Flaxseed Ingestion Alters Ratio of Enterolactone Enantiomers in Human Serum

Enterolactone (EL) is an enterolignan found in human subjects. In this pilot study, the enantiomeric ratios of serum EL were determined in serum from healthy adults during consumption of habitual diet, and after an 8-day supplementation with flaxseed (25 g/day). (−)EL dominated in all serum samples collected during habitual diet consumption. However, the ratio of (−)EL and (+)EL enantiomers differed markedly between individuals. Flaxseed ingestion increased significantly the proportion of (+)EL in all subjects. Moreover, a small but significant increase in serum (−)EL concentration was  measured. After flaxseed ingestion, (−)EL concentrations correlated with those of (+)EL suggesting that the stereochemistry of the parent plant lignan in flaxseed is not a major determinant of EL formation in human subjects. Comparison of EL concentrations obtained with the validated chromatographic methods (HPLC-MS/MS, HPLC-CEAD, and GC-MS) and the time-resolved fluoroimmunoassay (TR-FIA) revealed that the immunoassay method underestimates human serum EL concentrations after the flaxseed ingestion. (Authors abstract)
Enterolactone (EL) is an enterolignan produced by intestinal microbiota from several dietary plant lignans. In epidemiological studies, low serum and urine EL concentrations have been associated with an increased risk for breast, prostate, and colon cancers, and cardiovascular disease. EL is a chiral compound, and the enantiomers (one of two stereoisomers that are mirror images of each other) occurring are (8R, 8-R)-(−)EL and (8S, 8-S)-(+)EL. Whether this stereochemistry determines metabolic conversion capability of plant lignans to enterolignans is not known. In this pilot study, the concentrations of EL enantiomers in serum from adult volunteers consuming their habitual diet and after flaxseed ingestion was determined and whether the serum EL enantiomer profile was associated with the serum enterolignan concentrations was evaluated. The serum EL concentration levels obtained with various methods were compared  to evaluate the suitability of the immunoassay method for the analysis of serum samples after flaxseed ingestion. The results show that the ratio of (+) and (−)EL in human serum varies significantly between individuals with their habitual diet. However (−)EL predominated. These investigators have previously shown in rats that secoisolariciresinol diglycoside isolated from flaxseed is a precursor for (+)EL.  In this study, it was demonstrated that (+)EL dominates also in human serum after the flaxseed ingestion. A small increase in serum (−)EL concentration was observed after the flaxseed consumption. This is likely due to (−)secoisolaricireresinol, a minor lignan in flaxseed, that is a parent compound for (−)EL formation in vivo. This suggests that the stereochemical structure of the plant lignan precursors in flaxseed is not the major determinant for the EL production capacity of the intestinal microbiota in human subjects. The study shows that the ratio of human serum (+) and (−)EL enantiomers varies and can be significantly altered by dietary supplementation with flaxseed. It was also demonstrated that the immunoassay is not an optimal quantitation method for samples with very high EL concentrations or with significantly altered ratio of EL enantiomers like the ones collected after the flaxseed ingestion. The investigators suggest that chromatographic quantitation methods should be used in flaxseed intervention studies. (Editors comments)