Association between urinary zinc excretion and isoflavone-metabolizing enterotypes among Japanese females: a cross-sectional study

Background: Zinc absorption and competition among gut bacteria have been reported in animal studies. Thus, gut bacteria may modify zinc availability in humans. Metabolism of intestinal bacteria is known to be necessary for the activation of several phytoconstituents in the body. For example, equol, a typical substance of soybean isoflavone, is produced by intestinal bacteria metabolizing daidzein and the enterotype is one of distinct ones among Japanese population. The difference in the intestinal microflora can modify the bioavailability of zinc. In this study, we examined urinary zinc concentrations in adult female equol producers (EQPs). Methods: Urine samples from women participating in health examinations in Miyagi, Okinawa, Kyoto, Kochi, and Hokkaido prefectures were used; from total 17,484 samples, approximately 25 samples were randomly selected for each age group from 30 to 60 years per region (subsample: n = 520), and 520 samples with available urinary zinc concentration (determined by flame atomic absorption analysis) and enterobacterial type were analyzed. EQP was defined as log(equol/daidzein) >= -1.42, and urinary concentrations were corrected for creatinine concentration. Urinary zinc concentrations were compared by Student's t-test and multiple regression analyses. Results: The geometric mean urinary zinc concentration (mu g/g-Cr) was lower in EQP than in non-EQP (p = 0.0136 by t-test after logarithm transformation). On the other hand, there was no correlation between urinary zinc concentration with daidzein (r = -0.0495, P = 0.436) and equol concentrations (r = -0.0721, P = 0.256). There was a significant negative association between urinary zinc concentration and EQP (beta = -0.392, P = 0.0311) after adjusting with other potential confounding variables, such as daidzein intake. Conclusions: The results suggest that gut bacteria that produce equol are involved in the metabolism of zinc. Based on previous studies, the bacteria that affect the metabolism of both substances are thought to be Enterococcus. Future studies are expected to identify specific intestinal bacteria for zinc availability and understand individual differences in the effects of micronutrients.