Peter R. Cheeke, Ph.D.
Professor of Comparative Nutrition
OSU/LPI Affiliate Investigator

Saponins are natural detergents found in many plants, especially certain desert plants. Saponins are also present in small amounts in some foods, such as soybeans and peas. The two major commercial sources of saponins are Yucca schidigera, which grows in the arid Mexican desert country of Baja California, and Quillaja saponaria (soapbark tree), found in arid areas of Chile. Saponins have detergent or surfactant properties because they contain both water-soluble and fat-soluble components. They consist of a fat-soluble nucleus, having either a steroid or triterpenoid structure, with one or more side chains of water-soluble carbohydrates (sugars). Yucca saponins have a steroid nucleus (steroidal saponins), while the quillaja saponins have a triterpenoid nucleus. As a consequence of their surface-active properties, saponins are excellent foaming agents, forming very stable foams. Yucca and quillaja extracts are used in beverages, such as root beer and slurpies, to provide the foamy "head." Because of their surfactant properties, they are used industrially in mining and ore separation, in preparation of emulsions for photographic films, and extensively in cosmetics, such as lipstick and shampoo. Quillaja bark has been used as a shampoo in Chile for hundreds of years, and Native Americans used yucca to make soap. The antifungal and antibacterial properties of saponins are important in cosmetic applications, in addition to their emollient effects.

Mature desert Yucca

Yucca and quillaja saponins have both current and potential applications in animal and human nutrition. Yucca extracts are extensively used for ammonia and odor control in pig and poultry-raising facilities and in dog and cat foods. Yucca saponins, and perhaps other components of yucca as well, have ammonia-binding activity. When added to the diet, yucca saponins pass through the digestive tract unabsorbed and are excreted in the feces. In the excreta, the yucca components bind to ammonia and certain other odiferous compounds and prevent them from being released into the air. In recent studies in England, feeding of yucca extract to dogs and cats was shown to reduce fecal odor and reduce emission of volatile compounds contributing to fecal odor. Many pet foods and "kitty litter" products now contain yucca extract to reduce these noxious odors.

New applications for saponins in animal husbandry are being explored, especially the effect of saponins on protozoal diseases. Saponins form strong insoluble complexes with cholesterol. This has many important implications, including cholesterol-lowering activity in humans, discussed later in this article. Many protozoa enter the body via the digestive tract or cause their pathological effects in the gut. Saponins react with cholesterol in the protozoal cell membrane, causing the cell to rupture and lyse. Giardiasis (beaver fever), for example, is a disease with symptoms of severe diarrhea associated with the protozoan Giardia lamblia, often found in untreated drinking water, that can infect the small intestine. Research currently in progress at Agriculture Canada in Lethbridge, Alberta, has shown yucca extract to be very effective in killing Giardia trophozoites, which are the infective stages released in the gut when the oocytes, or eggs, sporulate, although no studies with humans have yet been done. Other important protozoal diseases of livestock, including coccidiosis and equine protozoal myoencephalitis, may be amenable to treatment with saponins. Ruminant animals (cattle, sheep and other cud-chewing animals with a complex stomach) have a large population of rumen protozoa. The rumen protozoa reduce the efficiency of fermentation in the rumen, and increases in animal performance often occur when the protozoa are removed (a process called defaunation). Yucca saponins are effective in suppressing rumen protozoa, again by reacting with cholesterol in the protozoal cell membrane, causing it to lyse.

The blood cholesterol-lowering properties of dietary saponins are of particular interest in human nutrition. One of the most prominent research programs on this subject was that of Dr. Rene Malinow at the Oregon Regional Primate Center, whose research (published in the American Journal of Clinical Nutrition in 1997) demonstrated unequivocally the cholesterol-lowering properties of saponins. This desirable effect is achieved by the binding of bile acids and cholesterol by saponins. Bile acids form mixed micelles (molecular aggregates) with cholesterol, facilitating its absorption. Cholesterol is continually secreted into the intestine via the bile, with much of it subsequently reabsorbed. Saponins cause a depletion of body cholesterol by preventing its reabsorption, thus increasing its excretion, in much the same way as other cholesterol-lowering drugs, such as cholestyramine.

Although there are reports of the development of synthetic saponins as drugs for treating high blood cholesterol, yucca and quillaja extracts are natural phytochemicals currently used in foods and beverages and as herbal products. Interestingly, recent research by scientists in Canada and Africa has suggested that the very low serum cholesterol levels of Masai tribes people in East Africa, who consume a diet very high in animal products, cholesterol, and saturated fat, are likely due to the consumption of saponin-rich herbs.

Masai village in East Africa

The binding of bile acids by saponins has other important implications. Bile acids excreted in the bile are called primary bile acids. They are metabolized by bacteria in the colon, producing secondary bile acids. Some of the secondary bile acids are promoters of colon cancer. By binding to primary bile acids, saponins reduce the formation of the secondary bile acids. Research at the University of Toronto has shown that feeding saponins to laboratory animals reduced the number of preneoplastic colon lesions in mice. The Canadian researchers also found that saponins had a dose-dependent inhibitory effect on growth of human carcinoma cells in culture. Major current interest in quillaja saponins concerns their effects on the immune system. Specially purified quillaja saponin fractions designated as Quil A are used as adjuvants for vaccines. (Adjuvants are substances that increase the effectiveness of vaccines.) Quillaja saponins increase the effectiveness of both injected and oral vaccines. In the case of injected vaccines, Quil A is used to prepare immunostimulating complexes (ISCOM). ISCOM's are prepared by attaching a portion of the protein envelope of a virus to Quil A. The association of the viral protein with saponin facilitates its transport across cell membranes. Quillaja saponin-based ISCOM's are presently being evaluated in development of experimental vaccines against HIV, the virus responsible for AIDS. Besides having adjuvant activity, quillaja saponins have a direct stimulatory effect on the immune system. For example, pretreatment of mice with quillaja saponins enhances their resistance to a disease challenge. Saponins enhance the effectiveness of oral vaccines by improving their absorption as a result of increasing gut mucosal permeability, which facilitates absorption of large molecules contained in vaccines.

The desert plants Yucca schidigera and Quillaja saponaria are rich storehouses of phytochemicals with many useful and important functions in human and animal nutrition. In many respects, we have just scratched the surface in our understanding of the many biological effects of steroidal and triterpenoid saponins and their potentials for improving human health.

Last updated May, 1998