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 isoflavones is affected by the food matrix is relatively scarce.
The new study, a collaboration between universities in the UK, Denmark and Spain and Unilever, UK and Agrotechnology and Food Innovations, Holland, and published in the Journal of Nutritional Biochemistry (Vol. 17, pp. 257-264) investigated the availability of the isoflavones daidzein and genistein from cookies, chocolate bars and juice.
The researchers added 455 milligrams of an isoflavone-rich extract (11 per cent isoflavone: SoyLife Extra) to the three different food products. Cookies, chocolate bars and fruit juices were chosen because of consumer preference from the sample group, the technological suitability as isoflavone carriers, and to examine the liquid-solid matrix effect.
Twelve postmenopausal women from the UK and Denmark tested the effect of food matrix on apparent bioavailability of the isoflavones. The women were not receiving hormone replacement therapy and did not consume soy products on a regular basis.
Blood and urine samples were taken to measure the availability of the isoflavones with blood samples taken after 4, 6, 8, 12, 24 and 48 hours after consuming one of the foods. Two repeat experiments were performed with at least one week between tests.
"After consumption of the test foods containing isoflavones, serum genistein concentrations increased to 160.9, 157.5, and 129.1 nanograms per millilitre of serum in the cookies, bars and juice, respectively," said lead author Sonia de Pascual-Teresa from Ciudad Universitaria, Madrid.
Serum daidzein concentrations reached a maximum of 116.8, 127.2 and 112.1 for the same test foods.
Total excretion of the isoflavones in the urine was always the lowest in the juice, showing that more of the isoflavone was bioavailable from this source. Genistein excretion was highest from the cookies, and daidzein excretion was highest from the bars, although all three foods had similar values.
"There was a lower total urinary recovery of genistein following ingestion of juice (61 per cent) than that of the other two foods.
These data suggest that levels of isoflavones attained, particularly the levels of genistein, may be altered depending on the food matrix consumed," said Pascual-Teresa.
This research has implications for using soy isoflavones as functional ingredients. The scientists recommended further research in order to determine the importance of the food matrix and to "ensure that safe and efficacious doses of these compounds can be delivered for potential health benefits."