Mammalian lignans such as enterolactone and enterodiol, which are produced in the colon from precursors in foods, have been suggested as playing a role in the cancer-protective effect of vegetarian diets. Despite this, very little is known regarding the amount that is produced from different food products. Therefore, the objective of this study was to determine the production of mammalian lignans from 68 common plant foods by using the technique of in vitro fermentation with human fecal microbiota, which simulates colonic fermentation. Results showed a wide range (21-67,541 microgram(s)/100 g sample) in the amount of lignans produced. On the average as a group, the oilseeds produced the highest amounts (20,461 +/- 12,685), followed by the dried seaweeds (900 +/- 247), whole legumes (562 +/- 211), cereal brans (486 +/- 90), legume hulls (371 +/- 52), whole grain cereals (359 +/- 81), vegetables (144 +/- 23), and fruits (84 +/- 22). The vegetables produced the second highest concentration of lignans (1,546 +/- 280) when the data were expressed on a moisture-free basis. Flaxseed flour and its defatted meal were the highest producers of lignans (mean 60,110 +/- 7,431). Lignan production with the in vitro method related well to the urinary lignan excretion observed in rats and humans. The data should be useful in the estimation of lignan production from a given diet and in the formulation of high-lignan-producing diet for the purpose of reducing the cancer risk.

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The cancer protective effects of flaxseed suggested by our previous short-term study were tested in a long-term tumorigenesis experiment. Feeding rats 5% flaxseed flour supplemented in a high-fat diet at the promotional stage of tumorigenesis, i.e., after 7,12-dimethylbenz[a]anthracene administration, significantly reduced by 66.7% the size of the tumours that occurred. Although flaxseed feeding at the initiation stage also tended to reduce the number of tumors per tumor-bearing animal, significant differences were seen only between the group fed flaxseed throughout the experiment and the promotional group. Therefore the effect of flaxseed on mammary tumorigenesis is not consistent. Although it was speculated that the effect may be related to the lignans enterolactone and enterodiol produced in such large quantities on the ingestion of flaxseed, further studies are required to clarify the role of lignans and other flaxseed constituents in mammary tumorigenesis.

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