb is commonly found in light, sandy soil in meadows throughout Europe and Asia. It is naturalized in North America, where its nitrogen-fixing properties are used in pasture renovation. The plant is low and bushy with several hairy stems rising from a taproot. Dense terminal heads with up to 125 fragrant flowers are borne at the end of the branched stems. The flowers range from magenta to white in color and are butterfly shaped. The leaves are in groups of 3 ovate leaflets, often notched at the tip, with a characteristic lighter water mark on their upper surface.1, 2, 3, 4
Dried red clover flowers have been used in traditional medicine to treat a wide spectrum of ailments, including jaundice, cancer, mastitis, joint disorders, and respiratory conditions (eg, whooping cough, bronchial asthma), and as a sedative. The plant was thought to purify the blood by promoting urine and mucus production, improving circulation, and stimulating secretion of bile. Red clover ointments have been used topically to accelerate wound healing and to treat psoriasis, eczema, and rashes. Respiratory complaints have been treated with an infusion; fomentations and poultices of the whole plant have been used as topical applications for cancerous growths.3, 4
The main chemical classes contained in red clover are carbohydrates, isoflavones, flavonins, and saponins. Other constituents include coumaric acid, fats, minerals, and vitamins. A volatile oil that includes methyl salicylate is distilled from the flowers.3 Isoflavones are often termed "phytoestrogens" because of their functional similarity to estrogens. The major isoflavones in red clover are biochanin A, formononetin, daidzein, and genistein; total phytoestrogen content is approximately 0.17%.1, 2, 3, 4
Uses and Pharmacology
Much of the interest in red clover originated from observations of positive health benefits derived from the use of soy products. Both soy and red clover are sources of isoflavones and have similar estrogenic activity shown in in vitro studies to be approximately 0.25% that of 17-beta-estradiol.5
The use of red clover as natural hormone replacement therapy has been proposed because of its estrogenic activity. Phytoestrogens appear to act as partial agonists in some tissues and antagonists in others, exhibiting hormonal and nonhormonal properties.6 They have a greater affinity for beta-estrogen rather than alpha-estrogen receptors. Weak estrogenic activity has been demonstrated in rats.7 Several nonhormonal mechanisms have also been demonstrated, including tyrosine kinase inhibition, antioxidant activity, and effects on ion transport.8
The availability of clinical trial data of red clover in cardiovascular conditions makes animal data less relevant; however, experiments in animals suggest an atheroprotective effect via low-density lipoprotein (LDL) reduction.9, 10
Beneficial effects of soy protein on blood lipid profiles have been demonstrated. However, results from studies of red clover have been mixed, with only some demonstrating modest improvements in lipid profile.11, 12, 13, 14, 15 Many small studies found decreased LDL and increased high-density lipoprotein,9, 16, 17 but a larger study (N = 401) reported no effect on lipid profile in menopausal women.18 However, significant improvements in both LDL (P = 0.005) and total cholesterol (P = 0.015) were seen in menopausal women who consumed red clover isoflavone 25 mg twice daily (predominantly formononetin and biochanin) for 2 years, compared with those who took placebo in a randomized, double-blind, placebo-controlled study (n = 147). LDL was 12% lower in the treatment arm compared with 2% for placebo.51
Arterial compliance, an index of the elasticity of large arteries, improved in a small, short-term study of postmenopausal women receiving red clover.13 These results were confirmed by a larger study of normotensive men and postmenopausal women.15 Ambulatory blood pressure remained unchanged, but total peripheral resistance improved in these patients. Participants received a supplement of red clover–derived isoflavone 80 mg/day containing mostly biochanin A or formononetin; improvements were greatest in the formononetin group.
In a small clinical trial (N = 23), red clover 86 mg/day isoflavone had no effect on homocysteine,19 while women receiving 40 to 80 mg daily of red clover isoflavones in another trial (N = 43) experienced decreased insulin sensitivity.20
Menopause, vasomotor symptoms
The availability of clinical trial data and meta-analyses of red clover in menopause makes animal data less relevant.
Several meta-analyses evaluating the efficacy of red clover and other phytoestrogens in reducing hot flushes have been published. A Cochrane meta-analysis21 included 7 clinical trials using red clover alone, while 1 study22 evaluated the effect of red clover therapy duration on outcomes and included 8 trials. Another study23 evaluated phytoestrogens (not only red clover) by the composite Kupperman Index outcome (7 trials) and by the incidence of hot flushes only (11 trials). The Cochrane meta-analysis21 further includes 4 trials in which a preparation containing high amounts of genistein (more than 30 mg/day) were used.
A nonsignificant mean difference of −0.93 (95% confidence interval [CI], −1.95 to 0.1) was found for red clover alone in reducing the incidence of hot flushes versus placebo, and a nonsignificant mean difference of 20.15% (95% CI, −12.1 to 52.4) was reported for reduction of hot flushes.21
For all phytoestrogens (7 trials), a nonsignificant effect was noted using the Kupperman Index. Although, a mean difference of 0.89 (95% CI, 0.26 to 1.52) was found for reduction in hot flushes compared with placebo for 10 studies.23
For red clover given over 3 to 4 months, a mean difference in hot flush frequency of −1.34 (95% CI, −1.9 to 0.77) was reported, whereas over 12 months, a nonsignificant difference was reported (0.89 [95% CI, −0.07 to 1.85]).22
The 3 meta-analyses,21, 22, 23 as well as other systematic reviews,1, 24, 25, 26 concluded that there is no compelling evidence of red clover efficacy in managing the vasomotor symptoms of menopause. The reports note heterogeneity among the clinical studies and a high risk of bias; a strong placebo effect (range, 1% to 59%) was also found.21 No adverse effects were reported over 2 years, including no evidence of estrogenic stimulation of endometrium or vaginal tissue.21 A 2016 systematic review (n=10) and meta-analysis (n=6) of randomized controlled trials found red clover to be more likely to result in significant reductions in hot flashes for women with lower estrogen levels (ie, postmenopausal women) and for women with at least 5 hot flashes/day. In contrast, a reverse effect was seen in women experiencing at least 3 hot flashes/day. Additionally, one trial reported a significant decrease in palpitations (from 58.5% to 17%) with red clover compared to only about a 10% reduction with placebo (P<0.05).53 Similarly, a double-blind, randomized, placebo-controlled trial conducted in 62 perimenopausal women in Denmark observed significant improvements in both objective and subjective vasomotor symptoms (P<0.01 and P<0.05, respectively) in the group who received a probiotic-rich red clover extract. The extract was standardized to 33.78 mg/day aglycones and given as 37.1 mg/day for 12 weeks. Women between the ages of 40 and 65 years with a body mass index (BMI) of 20 to 40 who experienced at least 5 hot flushes/day were eligible. The extract was well tolerated and no side effects were reported.56 A randomized, triple-blind, placebo-controlled trial in 72 postmenopausal Iranian women with a BMI up to 28 documented significant improvements in menopausal symptom scores for women who received red clover capsules daily (80 mg/day dried leaves) for 12 weeks compared to placebo. Total scores improved 10.33 points with red clover compared to 3.57 with placebo (P = 0.0001). Mean vegetative somatic scores (eg, hot flushes, heart discomfort, sleeping disorders) were significant (P = 0.002). No side effects were observed.52
The Society of Obstetricians and Gynaecologists of Canada revised clinical practice guidelines on managing menopause (2014) recommend that complementary and alternative medicine that has demonstrated efficacy for mild menopausal symptoms, including products containing isoflavones sourced from red clover, may be offered by health care providers (Level I-B).54 The Endocrine Society clinical practice guidelines for the treatment of symptoms of the menopause (2015) recommend counseling patients on the lack of consistent evidence for benefit of complementary medicine therapies, including red clover, as an alternative nonhormonal therapy for vasomotor symptoms (weak recommendation; low quality evidence).55
Menopause, CNS effects
In ovariectomized rats, red clover restored the pain threshold to levels matching those of the animals in the control group (intake ovary).27
Secondary outcomes of some clinical trials have included effects on CNS symptoms in postmenopausal women. A clinical trial using a crossover design was conducted with 109 women given 80 mg of red clover isoflavones or placebo for 90 days. Decreases of 70% were found for depression and anxiety compared with baseline. A reduction of 22% was found for the placebo.28 Another study (N = 66) reported no effect of 120 mg of red clover isoflavones on any cognitive measures in postmenopausal women.29 However, a randomized, triple-blind, placebo-controlled trial in 72 postmenopausal Iranian women with a BMI up to 28 documented significant improvements in menopausal symptom scores for women who received red clover capsules daily (80 mg/day dried leaves) for 12 weeks compared to placebo. Total scores improved 10.33 points with red clover compared to 3.52 with placebo (P = 0.0001). With the exception of vaginal dryness, no significant improvement was seen in urogenital subcategory scores; however, mean vegetative somatic scores (eg, hot flushes, heart discomfort, sleeping disorders) and differences in mean psychological scores (eg, anxiety, irritability, physical and mental exhaustion) were significant (P = 0.002, P = 0.0001, respectively). No side effects were observed.52
Menopause, sexual symptoms
Changes in sexual symptoms were not found to be significantly different between the red clover and placebo groups in 3 of 4 studies reviewed systematically; one study, however, reported significant decreases in dyspareunia and low libido with administration of red clover compared to placebo (P<0.05). Additionally, significant reduction in vaginal dryness was reported with red clover (41% fewer symptomatic patients) compared to placebo (5% fewer symptomatic patients) in the one study that assessed this outcome (P<0.05).53
Menopausal bone loss
The availability of clinical trial data and systematic reviews of red clover in menopause-related osteoporosis makes animal data less relevant.
Results from trials using red clover isoflavones have produced equivocal findings.30, 31 Subgroup analyses in studies suggest that some women might benefit from supplementation with red clover; however, most studies find no effect.30, 31, 53
A review of epidemiological studies among Asian populations suggested that consumption of soy foods is associated with a decreased risk of prostate cancer.33 However, a meta-analysis of 15 studies (through July 2008) on the effect of soy protein on free androgen index, sex hormone binding globulin, or testosterone found no evidence of effect.1, 34, 35 A petition to the US Food and Drug Administration (FDA) regarding the use of soy-protein products to reduce the risk of prostate and breast cancer was withdrawn.36
Formerly used as a sedative at doses of 4 g blossoms, red clover is now used chiefly as a source of isoflavones. Extracts standardized for isoflavone content have been used frequently in clinical trials.26 These tablets contain biochanin A, formononetin, genistein, and daidzein.7 A usual dosage is 40 to 80 mg/day of total isoflavones (1 to 2 tablets). Clinical trials have used higher doses (160 mg/day); however, long-term safety of such doses has not been established.21 Crude plant extract 240 mg approximates to 40 mg of isoflavones.1 Several commercial preparations are available.
Pregnancy / Lactation
Avoid use. The estrogenic effects of red clover are well documented and may have an effect on the fetus.
An additive effect may occur if red clover phytoestrogens are taken with hormonal therapies; avoid concurrent use with oral contraceptives, estrogen, or progesterone therapies.
Genistein content may negate the effects of tamoxifen and aromatase inhibitors such as letrozole.26
Red clover contains disputed amounts of coumarin derivatives (including coumarin, medicagol, and coumestrol); however, the risk of anticoagulant abnormalities is low.1, 3, 37, 38, 39 A case report of bleeding exists; however, the patient was taking a combination preparation that included dong quai and Siberian ginseng, as well as red clover.40
Contraindicated in patients with hormonal disorders or estrogen-dependent breast cancer (or risk of), and during pregnancy or lactation. Red clover supplementation is not advised in children younger than 12 years.1, 9, 36
Few adverse effects of red clover have been reported in clinical trials at doses of up to 160 mg isoflavones per day.1, 34, 41 Loss of appetite, pedal edema, and abdominal tenderness have been reported after high doses of isoflavones (4 or 8 mg/kg).42 Free and total genistein and daidzein disappeared rapidly from the plasma in this study and are unlikely to accumulate in the body with regular use of 2 to 3 times daily.
The phytoestrogens in red clover may be expected to act through estrogenic mechanisms with the associated risk of estrogen-like adverse effects, including increased incidence of endometrial, ovarian, and breast cancers36; however, whether phytoestrogens are beneficial or harmful to human health is unresolved.43 Red clover induced a proliferation of estrogen-sensitive breast cancer cells in an in vitro study.36, 44 Infertility and growth disorders have been observed in grazing animals, including sheep receiving high proportions of red clover in their feed. These effects have been attributed to the estrogenic activity of red clover.2 One report suggests that this estrogenism disease state is more likely to occur with Trifolium subterraneum than with Trifolium pratense.45
A systematic review of studies of the effect of red clover and risk of breast cancer suggests that at doses similar to those of a traditional Japanese diet (25 to 50 mg isoflavones), no risk is posed to breast cancer survivors. However, the reviewers note the need for more evidence of safety at doses greater than 100 mg/day of isoflavones.46 Another systematic review and meta-analysis of 8 clinical trials found no effect of isoflavones on breast density in postmenopausal women, but some evidence of increased density in premenopausal women (mean difference, 1.83% [95% CI, 0.25% to 3.4%]).47 The reviews include a study that found mammographic breast density to be unaffected by 40 mg daily of red clover isoflavones for up to 3 years in women who had at least 1 first-degree relative with breast cancer (N = 401).22 No change in endometrial thickness or follicle-stimulating hormone levels has been reported.18, 48, 49 A study not included in the reviews, reported that use of red clover among 1,183 breast cancer survivors resulted in fewer reports of weight gain, night sweats, or difficulty in concentration.50 None of the botanical products used by these women were associated with health-related quality of life measures, fatigue, or hormonally related symptoms.
The median lethal dose in mice is estimated to be approximately 4 g/kg intraperitoneally.3 As a flavor or seasoning, Trifolium species are GRAS classified under the Federal Food, Drug, and Cosmetic Act.3
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