Bioavailability of Isoflavones after Ingestion of Soy Beverages in Healthy Adults

Author: Mitsuyoshi Kano, Tomomi Takayanagi, Katsuhisa Harada, Seigo Sawada and Fumiyasu Ishikawa
Publication: J. Nutr. 136:2291-2296, September 2006

Many of the health benefits of soybeans are attributed to isoflavones. Isoflavones are polyphenolic antioxidants which may protect against hormone-dependent diseases, such as breast cancer, prostate cancer, menopausal symptoms and osteoporosis. The bioavailability of isoflavones aglycones (free isoflavones) and glycosides (isoflavones bound to sugar) are different, and may be influenced by the composition of the beverage. Isoflavones are naturally mainly present as glycosides. In the intestinal tract, the glycosides are converted to aglycones, which are much more absorbable. During the digestion, the isoflavones are further metabolized. For example, daidzein is converted into equol or O-desmethylangolensis. The aim of this study was investigate the effects of soymilk beverages on absorption, excretion, and metabolism of isoflavones. The following soymilk beverages were tested: normal soymilk, soymilk treated with the enzyme beta-glucosidase and fermented soymilk.

Twelve volunteers participated in this study. Their blood and urine was tested at regular intervals over a period of 24 hours after intake of the soymilk beverage. After intake of the standard soymilk, the isoflavones concentration reached a maximum of 0.94 micromol/liter after about 6 hours. The soymilk beverage which was treated with beta-glucosidase and fermented soymilk beverage showed maximum isoflavones concentration of 1.75 micromol/liter already after 1 hour. During the first eight hours after ingestion, the urinary excretion of isoflavones from these two beverages was also higher than from plain soymilk. There was no difference in levels of isoflavones in the blood of volunteers consuming fermented soy milk or soy milk with hydrolyzed isoflavones, indicating that typical fermentation products such as lactic acid or the method of hydrolyzation has no effect on isoflavones metabolism.

Of the 12 volunteers 5 were classified as equol producers. Equol producers secreted more urinary equol when consuming soymilk treated with the enzyme beta-glycosidase and fermented soymilk. Other studies have shown that the administration of bacteria, such as Lactobacillus gasseri, could influence the production of equol by other bacteria.

The study concluded that isoflavone aglycones were absorbed faster and in greater amounts than isoflavones glucosides. The metabolism of isoflavones might depend on the type of soymilk. Probiotic bacteria might have an influence on intestinal production of equol.