Influence of human gut microflora on dietary soya isoflavone phytoestrogen bioavailability in adults and children

It is known that the microflora (bacteria) in the human gut strongly influence the absorption and metabolism of phytoestrogens in the body. However, little detailed information about the processes involved is available.

This study aims to determine what effects the microflora in the gut have on the metabolism and levels of phytoestrogens in the blood and urine of adults and children.

The level of phytoestrogens and their metabolites will be measured in the blood, urine and faeces of 76 adults following one of two supervised diets (either high- or low-soya) for 10 weeks. The types of gut microorganisms and chemical processes that alter phytoestrogen availability will also be analysed.

In addition, babies and children (aged 4 months - 6 years), that were fed on soya milk formulas in early infancy will be recruited into 4 age groups (4-6 months, 6-12 months, 1-3 years, 3-6 years). Each group will consist of approximately 6 children. An equal number of age-matched controls that had been fed on cows' milk formula will also be recruited. The level of phytoestrogens and their metabolites will be measured in urine and faecal samples. The types of gut microflora will be analysed and compared to those in adults and breast-fed infants. The level of isoflavones in soya and cows' milk infant formulas will be measured.

Key Results

• Urine, faeces and blood of the adults on a high soya diet contained the isoflavones genistein and daidzein and their gut metabolites. By comparison the levels of these compounds in the urine faeces and blood of adults on the low soya diet were very low.
• Metabolism of the isoflavone daidzein to equol is used as an indication of the presence of certain gut microflora. Around one third of adults have the gut microflora to convert daidzein to equol.
• Babies fed soya milk formula excreted isoflavones in urine. This shows that they can absorb phytoestrogens from the gastrointestinal tract.
• Young infants acquired the gut microflora to undertake metabolism of some phytoestrogens. But over time the composition of the gut microflora altered to allow additional metabolic reactions to take place.
• The type of infant formula consumed (i.e. soya or cows milk based formulas) influenced the composition and form of the infant gut microflora, which affected the metabolism of isoflavones.

Final report is available from the FSA Library and Information centre. To obtain a copy, please contact the Enquiry Desk, Dr Elsie Widdowson Library and Information Services, Food Standards Agency, tel: 020 7276 8181/8182 or email: library&info@foodstandards.gsi.gov.uk

Hoey, L, Rowland, IR, Llyoyd, AS, Clarke, DB, Wiseman, H (2004). Influence of soya-based infant formula consumption on isoflavone and gut microflora metabolite concentrations in urine and faecal microflora composition and metabolic activity in infants and children. British Journal of Nutrition 91, 607-616.
Wiseman H, Casey K, Clarke DB, Barnes KA, Bowey E. The isoflavone aglycone and gluco-conjugate content of high soy and low soy UK foods used in nutritional studies. J Agric. Food Chem. (2002 in press).

Clarke DB, Lloyd AS, Botting N, Needs P, Wiseman H. Measurement of intact sulphate and glucuronide conjugates of phytoestrogens in human urine using direct injection HPLC and electrospray tandem mass spectrometry. Analyt. Chem. (2002 manuscript submitted to FSA for approval prior to publication)

Wiseman H, Casey K, Clarke DB, Lloyd AS, Rowland IR, Bowey E. Influence of soy food consumption on plasma, urinary and faecal isoflavone and gut microflora metabolite concentrations, faecal bacterial enzyme activity and conversion of daidzein to equol ex vivo in adult humans. Am J Clin Nutr (2002, manuscript in preparation)

Rowland IR, Hoey L, Bowey E, Wiseman H. Influence of infant formula soy milk consumption on urinary and faecal isoflavone and gut microflora metabolite concentrations and on faceal microflora composition in babies and children. Am J Clin Nutr (2002 manuscript in preparation).