Secoisolariciresinol diglycoside (SD), a mammalian lignan precursor found in high-fiber foods, was isolated from flaxseed and tested for effects on mammary tumorigenesis in rats fed a high-fat (20%) diet. Ingestion of purified SD at 1.5 mg/day for 20 weeks starting 1 week after treatment with the carcinogen dimethylbenzanthracene resulted in a 37% reduction (p < 0.05) in the number of tumors per tumor-bearing rat and a 46% reduction (p < 0.05) in the number of tumors per tumor-bearing rat and a 46% reduction (p < 0.05) in the number of tumors per number of rats in each group. Urinary mammalian lignan excretion significantly increased (p < 0.0001) with SD treatment, indicating the conversion of SD to mammalian lignans. No enlargement or gross abnormalities of the major organs were observed in the SD-treated rats. This study showed, for the first time, that SD has an antitumor effect when provided at the early promotion stage of tumorigenesis and may contribute to the health benefits of high-fiber foods.

Flaxseed, a rich source of mammalian lignan precursor secoisolariciresinol-diglycoside (S.D.) and alpha-linolenic acid (ALA), has been shown to be protective at the early promotion stage of carcinogenesis. The objective of this study was to determine whether supplementation with flaxseed, its lignan or oil fractions, beginning 13 weeks after carcinogen administration, would reduce the size of established mammary tumors (present at the start of treatment) and appearance of new tumors in rats. Dietary groups consisted of the basal diet (BD, 20% corn oil) alone or supplemented with a gavage of 2200 nmol/day S.D. [S.D., equal to level in 5% flaxseed (F)], 1.82% flaxseed oil (OIL, equal to level in 5% F) or 2.5% or 5% flaxseed (2.5% F and 5% F, respectively). After 7 weeks of treatment, established tumor volume was over 50% smaller in all treatment groups (OIL, 2.5% F, 5% F, P < 0.04; S.D., P < 0.08) while there was no change in the BD group. New tumor number and volume were lowest in the S.D. (P < 0.02) and 2.5% F (P < 0.07) groups. The combined established and new tumor volumes were smaller for the S.D., 2.5% F and 5% F groups (P < 0.02) compared to the OIL and BD groups. The high negative correlation (r = -0.997, P < 0.001) between established tumor volume and urinary mammalian lignan excretion in the BD, S.D., 2.5% F and 5% F groups indicates that the reduction in tumor size is due in part to the lignans derived from the S.D. in flaxseed. However, there was no relationship between new or total tumor development and urinary lignan levels. The effect of flaxseed oil may be related to its high ALA content. In conclusion, the S.D. in flaxseed appears to be beneficial throughout the promotional phase of carcinogenesis whereas the oil component is more effective at the stage when tumors have already been established.

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