They foam like soap, taste bitter, and are hidden in hundreds of everyday foods: saponins are among the most versatile secondary plant compounds there are. Their name comes from the Latin "sapo" (soap) – a reference to their ability to form a stable foam in water. For a long time, they were seen mainly as raw materials for washing or as fish poisons. Modern research paints a more nuanced picture: from a vein-supporting effect that is well established clinically, to immunological effects, to promising but still early findings in cell culture.
What saponins are, and why plants produce them
Saponins are glycosidic compounds found in more than 90 plant families. Chemically, they consist of a water-loving (hydrophilic) sugar component and a fat-loving (lipophilic) aglycone, known as the sapogenin. This dual structure makes them natural surfactants: they can emulsify fats and form a stable foam when shaken with water – visible, for example, when cooking chickpeas or lentils.
Plants produce saponins primarily as a natural defence against fungal attack, insect feeding and other predators. During the ripening of nightshade plants such as tomatoes and potatoes, initially formed steroidal alkaloid saponins (such as solanine) are enzymatically converted into toxicologically less problematic steroidal saponins.
Three main groups are distinguished:
| Group | Core structure | Key representatives |
|---|---|---|
| Triterpenoid saponins | C-30 core (oleanane) | Horse chestnut (aescin), ginseng (ginsenosides), soapwort |
| Steroidal saponins | C-27 core (spirostane, furostane) | Butcher's broom, yam, lily of the valley |
| Steroidal alkaloid saponins | C-27 core with nitrogen | Nightshades (tomatoes, potatoes) |
How saponins act: foaming, haemolysis and fish toxicity
Foam formation and surface activity are the defining, name-giving properties of saponins. This surfactant effect has made them useful as natural detergents for centuries – the names of the South American soapbark tree (Quillaja saponaria), soapnut and soapwort (Saponaria officinalis) all refer to this tradition.
Saponins also have haemolytic activity: they form complexes with cholesterol in cell membranes, which is toxic if they enter the bloodstream directly. When taken orally through food, however, they are largely harmless to warm-blooded animals, since they are only poorly absorbed in the gastrointestinal tract. Towards fish, by contrast, saponins are highly toxic because they impair gill function – a reminder that biological effects always need to be assessed in context.
What research says about the pharmacological effects of saponins
The pharmacological spectrum of saponins is broad. An important quality marker of rigorous saponin research is the distinction between in-vitro findings, animal studies and clinical human trials – three levels of evidence with very different weight.
| Area of effect | Level of evidence | Key sources |
|---|---|---|
| Vein toning, vasoprotective | Clinically proven (Cochrane review) | Horse chestnut (aescin) |
| Expectorant (mucus-thinning) | Clinically proven | Ivy (α-hederin), primrose |
| Cholesterol-lowering | Laboratory and human studies | Legumes, soy extract |
| Immunomodulatory / adjuvant | Proven in animal models and in vitro | Quillaja, soy |
| Antiviral, antimicrobial | Proven in vitro | Various plant extracts |
| Antitumour | In vitro (cell culture), preclinical | Soapwort, ginseng |
| Anti-inflammatory | In vitro and animal models | Glycyrrhizin (liquorice), ginsenosides |
| Neuroprotective | Preclinical, limited human data | Ginseng |
Horse chestnut: the most thoroughly studied saponin plant
The saponin mixture aescin, extracted from horse chestnut seeds (Aesculus hippocastanum), is the most thoroughly studied venous therapeutic agent in phytotherapy. Aescin inhibits lysosomal enzymes in the vessel wall, thereby reducing capillary permeability and exerting anti-exudative and vessel-sealing effects.
A Cochrane review (Pittler & Ernst, 2012), pooling data from 17 randomised controlled trials, confirmed an improvement in leg pain, oedema and itching with standardised horse chestnut seed extracts compared with placebo in chronic venous insufficiency, with adverse events generally mild and infrequent. In one of the underlying trials – a randomised, partially blinded, placebo-controlled study in 240 patients over 12 weeks (2 × 50 mg aescin daily) – lower leg volume decreased by an average of 43.8 ml, compared with 46.7 ml under compression therapy, while it increased by 9.8 ml under placebo; both active treatments were therefore superior to placebo and roughly equivalent to each other. The review also notes that larger, more methodologically robust trials would be desirable. Only dry or liquid extracts standardised to aescin content are therapeutically relevant – not the raw seed or home-made preparations.
Ivy and primrose: plant-based expectorants for respiratory complaints
Ivy leaves (Hedera helix) contain saponins such as α-hederin and hederacoside C, which act as secretolytics and spasmolytics. Ivy extracts are among the most widely used herbal cough remedies in Europe and have been studied in clinical trials in children and adults with bronchitis. The roots of various primrose species (Primula spp.) also contain saponins that can thin thick mucus and make coughing easier – an indication that is among the classic cold remedies in phytotherapy.
Liquorice and ginseng: benefits and limits of two well-known saponin plants
Glycyrrhizin, the main saponin in liquorice root (Glycyrrhiza glabra), tastes sweet – about 50 times sweeter than sucrose – and shows anti-inflammatory properties in laboratory and animal studies. At the same time, regularly consuming large amounts can raise blood pressure and lower potassium levels; pregnant women should avoid large quantities of liquorice. This is a clear example of how strongly the benefit-risk balance of plant compounds depends on the amount consumed.
Ginseng saponins, known as ginsenosides, belong to the triterpenoid saponins. In vitro, they show antioxidant and anti-inflammatory properties; in human studies, they are investigated as adaptogens to support physical and mental performance. The clinical evidence is heterogeneous – depending on the extract, dose and study design – but overall tends to be positive.
Butcher's broom and field horsetail: two lesser-known saponin plants
Ruscogenins and neo-ruscogenins from butcher's broom (Ruscus aculeatus) have vessel-stabilising and astringent effects and are used for venous conditions, often in combination with other venous plants. The saponin equisetonin from field horsetail (Equisetum arvense) is traditionally used to support connective tissue; the clinical evidence here is considerably more limited than for horse chestnut or ivy.
What a recent study on soapwort shows – and what it doesn't
A study published in 2024 in the journal Plants (MDPI) by a research group at Frederick University in Nicosia investigated root extracts of common soapwort (Saponaria officinalis) using UHPLC/Q-TOF-MS analysis. Six major saponins were identified – including gypsogenin and gypsogenic acid derivatives as well as saponariosides C, D and E – along with six phenolic compounds (rutin, quercetin galactoside, syringic acid, apigenin, protocatechuic acid, vanillic acid). All extracts tested showed antioxidant capacity. The acetone extract showed the strongest antibacterial activity against four tested strains: Escherichia coli, Staphylococcus aureus, Enterococcus faecalis and Salmonella enteritidis.
In cell-culture experiments, the acetone extract inhibited the proliferation of A375 melanoma cells in a concentration-dependent and significant manner; the effect on healthy keratinocytes (HaCaT) was markedly weaker. No protective effect was seen against neurotoxic Aβ25–35 peptides. Important for interpretation: these findings come exclusively from experiments on isolated cell lines in culture. No conclusions about an effect in humans – for example from taking soapwort preparations – can be drawn from this.
Saponins as vaccine adjuvants
Outside classical phytotherapy, certain saponins play an established role in modern vaccine development. As early as 1980, an animal study showed that, among several adjuvants tested, saponin produced by far the strongest enhancement of the antibody response against sheep red blood cells in mice. Quillaja saponins are now part of approved adjuvant systems, including vaccines against herpes zoster and malaria. The mechanism involves destabilisation of cholesterol-containing lipid membranes and the resulting activation of immune cells.
A study in Biochimica et Biophysica Acta – Biomembranes (2017) also examined, in human lymphoma cells in the laboratory, how membrane cholesterol affects the augmentation, by triterpenoid saponins, of a saporin-based immunotoxin directed against the surface molecule CD19 – an experimental contribution to basic research on therapeutic targets in cancer treatment, not evidence of an effect in patients.
Saponins in the diet: occurrence, preparation and bioavailability
Saponins are found in many everyday foods, without most consumers being aware of it.
| Food group | Examples |
|---|---|
| Legumes | Soybeans, chickpeas, lentils, peas, green beans |
| Vegetables | Spinach, asparagus, beetroot, tomatoes |
| Grains and pseudocereals | Oats, quinoa |
| Spices | Garlic |
| Foods and beverages | Liquorice, certain teas |
Legumes are considered the main dietary source of saponins. During cooking, losses of up to 50% must be expected, since saponins are water-soluble and pass into the cooking water. That cooking water can be reused in soups or sauces. No scientifically defined recommendation for an optimal daily intake currently exists.
Safety: what to consider with concentrated saponin extracts
Saponins must never enter the bloodstream – haemolytic toxicity following parenteral administration is well documented; this risk does not apply to dietary intake or usual oral doses. In cases of existing inflammation of the intestinal wall, saponins may increase the permeability of the gut mucosa. Glycyrrhizin from liquorice can, with regular consumption of large amounts, promote high blood pressure and lower potassium levels; pregnant women should avoid large quantities. For aescin preparations, possible interactions with anticoagulants and antidiabetic drugs are known – anyone on such long-term medication is advised to consult a doctor or pharmacist before regularly taking saponin-containing preparations.
Conclusion: what is clinically proven, and what remains a research question
Saponins are a chemically diverse group of secondary plant compounds whose potential varies considerably depending on the substance class and plant origin. Aescin for chronic venous insufficiency and ivy saponins for respiratory complaints are well established clinically; the role of certain saponins as vaccine adjuvants is likewise established. Other areas – including antitumour and neuroprotective effects, currently being studied for example in soapwort – remain at the preclinical research stage and cannot be extrapolated to an effect in humans. A varied diet with legumes, asparagus, spinach and quinoa provides, as a side benefit, a natural share of these versatile plant compounds.
This article is intended for general information about saponins and does not replace medical advice, diagnosis or treatment. If you have health concerns, please consult a doctor or pharmacist.
Sources
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