In this large prospective study, women who had the highest dietary intakes of total plant lignans, and of lariciresinol in particular, had a statistically significant lower risk of postmenopausal breast cancer compared with women who had the lowest intakes. These inverse associations were restricted to ER+
breast cancers, for which we also observed inverse associations between breast cancer risk and intakes of enterolignans. All studies published to date that have shown that higher levels of circulating or excreted enterolignans are associated with a statistically significant reduction in the risk of postmenopausal breast cancer have been biomarker studies (13
). To our knowledge, this is the first prospective study using dietary questionnaire data to show statistically significant inverse associations between dietary lignan intakes or estimated levels of metabolized enterolignans and breast cancer risk in postmenopausal women.
The inverse associations between dietary lignan intakes and postmenopausal breast cancer risk in Western populations have been investigated in two prospective (45
) and four case–control (5
) studies. None of these studies found an overall association, but two studies (45
) reported a suggestive trend for an inverse association on the basis of subanalyses of postmenopausal participants. In all six study populations, the levels of dietary lignan intake were defined as the sum of the secoisolariciresinol and matairesinol intakes, which were of the same order of magnitude as the combined secoisolariciresinol and matairesinol intakes in our study (approximately 0 to 0.8 mg/day). Intakes of lariciresinol and pinoresinol were not evaluated in those six previous studies, whereas in our study they accounted for 83% of the plant lignan intake. It is therefore likely that all six previous studies underestimated dietary lignan intake, which may explain why no association with breast cancer risk was detected.
In our study, the median total plant lignan intake was 1.1 mg/day and ranged from approximately 0 to 5 mg/day. The lignans consumed by our cohort originated from a wide variety of sources including fruits, vegetables, tea, coffee, and cereal products, similarly to diets estimated in other studies of Western countries (33
); in particular, in this study, the main sources of lariciresinol were cruciferous vegetables, green beans, citrus fruits, pears, tea, coffee, and bread. As for the food sources of lignans, we note that the traditional French diet does not contain flaxseed, which is the food containing the highest concentration of lignans. However, lignan intakes are probably increasing in France, as they are in other Western countries such as Finland and North America, because the recent introduction of flaxseeds in multigrain bread making may provide a major source of lignans (52
). Although the main sources of lignans in Western countries probably vary according to the foods eaten there, assessments of more types of lignans in dietary studies might well reveal more consistent associations between phytoestrogen intakes and the risk of breast cancer.
Our findings of inverse associations between enterolignan exposure and the risk of postmenopausal breast cancer are consistent with those of the four case–control studies of serum or urinary biomarkers conducted to date (13
), even though ours are based on indirectly estimated enterolignan levels. Of six prospective biomarker studies (6
), only one (6
) reported an inverse association between plasma enterolactone concentrations and postmenopausal breast cancer risk; it is unclear why the five other studies (8
) did not detect any association, especially because the level of enterolignan exposure was consistent across all biomarker studies.
It may be argued that lignan and enterolignan information is redundant in our study and that lignans alone should be considered because those are compounds provided by the diet. Nevertheless, we examined enterolignans as well as plant lignans to provide two different approaches to study the effects of dietary lignans and also because enterolignans are the bioactive components relevant to cancer prevention (1
). The amounts of enterolignans that are produced from different foods have been measured by a technique that involves in vitro fermentation of plant foods with human fecal microbiota, which simulates colonic fermentation (7
). In this study, we used enterolignan values estimated by this technique to indirectly estimate all lignans (precursors of the enterolignans) present in foods. Total enterolignan values estimated by this technique integrate other lignans and unknown precursors that cannot be assessed otherwise due to the unavailability of food composition data. For example, the lignins, which are polyphenolic plant constituents responsible for the rigidity of wood, were suggested to be major dietary precursors of enterolignans, at least in rats (53
), but have never been analyzed in foods.
Our finding that all of the statistically significant inverse associations we observed between phytoestrogen intakes and postmenopausal breast cancer risk were restricted to ER+
breast cancers supports a differential role of phytoestrogens or their food sources according to hormonal receptor status as has been previously suggested (1
). This finding is consistent with that of a large case–control study (19
) that reported a reduced risk of postmenopausal ER+
breast cancer associated with higher intakes of leafy or yellow vegetables. The authors of that study found no clear association between the risk of postmenopausal ER+
breast cancer and the carotenoids contained in such vegetables. By contrast, our results suggest that lignans, especially lariciresinol, might be among the bioactive plant compounds involved in reducing postmenopausal ER+
breast cancer risk. These results differ from those of a large prospective study (21
), which showed a reduced risk of ER−
postmenopausal breast cancer with higher intakes of fruits, vegetables, and whole grains after adjustment for hormonal exposure factors, and from another prospective study, which reported a reduced risk of ERα-negative postmenopausal breast cancer associated with a higher consumption of fruits and vegetables (22
) or higher plasma enterolactone concentrations (6
). One case–control study (49
) and two prospective studies (11
) found no association between postmenopausal ER+
breast cancer risk and dietary, plasma, or urinary lignans; however, those studies had limited statistical power to detect such associations because the dietary lignan intakes were relatively low and too homogeneous (46
) or because of a small study size (11
We cannot conclude definitely from our results that lignans have specific biologic effects that influence their association with breast carcinogenesis or that they are good biomarkers for particular nutrients or food sources. However, our finding that adjustments for fiber and vitamin C intakes did not remove the statistical significance for the association between lignan intake and postmenopausal breast cancer risk suggests that lignans have specific biologic effects. Furthermore, the inverse associations we observed between lignan intakes and risk of ER+
breast cancer suggest that the biologic effects may be mediated through hormonal receptors, a plausible interpretation given that phytoestrogens have hormone-like properties whereas vitamins, which did not show any association with breast cancer risk (19
), do not (54
). Because of their structural similarity to 17 β-estradiol, phytoestrogens are natural ligands of ERs and are believed to be naturally existing SERMs (1
). They might therefore act as anticarcinogens, either through antiestrogenic actions (e.g., by competing with estradiol to bind ERs) or by initiating their own anticarcinogenic effects (e.g., by recruiting specific transcriptional coregulators to phytoestrogen-activated ERs). Finally, phytoestrogens or their plant sources might modulate ER protein expression and degradation and therefore influence the hormonal status of both normal tissues and tumors, as has been previously suggested (25
), just as estradiol decreases expression of ER (56
This study found no statistically significant association between lignan or enterolignan intakes and ER+
postmenopausal breast cancer risk. Given the small number of ER+
tumors and the magnitude of the risks for these cases, low statistical power is probably a large part of the reason for these non–statistically significant associations. An alternative explanation would be based on molecular aspects. In this study, ER status most likely referred to the expression of the nuclear α subtype of the receptor, the predominant subtype in breast tissue and the subtype commonly detected by the analytical methods used in clinical practice. Because PR expression mostly depends on ERα activity (57
), our observation that there were no inverse associations between phytoestrogen intakes and the risk of postmenopausal breast cancer in the absence of PR despite ERα expression (i.e., ER+
tumors) agrees with the molecular theories that, in the absence of PR expression, ER is nonfunctional (57
) or other growth factor receptor pathways are activated (58
). In other words, it is possible that ER+
tumors are “resistant” to phytoestrogens, just as they are to SERM therapy (57
The strengths of our study include the large population, the long follow-up, and the use of a comprehensive food composition table for lignans and enterolignans. The resulting large range of intakes provided increased statistical power to detect associations other than those that occurred by chance. Because we included lariciresinol and pinoresinol, whose associations with breast cancer risk were evaluated here for the first time, to our knowledge, the estimated lignan intakes were much higher than those reported in previous dietary studies. The prospective design of our study precluded differential recall bias between case subjects and non– case subjects. Finally, we adjusted for a large number of hormonal factors that could have acted as potential confounders, including age at menarche and at menopause, parity, age at first full-term birth, and lifetime use of exogenous hormones (oral contraceptive and hormone replacement therapy).
Our study also has limitations related to exposure misclassification, confounding, endpoint misclassification, and generalizability of the results. First, we cannot exclude possible misclassification bias arising from reporting error although this would most likely result in the underestimation of the true association. In general, we assumed that the diet reported over the previous year reflected long-term dietary habits. However, we were not able to assess possible changes in dietary habits during the follow-up period. As for lignans specifically, our use of a simple summation of four lignans to provide a total lignan intake did not account for other lignans or other potential enterolignan precursors, such as lignins, for which food composition data are not available; therefore, we also evaluated each individual lignan in risk analyses. Our estimates of lignan dietary intakes were also potentially limited by the food database used in this study. We had to rely on data compiled from various sources because, at present, there is no complete food composition database for lignans. The values assigned to the foods were based on food supplies that originated from different countries and on different analytical techniques and were estimated in different laboratories. Although values for the same foods may vary in the literature according to the source of data, most of the different values available for one food were consistent with each other. As for enterolignans, our indirect estimation of enterolignan exposures from the dietary intake data did not take into account interindividual differences in the characteristics of intestinal microflora or in enterolignan metabolism, such as absorption and excretion rates (1
); however, enterolignan production obtained via the in vitro method has been shown to be well correlated with urinary lignan excretion (7
). Also, we do not know the relative bioactivity of each enterolignan but the summation of all enterolignans would likely result in misclassification bias and would decrease the statistical power.
Second, as in other observational studies, we cannot rule out that the associations we observed have resulted from confounding bias, although we adjusted the analyses for known risk factors of breast cancer, in particular hormonal factors. With respect to phytoestrogens specifically, we note that our dietary questionnaire did not cover soy foods. However, given that soy foods were probably consumed at very low levels by our study population (59
), it is unlikely that their omission from the dietary questionnaire affected our assessment of lignan intake or was a potential confounding factor in this study of lignan intake and breast cancer risk. Third, endpoint misclassification may have hampered our analysis of breast cancer by receptor status because assessment of ER status probably referred only to the expression of ERα and not ERβ, which is the subtype of ER that phytoestrogens preferentially bind (60
). However, our results are relevant to breast carcinogenesis because ERα is the predominant form of the receptor expressed in normal breast tissue and its level increases dramatically in premalignant tissues (61
). Fourth, the women in our cohort were self-selected volunteers who were recruited from among employees of the public education health system or their families and thus were not a representative sample of the general French population. The cohort members were highly educated and, compared with nationally representative samples of the French population, had higher rates of breast cancer (62
) even though they may have had more health-conscious dietary practices. Thus, as is true for many other large cohort studies based on selected populations, it may not be possible to extrapolate our findings to the general population. However, if the association between dietary lignan intakes and breast cancer risk in our population reflects a true biologic mechanism, this mechanism may be relevant to the general population.
In conclusion, the results of this large prospective study of French women showed that higher dietary intakes of lignans were associated with a reduction in the risk of postmenopausal breast cancers, particularly those positive for both ER and PR. This finding is potentially important for public health policies because the increasing ER ER+
subtype incidence rates are thought to explain most of the increasing incidence of breast cancer in Western countries (63
) and the higher rates in Western countries than in Asia (64
). Although the possible role of plant foods in breast cancer prevention is still debated, increasing dietary lignan intake may be an interesting potential preventive approach. In particular, individual differences in the metabolism of plant lignans into enterolignans and the mechanisms behind the potential biologic actions of enterolignans in breast carcinogenesis need to be better understood. In view of the epidemiologic results of this study, the recommendation that women should consume diets that consist largely of fruits, vegetables, and cereals (65
) — all foods rich in lignans— should continue.