Flaxseed and its lignan precursor, secoisolariciresinol diglycoside, affect pregnancy outcome and reproductive development in rats.

Flaxseed is the richest source of the mammalian lignan precursor secoisolariciresinol diglycoside (SDG). Because lignans have estrogen agonist or antagonist properties, the objective of this study was to determine whether feeding flaxseed to rats during a hormone-sensitive period has reproductive effects. Rat dams were fed a basal diet or the basal diet supplemented with 10% flaxseed, 5% flaxseed or SDG at the level in 5% flaxseed during pregnancy and lactation. At weaning, the offspring were fed the basal diet. Flaxseed had no effect on pregnancy outcome except that the 10% flaxseed diet lowered birth weight (P < 0.05), compared with other treatments, and produced hormonal effects. The female offspring had shortened anogenital distance, greater uterine and ovarian relative weights, earlier age and lighter body weight at puberty, lengthened estrous cycle and persistent estrus (P < 0.05), whereas the males had reduced postnatal weight gain and, at postnatal d 132, greater sex gland and prostate relative weights (P < 0.05), suggesting estrogenic effects. In contrast, compared with the basal diet, 5% flaxseed reduced immature ovarian relative weight by 29% (P < 0.05), delayed puberty by approximately 5 d (P < 0.05) and tended to lengthen diestrus, indicating an antiestrogenic effect. The SDG produced results similar to those of 5% flaxseed, suggesting that lignans were responsible for the observed effects. Lignans were transferred to the offspring via rat dam's milk as indicated by the recovery of radioactivity in the offspring of lactating dams given 3H-SDG. Because flaxseed affects the reproductive development of offspring, caution is suggested when consuming flaxseed during pregnancy and lactation.