Key Findings
The objective of this study was to examine the effect of dietary intake of a lignan-rich flaxseed on urinary lignan excretion in postmenopausal women consuming a semi-controlled diet. The consumption of 5 or 10 grams of flaxseed by postmenopausal women on a semi-controlled diet increased the excretion of enterolactone, enterodiol, and total lignans in a dose-dependent manner without changing the ratio of enterodiol: enterolactone excretion, suggesting that urinary excretion of lignan metabolites can be used as a dose-dependent biomarker of flaxseed consumption. Because flaxseed and its mammalian lignan products enterolactone and, to a lesser extent, enterodiol have been shown to influence the early risk markers for and incidence of mammary and colonic carcinogenesis in animal models, decrease cell proliferation in vitro, and influence factors that effect the hormone concentrations in humans, increases in the metabolism and excretion of these compounds may offer increased protection against hormone-dependent cancers. Wide variations in the excretion of enterodiol, enterolactone, and total lignans and the enterodiol:enterolactone ratio suggest that the factors that may influence individual variations in mammalian lignan formation, absorption, and excretion need to be examined so that we may have a better understanding of the metabolism of these compounds.
ABSTRACT
Dietary estrogens, such as lignans, are similar in structure to endogenous sex steroid hormones and may act in vivo to alter hormone metabolism and subsequent cancer risk. The objective of this study was to examine the effect of dietary intake of a lignan-rich plant food (flaxseed) on urinary lignan excretion in postmenopausal women. This randomized, cross-over trial consisted of three 7-week feeding periods during which 31 healthy postmenopausal women, ages 52-82 years, consumed their habitual diets plus 0, 5, or 10 grams of ground flaxseed per day. Urine samples collected for 2 consecutive days during the last week of each feeding period were analyzed for lignan content (enterodiol, enterolactone, and matairesinol) by isotope dilution gas chromatography/mass spectrometry. Compared with the 0-gram flaxseed diet, consumption of 5 or 10 grams of flaxseed significantly increased excretion of enterodiol by 1,009 and 2,867 nmol/day, respectively; significantly increased excretion of enterolactone by 21,242 and 52,826 nmol/day, respectively; and significantly increased excretion of total lignans (enterodiol + enterolactone + matairesinol) by 24,333 and 60,640 nmol/day, respectively. Excretion of matairesinol was not significantly altered by flaxseed consumption. Consumption of flax, a significant source of dietary estrogens, in addition to their habitual diets increased excretion of enterodiol and enterolactone, but not matairesinol, in a dose-dependent manner in this group of postmenopausal women. Urinary excretion of lignan metabolites is a dose-dependent biomarker of flaxseed intake within the context of a habitual diet.