Different seed matrices or rye and flax affect the bioavailability of enterolactone precursors

Rye and flax are important sources of dietary phytoestrogens, the intake of which results in increased urinary and plasma enterolactone levels. Enterolactone levels in plasma have been associated with the reduced risk of breast cancer and acute coronary events. Furthermore, lignan-rich diets have shown inhibition of colon cancer growth in animal trials. Enterodiol and enterolactone are formed by bacterial flora in the colon from several precursors from rye, and mainly from secoisolariciresinol from flax. Enterodiol formation occurs concomitantly with the carbohydrate consumption, whereas subsequent enterolactone conversion is a slower process. Plant lignans are attached to the fibre matrix. As carbohydrates are degraded in the colon, lignans are liberated to be available for colonic bacteria. In the present study enterodiol and enterolactone formations from rye and flax were compared using an in vitro fermentation model (Karppinen et al. 2000). The samples were digested enzymatically prior to the fermentation with human faecal flora. Compared to rye flax carbohydrates were degraded to a lower extent. Also, only 45 % of secoisolariciresinol was converted to enterodiol, which was subsequently oxidized to enterolactone completely. This can be explained by several layers of seed coat in flax making the matrix rigid and slowly fermentable. Carbohydrate components of rye are cellulose, arabinoxylans, glucans and fructans and they contain also soluble, easily fermentable polysaccharides. Even though the amount of plant lignans is lower in rye than in flax seed, the bioavailability of enterolactone precursors is fairly good due to its easily fermentable carbohydrate matrix. Therefore, enterolactone formation can be detected in vivo (Juntunen et al. 2000). More work is needed to fully confirm all the enterolactone precursors in rye. Even though seed structure of flax affects strongly the bioavailability of plant lignans, the abundance of secoisolariciresinol makes flax an excellent source of enterolactone precursors. References Karppinen S, Liukkonen K, Aura, A-M, Forssell P, Poutanen K (2000) J. Sci. Food Agric. 80: 1469-1476 Juntunen KS, Mazur WM, Liukkonen KH, Uehara M, Poutanen KS, Adlercreutz HCT, Mykkanen, HM (2000) Br. J. Nutr. 84: 839-846