Saponins are glycosides with a distinctive foaming characteristic. They are found in many plants, but get their name from the soapwort plant (Saponaria), the root of which was used historically as a soap (Latin sapo ---> soap). They consist of a polycyclic aglycone that is either a choline steroid or triterpenoid attached via C3 and an ether bond to a sugar side chain. The aglycone is referred to as the sapogenin and steroid saponins are called saraponins. The ability of a saponin to foam is caused by the combination of the nonpolar sapogenin and the water soluble side chain. Saponins are bitter and reduce the palatability of livestock feeds. However if they have a triterpenoid aglycone they may instead have a licorice taste as glucuronic acid replaces sugar in triterpenoids. Some saponins reduce the feed intake and growth rate of nonruminant animals while others are not very harmful. For example, the saponins found in oats and spinach increase and accelerate the body's ability to absorb calcium and silicon, thus assisting in digestion. Certain pasture weeds contain substantial quantities of dangerous saponins and result in life threatening toxicities for certain animal species.
Saponins are generally not a problem in tropical forage legumes. However, they are common in several temperate forage legumes. The use of alfalfa, (Medicago sativa, in supplemental protein meals for swine and poultry is limited by its saponin content. Although alfalfa contains several saponins (medicagenic acid, soyasapogenol A, soyasapogenol B, lucernic acid ), medicagenic acid appears to be the one responsible for its antinutritional effects. Saponin content in alfalfa foliage is low in spring and fall and high in midsummer. Low-saponin cultivars of alfalfa have been developed. The seeds and foliage of chickpeas (Cicer arietinum), soybeans ( ), and common beans ( ) also contain saponins. Several rangeland weeds in the US including corn cockle (Agrostemma githago, soapwort (Saponaria officinalis), cow cockle (Saponaria vaccaria), and broomweed (Gutierrezia sarothrae) cause serious toxicity problems for grazing livestock because of their saponins. Alfombrilla (Drymaria arenaroides) is a weed in northern Mexico containing @3% saponins that is responsible for cattle losses in Mexico and has potential for spread to the southwest U.S. Yucca contains sarsaponins and is occassionally grazed by cattle. However, research indicates that sarsaponins might actually be beneficial to rumen digestion. Other plants containing saponins include Christmas Rose (Helleborus niger), Horse Chestnut trees (Aesculus hippocastanum), Asparagus fern (Asparagus officinalis), and Daisies (Bellis perennis)
Alfalfa poisoning in poultry and swine:
- irritated mucous membranes of the mouth and digestive tract -->
- reduced feed intake,
- low dietary protein quality (supplemental methionine will counteract this).
- the above factors lead to--> decreased performance and growth rate.
- increased excretion of cholesterol.
Corn cockle, soapwort, cow cockle, and broomweed poisoning
- weight loss
- rough hair coat
- gastroenteritis and diarrhea
- in the case of broomweed, possibly abortion.
- same symptoms as above plants, but progressing rapidly to -->
- arched back -->
- coma -->
As well as irritating the membranes of the respiratory and digestive tract, the aglycones in certain saponins increase the permeability of the membranes of red blood cells. In severe cases, the membranes are destroyed and their hemoglobin escapes into the bloodstream. This hemolytic effect varies considerably between different plant species.
Historically, saponins have been blamed for the incidence of bloat in ruminants consuming fresh alfalfa. Bloat occurs in animals grazing temperate legumes that contain saponins but not in livestock grazing tropical legumes or temperate legumes like birdsfoot trefoil that do not contain saponins. However, low-saponin cultivars of alfalfa can cause bloat. Current research blames bloat on cytoplastmic protein fractions that are also present in the plants..
Humans generally do not suffer severe poisoning from saponins. Our cholesterin inactivates them so that only our mucus membranes are affected. Because of this, saponins have been used in sneezing powders, emetics, and cough syrups to facilitate expectoration. Most saponins are also diuretic. In humans, this effect disappears within a week following the neutralizing action of cholesterin.
Alfalfa saponins may have potential in human health issues because they reduce serum cholesterol by preventing its reabsorption after it has been excreted in the bile. It is hypothesized that the saponins either bind with bile salts or cause the bile salts to bind to the polysaccharides in dietary fiber. Either way the bile salts are unavailable to bind with cholesterol. Unfortunately, the feeding of alfalfa saponins to hens has not resulted in low cholesterol eggs!