Soybeans are not only an important source of high quality proteins but contain also health promoting phytochemicals including isoflavones, trypsin inhibitors and saponins. Saponins are phytochemicals consisting of a triterpenoid or steroid bound to a sugar molecule. Soybeans contain 3 groups of saponins: A, B and E. The main isoflavones in soybeans are genistein, daidzein and glycetein and their glycosides genistin, daidzin and glycetin. Selection on new soybean cultivars is mainly based on yield and resistance to pests but there is growing interest for cultivars with improved taste, functionality and health benefits. Several studies suggest that the intake of soy may reduce colon cancer risk. Soy isoflavones are often used by menopausal women to ease their symptoms. Animal studies have shown that isoflavones and soy protein have a protective role against the development of colon cancer. There is no consensus regarding the influence of soy isoflavones on the breast cancer risk.
In the first part of the study the researchers quantified isoflavone and saponin levels in soybean cultivars grown under different conditions. Among the cultivars there were big difference in isoflavone levels (from 0.9ppm to 2.6ppm) and saponin levels (from 2.2ppm to 5.8ppm). There was a positive relation between levels of isoflavones and saponins. The researchers also determine the influence of isoflavones and saponins on carcinogenic process of cultured human colon cancer cells, which were incubated during 3 days in the presence of isoflavones. Pure genistein, daidzein and a soy protein enriched in isoflavones (Novasoy) were used as isoflavone source. The researchers found a dose-dependent inhibitive effect of isoflavones (mainly genistein) and saponins (mainly aglycones) on the colon cancer cells. The researchers could not find any adverse effects of the intake of pure saponins on growth, organ weight or intestinal morphology. Saponins could not be found in the urine or serum of the mice, in line with previous studies that showed that saponins are not absorbed by rodents.
The study concluded that soy isoflavones and saponins may protect against colon cancer and are well tolerated. The researchers further stressed that result obtained with purified isoflavones or saponins can not always be extrapolated to whole soy foods as the interaction between the different phytochemicals may affect biological response.
Source: MacDonald RS, Guo J, Copeland J, Browning JD Jr, Sleper D, Rottinghaus GE and Berhow MA. Environmental influences on isoflavones and saponins in soybeans and their role in colon cancer.. Journal of Nutrition. 2005 May;135(5):1239-42