Adding probiotic strains to soymilk may have extra benefits by improving the bioavailability of the isoflavones in the milk, suggests new research.
Writing in the Journal of Food Science, researchers from Victoria University in Australia report that addition of the probiotic bacteria to soymilk led to a conversion of isoflavones from the less bioavailable glycoside form to the aglycone form.
The research 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.
A study by Yakult last year however (The Journal of Nutrition, Vol. 136, pp. 2291-2296) reported that isoflavones in the their aglycone form are absorbed faster and in bigger amounts from soy milk than the glucoside form.
The new study tested the efficacy of two probiotic strains, Bifidobacterium animalis A and B, to transform soy isoflavone glycosides, the predominant form in unfermented/natural soymilk made from soy protein isolate, to the aglycone form in the presence of skim milk powder (SMP). The probiotics were also added to soymilk with no SMP added - unsupplemented soymilk (USM) - and reconstituted skim milk powder (RSMP).
During fermentation isoflavones change from the glucoside form to the aglycone form, with the unsupplemented soymilk isoflavones found to undergo to 74 and 73 per cent transformation for B. animalis A and B, respectively. However, when SMP was added to the probiotic-containing soymilk, biotransformation was increased to 84 and 85 per cent, respectively.
"The biotransformation of isoflavone glycosides in soymilk supplemented with SMP was significantly increased by approximately 10 per cent with by both B. animalis A and B," wrote the authors.
Comparing lactose transformation between the soymilk containing SMP (SSMP) and the RSMP, the researchers report that lactose utilisation increased by three to five milligrams per millilitre in SSMP compared to that in RSMP by both probiotic strains.
"However, the viable counts of B.animalis in SSMP were slightly lower than those in RSMP, but significantly higher than those in USM. Therefore, addition of SMP appears to stimulate the growth of B. animalis A and B as well as the biotransformation of isoflavone glycosides to aglycones," they concluded.
Last year's study by Yakult stated that it made sense that aglycones 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).
Source: Journal of Food Science
Published on-line ahead of print, doi: 10.1111/j.1750-3841.2007.00476.x
"Biotransformation of Isoflavone Glycosides by Bifidobacterium animalis in Soymilk Supplemented with Skim Milk Powder"
Authors: T.T. Pham and N.P. Shah