By Zheng Y, Lee S-O, Verbruggen MA, Murphy PA, Hendrich S. Apparent absorption of isoflavone glucosides and aglycones is similar in women and increased with rapid gut transit time and low fecal isoflavone degradation. Am. Society for Nutr. Sc. 5 (2004): 2534-2539.
“Bioavailability” indicates to what extent active compounds are absorbed. In the case of isoflavones: glycosides are absorbed through the gut wall after substantial bacterial metabolism, whereas aglycones might be directly absorbed through the intestinal wall. Either route will lead to circulatory metabolites, finally excreted in the urine. Isoflavones excreted in the faeces were either not absorbed or were returned in the gut through the bile (enterohepatic circulation).
Controversy exists as to which (form of) isoflavone shows the best bioavailability. Apart from interpersonal differences, product-related factors of influence might be the bonding of isoflavones to a glucose molecule (aglycone versus glucoside) and the type of isoflavone (genistein, glycitein or daidzein). In this study, the bioavailability of isoflavones was compared between aglycones and glucosides, from a soy source rich in glycitin (SoyLife from soy germ) vs soy rich in genistin (Novasoy from soy molasses).
To exclude interpersonal variation, a group of young healthy women consumed an isoflavone-rich diet for 2 weeks. Based on the recovery of isoflavone metabolites in their urine, they were classified as low, moderate or high excreters of isoflavones. Of this total group, 26 moderate excreters were consequently randomly assigned to one of three treatments for 7 days:
- unfermented SoyLife (US) – rich in glucosides and relatively rich in glycitin
- fermented SoyLife (FS) – rich in aglycones and relatively rich in glycitein
- Novasoy (NS) – rich in glucosides and relatively rich in genistin.
Total urine was collected over 0-12h and 12-24h on the 1st and 7th day. Urinary isoflavone contents were expressed as a percentage of ingested dosage.]
In the first 12 hours, FS isoflavones (aglycones) were excreted to a greater extent than isoflavones from US (glycosides). In the second 12 hours of day 1, isoflavones from US were excreted to a greater extent than were FS isoflavones.
Total urinary excretion of isoflavones over the first 24h and after 7 days was greater for SoyLife than for Novasoy, but did not differ between fermented and unfermented SoyLife.
This study showed that aglycones were absorbed more rapidly than glucosides, but that the total urinary isoflavone excretion over 24 hours was similar between aglycones and glucosides.
Aglycones might therefore be seen as “fast-release” isoflavones, while glucosides show a more “slow release” absorption pattern. The study also shows that after 7 days of consuming isoflavones, the body’s efficiency to absorb and use isoflavones has increased on day 7 compared to day 1. In addition, a source rich in glycitein appeared to be better bioavailable compared to a source rich in genistein.