Saponins are a group of secondary metabolites found in many plants as well as some marine organisms. They are composed of a sterol based structure bound to one or more units of a sugar. These natural products are of great interest as dietary compounds because of potential health benefits. One must be careful with ingesting them, however, since some are known poisons.
When shaken in water, saponins act like a soap. Originally the roots of the soapwood plant, Saponaria, were used as a substitute for soap. This genus provided the name for these compounds. Several well-known members of this class include the heart medication digitoxin, produced from foxglove that is toxic in high concentrations. The deadly nightshade family is well known for producing the toxin solanine.
Surfactants are compounds that can mix with oil and water. Saponins possess this capability because of their structure with differing types of molecules at each end of the chemical. One end of the compound has one or more units of sugar, which makes that part of the molecule soluble in water. The other end can dissolve in membranes.
Membrane solubility derives from the sterol ring structure of these chemicals. Frequently, this sterol group is a natural product known as a triterpene. This is a particular type of sterol that contains several oxygen molecules. Although cholesterol fits the chemical definition of a triterpene, plants produce phytosterols instead. Contrary to popular belief, plants do not contain cholesterol.
Saponins have been studied for their role in plant defense for many years, and are thought to protect the plant against infection by a variety of plant pathogens. For years it was thought that these compounds are stored inactive until the plant is attacked and, upon attack, the molecule is activated by sugar cleavage. Recent studies suggests that some of these compounds might be induced by infection instead of always being present in an inactive form.
After the sugar has been cleaved off the saponin, the remaining sterol compound is known as an aglycone. It can insert into membranes, causing a variety of biological effects. It can change membrane fluidity and can cause channels to open in membranes. Inside the human digestive system, bacteria can cleave off the sugars and generate the active aglycone.
A number of test tube studies have suggested the various saponins have anti-cancer capabilities and abilities to enhance the immune system. Saponins from soybeans are of particular interest. The ability to protect against infection is thought to extend to mammals that have eaten plants containing these secondary metabolites. There is a lot of interest in consuming these compounds as phytonutrients to enhance health. It may be wise to avoid consuming saponins until more studies have been conducted on the safety and efficacy of these compounds.