The plant lignans, secoisolariciresinol (SEC) and matairesinol (MAT) are converted to the metabolites, enterodiol (ED) and enterolactone (EL), known as the mammalian lignans in the gastrointestinal tract. In vitro, mammalian lignans may have both estrogenic and antiestrogenic properties. Increased consumption of lignan-rich foods through the addition of brans, oilseeds and cereals in bread, muffins, health bars and breakfast cereal or emphasis on fruits and vegetables in a balanced diet has lead to the need for reliable data on lignan content in foods. The quantitative determination of the lignans to date by a direct method has involved hydrolysis of the glycoside and analysis of the resulting aglycones (SEC and MAT). The in vitro fermentation methodology, which simulates colonic fermentation with fecal microflora, has been utilized in the indirect analysis of foods where ED and EL are measured as an indication of the plant lignans. A compilation of the available data on the lignan content of various food groups and the assessment of these data by contrasting two different analytical methodologies are given. In general, in vitro fermentation gave higher values, compared to direct analysis, for most foods for which comparisons could be made.
J Food Comp Analysis. , 2000., 13, 935. doi.10.1006/jfca.2000.0932
Accurate lignan determination requires complete extraction of lignans from the plant matrix; formation of the lignan aglycones; adequate sample clean-up; selection of a sensitive analytical method for separation and quantitation; precautions to avoid artifact formation and extensive losses during analysis. The aim of this study was to compile the available data on the lignan content of various food groups, and assess these data by contrasting two different analytical methodologies. The direct analysis of the plant lignans, SEC and MAT contrasted with the concentrations of ED and EL, the reduced metabolites, determined by the indirect in vitro fermentation method. Flaxseed contains the highest concentrations of SEC (28 800+/-369 000ug/100 g) of any food. The concentration of ED and EL and residual SEC was found to range from 28 800 to 94 500 ug/100 g when 10 flaxseed varieties were grown in the same location, from 28 800 to 82 800 ug/100 g when four locations were chosen for planting (Manitoba, Saskatchewan and Alberta, Canada), and from 28 800 to 70 200 ug/100 g when planted in three different years (1987, 1988, 1989). The results indicate that the concentration of lignans in flaxseed is variable. The data presented indicate that the total lignan concentration of foods (SEC and MAT) is higher when measured by the indirect method as ED and EL than by enzyme hydrolysis with b-glucuronidase in the direct method. Further research contrasting the direct and the indirect in vitro fermentation method of the same food samples needs to be conducted for accurate comparisons of lignan data. Investigation of organic extraction and the use of enzymes may prove useful in the analysis of lignans.