Soy SaponinsMany health benefits of soybeans are attributed to their isoflavones but soy saponins, which are less studied, may also play an important role. Tempeh and other soy products, except soy protein extracted with alcohol, contain high levels of saponins. Raw soybeans contain between 2 and 5 g saponins per 100 g. Saponins are glycosides of steroids, steroid alkaloids or triterpenes found in many plants such as asparagus, soybeans, quinoa, olives, grapes and many herbs including ginseng and soapwort. When dissolved in water they form a stable soapy froth. This can easily be observed when you add water to soybean grits (see pictures above). Because of the presence of both a hydrophilic (sugar) and hydrophobic (steroid) they act as emulsifiers and foaming agents. Soy saponins are divided in 2 groups: group A saponins have and undesirable astringent taste, typical for some soy products, and group B saponins have the health promoting properties. The soybean germ contains most group A saponins, whereas group B saponins are found in the both the soybean germ and cotyledons. The soybean hulls, which are removed during the tempeh production, contain little saponins. Some types of saponins are poisonous but the ones found in soybeans and tempeh are eatable and actually possess health benefits.
Health Benefits of Soy SaponinsSoy saponins may contribute to cholesterol lowering by increasing bile secretion and may reduce the risk of colon cancer.
Cholesterol Reduction - Already in 1979 Potter et al discover the cholesterol lowering property of soy saponins. The ability of saponins to form emulsions or micelles with bile salt in the intestine probably explains their cholesterol lowering activity. Bile salt forms small micelles with cholesterol facilitating its absorption. Because soy saponins bind with bile salt, the absorption of cholesterol is reduced. Some saponins form insoluble complexes with cholesterol, resulting in inhibition of absorption cholesterol.
Anticancer - Scientific studies have illustrated that saponins can lower the risk of colon cancer. Cancer cells also need cholesterol to grow and saponins bind cholesterol rich cancer cell membranes, thereby limiting their growth. A study by Roa et al published in The Journal of Nutrition (1995, 125, 717s-724S) concluded that saponins may help to prevent colon cancer.
Inhibition of HIV - The Japanese study entitled "Inhibitory effect of glycosides like soy saponins on the infectivity of HIV in vitro" and published in AIDS (1989 Oct) concluded that soy saponins, especially saponin B1, may have inhibitory activity against HIV infection. It should be stressed that this was an in-vitro study using cultured cells and that the consumption of soy saponins is not a medicine to cure HIV.