The new study, published in The Journal of Nutrition (Vol. 136, pp. 2291-2296), has implications for using soy isoflavones as functional ingredients, and also reports that adding probiotics to the soymilk affects the absorption and metabolism of the isoflavones in the gut.
Soy isoflavones are well known phytoestrogens - active substances derived from plants that have a weak oestrogen-like action. They have been studied for their role in cancer prevention and slowing down the ageing process in peri-menopausal women, and have proved to be a popular alternative to hormone replacement therapy for those wishing to control menopause symptoms without resorting to drugs.
Although significant research has been done into the benefits of soy isoflavones, information about how the bioavailability of the form of the isoflavone (glucoside versus aglycone) is controversial and contradictory.
Some studies have reported no difference between bioavailability of the isoflavone type, whereas others have reported higher bioavailability for aglycones.
The new research, led by Mitsuyoshi Kano from the Yakult Central Institute for Microbiological Research, Kunitachi, Tokyo, sought to end this controversy by supplementing the diets of 12 healthy volunteers with three kinds of soymilk: untreated/normal soymilk, beta-glucosidase-treated soymilk, and fermented soymilk.
The fermented soymilk is rich is aglycone, while the unfermented/natural milk is rich in glycosides. The milk treated with the beta-glucosidase enzyme converts the naturally found glycoside to the more absorbable aglycones.
Blood samples were taken every hour for eight hours and then after 24 hours, and showed that, after ingestion of the normal soymilk, the total serum concentration of isoflavones increased gradually and peaked at 0.94 micromoles per litre after six hours.The soymilks treated with beta-glucosidase and the fermented soymilk raised the serum isoflavone concentrations faster, peaking at 1.75 micromoles per litre and 2.05 micromoles per litre, respectively, after only one hour.
This result suggested that the probiotics, Bifidobacterium breve and Lactobacillus mali, enhanced the absorption of the isoflavones. Previous reports have also suggested that pre- and probiotics can enhance calcium absorption and could boost bone health.
Kano and his associates also report that the total and individual serum and urine concentrations of isoflavones did not differ after consumption of either aglycone-enriched soymilks.
"These results demonstrated that the isoflavone aglycones of soymilk were absorbed faster and in greater amounts than their glucosides in healthy adults," concluded the researchers.
It makes sense, said the Yakult researchers, that aglcyones are more bioavailable since they are smaller molecules and are more hydrophobic, and so do not dissolve as easily in water and thus rapidly excreted.
A recent collaboration between universities in the UK, Denmark and Spain and Unilever, UK and Agrotechnology and Food Innovations, Holland, investigated the availability of the isoflavones daidzein and genistein from cookies, chocolate bars and juice and reported that the body's ability to absorb and benefit from soy isoflavones is greater when taken from juice but not cookies (Journal of Nutritional Biochemistry, Vol. 17, pp. 257-264).