Serum isoflavones and soya food intake in Japanese, Thai and American end-stage renal disease patients on chronic haemodialysis

BACKGROUND Soya foods, a staple in several Asian countries, have received increasing attention because of their nutritional properties and their high isoflavone content. We have shown recently abnormal pharmacokinetics of soya isoflavones following acute oral intake, in soya-naive end-stage renal disease (ESRD) patients. No information is available, however, about blood levels of soya isoflavones in ESRD patients with habitual soya intake. Additionally, no information is available about the conjugation profile of these compounds in ESRD patients. METHODS To assess the relationship between habitual soya intake on blood isoflavone levels in ESRD patients, we recorded dietary soya food intake and analysed circulating levels of soya isoflavones in randomly selected, clinically stable haemodialysis patients from the United States (n = 20), Thailand (n = 17) and Japan (n = 20). Dietary records and three weekly blood samples were collected from each participant. Combined isoflavones and individual genistein, daidzein, glycitein and O-desmethylangolensin (DMA) were analysed in serum by liquid chromatography/mass spectrometry. Lipid phase micronutrients, including tocopherols, carotenoids and retinol were also measured to compare ethnic differences in isoflavones with those of more common lipid soluble antioxidant micronutrients. RESULTS Soya intake was higher in Japanese than in Thai patients and it was negligible in the US patients. Blood levels of genistein were very elevated and significantly higher in the Japanese patients (1128 +/- 205 nM), as compared with the Thai and US patients (258 +/- 64 and 168 +/- 49 nM, respectively; P < 0.001). The other isoflavones followed the same trend. Daidzein was more concentrated than genistein in the dialysis patients. Robust correlation was present between weekly soya intake and blood isoflavone levels (r = 0.56, P < 0.001). Despite very high total isoflavone concentrations, the levels of unconjugated and sulphated isoflavones in the Japanese patients were comparable to those described in healthy subjects. Compared with the striking difference in isoflavones, more easily accessible dietary antioxidants, including tocopherols, carotenoids and retinol, differed only minimally or not at all in the three groups. CONCLUSIONS ESRD patients appear to accumulate isoflavones as a function of dietary soya intake, resulting in blood concentrations that are higher than those reported in subjects with preserved kidney function. Even in the presence of very elevated total isoflavone levels, the concentrations of the unconjugated and sulphated fractions are comparable to those of healthy subjects. A discrepancy is noted between accumulation of soya isoflavones and other more common lipid-soluble antioxidant micronutrients.