The Isoflavones in Red Clover: Formononetin, Biochanin A and the Rest

Red clover is often described simply as being "rich in isoflavones." The statement is accurate, but it leaves out most of what matters. Isoflavones are a family of related compounds, not a single substance. Their identity, chemical form, relative abundance and concentration can vary considerably between batches of Trifolium pratense. Those differences determine how useful an extract is for formulation and how reliably it can be characterised.

This article looks at the chemistry behind red clover extracts. It explains what isoflavones are, why some of them are classified as phytoestrogens, which compounds dominate red clover, how those compounds are stored in the living plant, what affects their concentration and how laboratories measure them. The focus is on the primary literature and on details that matter to formulators, pharmacists and technically minded readers.

What an isoflavone actually is

Isoflavones belong to the flavonoid family, a large class of plant polyphenols involved in colour, bitterness, signalling and chemical defence. Their basic structure contains three rings: two aromatic rings, known as the A and B rings, connected through an oxygen-containing C ring.

The difference between a flavone and an isoflavone is the position of the B ring. In a flavone, the B ring is attached at position 2 of the C ring. In an isoflavone, it is attached at position 3.

That positional change alters the overall geometry of the molecule. It is one reason some isoflavones can interact with biological targets that ordinary flavones do not engage in quite the same way.

Isoflavones are not distributed evenly across the plant kingdom. They are concentrated mainly in the Fabaceae, or legume family. This is why dietary discussions of isoflavones usually centre on plants such as soy, chickpeas, lupin, alfalfa and clover.

Why these compounds are considered phytoestrogens

Some isoflavones are classified as phytoestrogens because their size, shape and hydroxyl-group arrangement allow them to interact with estrogen receptors.

The geometry of certain isoflavones places terminal hydroxyl groups at a distance broadly comparable with the 3-hydroxyl and 17-hydroxyl groups of 17β-estradiol. The molecules are also relatively planar and similar in overall size. These structural similarities allow some isoflavones to fit, with much lower affinity than estradiol, into estrogen-receptor binding sites.

The differences in affinity are substantial. In the analysis of a clinical red clover extract by Booth and colleagues, competitive binding assays using recombinant human estrogen receptors produced IC₅₀ values of approximately 0.30 μM at ERα and 0.020 μM at ERβ for genistein. Daidzein produced values of roughly 17 μM and 1.2 μM respectively.

Biochanin A showed weaker binding, with values around 35 μM at ERα and 4.1 μM at ERβ. Formononetin was weaker again, at approximately 104 μM for ERα and 60 μM for ERβ, and it did not show estrogenic activity in the cell-based assay reported by the same research group.

These results highlight two useful points. First, several red clover isoflavones show greater affinity for ERβ than for ERα. This receptor preference is one reason isoflavones are often discussed in terms of selective or tissue-dependent activity rather than as simple substitutes for estrogen. Our Phytoestrogens 101 guide explains that distinction in more detail.

Second, red clover's two most abundant isoflavones, formononetin and biochanin A, are relatively weak direct receptor binders in their original forms. Their importance after oral consumption depends heavily on metabolism. That relationship is covered in our companion article, Red Clover vs Soy Isoflavones: What Actually Differs.

Red clover's four main isoflavones

Red clover contains a number of isoflavones, but four account for most of the total profile: formononetin, biochanin A, daidzein and genistein.

These compounds form two closely related pairs.

• Formononetin is the 4′-O-methyl ether of daidzein. The two share the same core structure, but formononetin carries a methyl group at the 4′ hydroxyl position.
• Biochanin A is the 4′-O-methyl ether of genistein. It has the same relationship to genistein.

Removing those methyl groups converts formononetin into daidzein and biochanin A into genistein.

In red clover, the methylated forms dominate. In a Lithuanian genotype survey conducted at flowering, formononetin accounted for about 51% of total isoflavones and biochanin A for around 40%. Together they represented approximately 91% of the measured total.

Genistein and daidzein were present only in much smaller amounts. Soy generally shows the opposite pattern, with genistein and daidzein among the dominant compounds.

This difference in starting composition is one of the main reasons evidence from soy cannot simply be transferred to red clover.

Red clover also contains smaller quantities of other isoflavones. In the clinical extract analysed by Booth and colleagues, irilone accounted for about 3.2% of the preformulated extract. Pratensein, pseudobaptigenin, prunetin and calycosin were also detected at lower concentrations.

Glycosides and aglycones: ononin and sissotrin

In the living plant, many isoflavones are stored as glycosides. These are isoflavone molecules attached to a sugar, commonly as 7-O-glucosides and often with an additional malonyl group.

Glycosylation changes properties such as water solubility, chemical reactivity and storage within plant cells.

Two glycosides are particularly relevant in red clover:

• Ononin is formononetin 7-O-glucoside.
• Sissotrin is biochanin A 7-O-glucoside.

The corresponding molecules without the sugar group are called aglycones.

This distinction matters analytically because glycosides and aglycones differ in solubility, extraction behaviour, chromatographic retention and absorption.

The balance between the two forms can also change after harvest. Tsao and colleagues examined thirteen cultivars and reported that samples stored at −5 °C for several days before freeze-drying contained mainly aglycones by the time they reached the analytical stage. Enzymatic hydrolysis during sample handling had altered the original profile.

As a result, two certificates of analysis that both report "total isoflavones" may not be directly comparable. One laboratory may hydrolyse the sample and report aglycone equivalents, while another may quantify the native glycosides separately.

How much is actually in the plant

Lemežienė and colleagues measured red clover genotypes at the flowering stage and reported isoflavone concentrations on a dry-matter basis.

The range is as important as the average. Total isoflavone content varied by roughly 1.5-fold between genotypes grown under the same trial conditions and harvested at the same stage.

Formononetin averaged about 3.4 mg/g across the genotypes studied, while biochanin A averaged around 2.7 mg/g. Daidzein was present at much lower concentrations.

The ratios are also informative. Red clover contains far more formononetin than daidzein, which is why describing the plant mainly as a source of daidzein and genistein gives an incomplete picture of its chemistry.

What affects isoflavone content

Plant part

Isoflavones are not distributed evenly throughout the plant.

In the thirteen-cultivar study by Tsao and colleagues, leaf tissue showed the highest overall concentration. Stem, petiole and flower contained different amounts, and other studies have reported somewhat different rankings among the remaining aerial parts.

The most consistent finding is that leaves are among the richest tissues. The exact profile of flowers, stems and other aerial parts varies with cultivar and study conditions.

This is why a label that specifies only "aerial parts" provides less information than one that identifies the exact plant tissue used.

Harvest timing

Isoflavone concentration and composition change as the plant develops.

Studies comparing early bud stages with later flowering stages have found measurable changes both in total isoflavone content and in the balance between individual compounds.

Harvest timing therefore affects the chemistry of the raw material and should be considered part of the extraction specification rather than simply an agricultural detail.

Cultivar and genetics

Genetic variation alone can produce substantial differences.

In the genotype survey described above, plants grown under the same conditions and harvested at the same stage still showed up to roughly a 1.5-fold difference in total isoflavone concentration.

Drying, storage and processing

Post-harvest handling can change both the amount and the chemical form of the isoflavones.

Storage conditions may allow plant enzymes to convert glycosides into aglycones, while excessive heat or poorly controlled drying can degrade sensitive compounds.

For this reason, handling between harvest and extraction can be just as important as the original composition of the plant.

How preparation changes the profile

Extraction does not simply remove everything from the plant in equal proportions. Solvent choice, solvent concentration and extraction time determine which compounds are recovered most efficiently.

Malca-Garcia and colleagues compared aqueous infusions, decoctions and 45% ethanolic tinctures of red clover using quantitative NMR, LC-MS/MS and UHPLC-UV.

The preparations produced clearly different chemical profiles.

Both the infusions and decoctions contained higher concentrations of the glucosides ononin and sissotrin than the 45% ethanolic tinctures. This is consistent with the greater water solubility of the sugar-conjugated forms.

The researchers also found that the tincture changed chemically over time. Biochanin A and formononetin concentrations varied during a one-month observation period and reached their highest measured levels at around six days.

The authors described this changing composition as "dynamic residual complexity."

For formulation purposes, the practical conclusion is straightforward. Solvent system, solvent ratio and extraction time affect the final isoflavone profile and should be treated as part of the raw-material specification.

Clovamide and other compounds

Isoflavones receive most of the attention, but red clover contains other phenolic compounds in meaningful amounts.

Clovamide, a caffeic acid-DOPA conjugate, was measured by Tava and colleagues at 15.6 ± 0.6 mg/g dry weight in leaves from red clover grown in Italy.

The same material contained 24.6 mg/g of isoflavones and 13.2 mg/g of flavonols, with total phenolic content measured at 53.7 mg/g.

Clovamide therefore represented a substantial part of the measured phenolic fraction, even though it would not appear on a certificate of analysis focused only on total isoflavones.

Other reported constituents include several additional classes of plant compounds.

• Flavonols: predominantly quercetin derivatives, including glycosides and malonylated glycosides.
• Coumestans: including coumestrol and medicagol. In the Booth analysis, coumestrol showed strong estrogen-receptor binding, with IC₅₀ values of approximately 0.06 μM at ERα and 0.02 μM at ERβ, but it represented no more than 0.01% of the extract.
• Pterocarpans: including maackiain and its glucoside trifolirhizin. Pterocarpans represented about 0.06% of the clinical extract analysed by Booth and colleagues.
• Coumarins: reported at no more than 0.03% in the same extract.
• Saponins: triterpene glycosides described in broader reviews of the Trifolium genus.

The phase II clinical extract characterised by Booth and colleagues contained 35.54% isoflavones, 1.11% flavonoids, 0.06% pterocarpans, no more than 0.03% coumarins and no more than 0.03% tyramine.

This illustrates an important point: even a carefully standardised extract contains many constituents beyond the marker compounds highlighted on a product specification.

How isoflavones are measured

Most published red clover analyses rely on a small number of established analytical techniques.

HPLC-UV

HPLC-UV is widely used for routine isoflavone quantification. Isoflavones absorb in the ultraviolet range, commercial reference standards are available for many of the major compounds and the necessary instrumentation is common in quality-control laboratories.

Lee and colleagues published a validated HPLC-UV method specifically for the quantification of formononetin and biochanin A in Trifolium pratense extract.

LC-MS/MS

LC-MS/MS combines chromatographic separation with mass-selective detection.

It is particularly useful when compounds co-elute, when minor constituents need to be measured alongside much more abundant compounds or when the sample matrix is complex.

This can be useful for components such as irilone or coumestrol when they are present at much lower concentrations than formononetin and biochanin A.

Quantitative NMR

Quantitative NMR, or qNMR, offers a different advantage.

Because the NMR signal can be related directly to the number of observed nuclei, qNMR can sometimes quantify compounds without requiring a matched reference standard for every individual analyte.

This is particularly useful for minor plant constituents for which purified commercial standards may not be readily available. Malca-Garcia and colleagues used quantitative NMR alongside chromatographic methods when examining traditional red clover preparations.

One analytical detail is essential when comparing published results. A hydrolysed sample and a non-hydrolysed sample are not reporting the same chemical forms.

If the sample is hydrolysed before analysis, the result may be expressed as aglycone equivalents. If the native compounds are measured without hydrolysis, glycosides and aglycones may be reported separately.

Always check which method was used before comparing two "total isoflavone" figures.

Why standardisation matters

The chemistry described above leads to a practical problem: red clover extracts can differ substantially from one another, while a product label may provide very little information about those differences.

Wang and colleagues analysed marketed red clover products and found significant variation in measured isoflavone content. They also reported ambiguity in product labelling.

The researchers then examined the products in a Caco-2 intestinal cell model. Absorption rates and permeability for compounds including biochanin A and formononetin varied between products. The surrounding product matrix also influenced the results, showing that composition mattered beyond the nominal amount of isoflavones.

The study used a cell-culture model rather than human participants, so it should not be interpreted as evidence of clinical effects. Its relevance is narrower: products carrying the same botanical name can differ chemically and behave differently in an experimental intestinal model.

That is why "contains red clover extract" is not, by itself, a useful specification.

A more informative specification identifies the plant part, extraction solvent, marker compounds, analytical method, acceptable concentration range and whether the result is expressed as native glycosides or aglycone equivalents.

On the regulatory side, red clover ingredients such as Trifolium Pratense Flower Extract appear in the EU CosIng cosmetic ingredient database.

CosIng is an inventory of cosmetic ingredients and their declared functions. It is not an approval list, and inclusion in the database does not demonstrate the effectiveness of a finished product.

Minerva108 produces its red clover extract in-house rather than relying on a generic commodity extract. This gives us direct control over extraction and post-harvest handling, both of which influence the resulting isoflavone profile.

You can read more about the plant and our sourcing on our red clover ingredient page. For a broader review of the skin and hair literature, see Red Clover: The Anatolian Flower Science Keeps Coming Back To.

Frequently asked questions

What are the main isoflavones in red clover?

The four main compounds are formononetin, biochanin A, daidzein and genistein.

Formononetin and biochanin A are much more abundant. In one flowering-stage survey they represented approximately 51% and 40% of total isoflavones respectively.

Red clover also contains smaller amounts of irilone, pratensein, pseudobaptigenin, prunetin and calycosin.

What is the difference between formononetin and daidzein?

Formononetin is the 4′-O-methyl ether of daidzein. The two compounds share the same core molecular structure, but formononetin has a methyl group attached at the 4′ hydroxyl position.

Biochanin A has the same relationship to genistein.

In receptor-binding assays, the methylated forms showed substantially weaker affinity for estrogen receptors than their unmethylated counterparts.

What are ononin and sissotrin?

They are glycoside forms of two major red clover isoflavones.

Ononin is formononetin 7-O-glucoside, while sissotrin is biochanin A 7-O-glucoside.

Aqueous infusions and decoctions have been reported to contain higher concentrations of these glucosides than 45% ethanolic tinctures.

How much isoflavone does red clover contain?

In one genotype survey at flowering, total isoflavone concentration ranged from 5.40 to 8.09 mg/g of dry matter.

Formononetin ranged from 2.61 to 4.40 mg/g, while biochanin A ranged from 1.79 to 3.32 mg/g.

The exact figure depends on cultivar, plant part, harvest timing, growing conditions and post-harvest processing, so there is no single concentration that accurately describes every red clover sample.

Why can two red clover extracts produce different laboratory results?

Several factors can contribute.

Different cultivars can produce different isoflavone concentrations even when grown under similar conditions. Plant parts also differ in composition, while solvent choice and extraction method affect which compounds are recovered.

Post-harvest handling can alter the balance between glycosides and aglycones, and laboratory results may differ depending on whether the sample was hydrolysed before analysis.

For that reason, two extracts labelled simply as "red clover extract" can have meaningfully different chemical profiles.

This article is for general information and education only. It is not medical advice, and nothing here is intended to diagnose, treat, cure or prevent any disease. If you are pregnant, breastfeeding, taking medication or managing a health condition, speak with a healthcare professional before using red clover supplements.

References

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2. Booth NL, Overk CR, Yao P, et al. The chemical and biologic profile of a red clover (Trifolium pratense L.) phase II clinical extract. Journal of Alternative and Complementary Medicine, 2006;12(2):133–139. doi.org/10.1089/acm.2006.12.133
3. Malca-Garcia GR, Zagal D, Graham J, et al. Dynamics of the isoflavone metabolome of traditional preparations of Trifolium pratense L. Journal of Ethnopharmacology, 2019;238:111865. doi.org/10.1016/j.jep.2019.111865
4. Tsao R, Papadopoulos Y, Yang R, Young JC, McRae K. Isoflavone profiles of red clovers and their distribution in different parts harvested at different growing stages. Journal of Agricultural and Food Chemistry, 2006;54(16):5797–5805. doi.org/10.1021/jf0614589
5. Tava A, Pecio Ł, Stochmal A, Pecetti L. Clovamide and flavonoids from leaves of Trifolium pratense and T. pratense subsp. nivale grown in Italy. Natural Product Communications, 2015;10(6):933–936. pubmed.ncbi.nlm.nih.gov/26197520
6. Lee JH, Paje LA, Kim JY, et al. Validation of an optimized HPLC–UV method for the quantification of formononetin and biochanin A in Trifolium pratense extract. Applied Biological Chemistry, 2021;64:57. doi.org/10.1186/s13765-021-00630-5
7. Wang SW, Chen Y, Joseph T, Hu M. Variable isoflavone content of red clover products affects intestinal disposition of biochanin A, formononetin, genistein, and daidzein. Journal of Alternative and Complementary Medicine, 2008;14(3):287–297. doi.org/10.1089/acm.2007.0617
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