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LETTER TO THE EDITOR |
Julius Center for Health Sciences and Primary Care
Room D01.335
PO Box 85500
University Medical Center
3508 GA Utrecht
Netherlands
E-mail: p.h.m.peeters{at}umcutrecht.nl
Dear Sir:
We thank Dr Messina for taking the time to critically read our article and comment on it. However, we feel we extensively discussed in our article most of the issues raised in his letter.
The primary goal of our study was to investigate the effects of the habitual intake of phytoestrogens—not only isoflavones, but also lignans—on breast cancer risk in Dutch women whose diet is a typically Western diet (1). Thus, our data were based on the computed daily intakes of isoflavones and lignans, which were indeed low, but which reflected the usual intakes in the study population (2). There is no point, therefore, in comparing these figures with the habitual isoflavone intake from traditional Asian diets (
50 mg/d) or with the usual doses given in clinical trials (40–90 mg/d). Furthermore, we speculated that the role of lignans in breast cancer prevention might be more relevant than that of isoflavones in Western populations, because of the very low intake of soy products (ie, isoflavones) in these populations and their higher access to lignan-containing foods (2).
We are fully aware that exposure misclassifications are possible for both isoflavones and lignans. However, because complete food-composition data are lacking and because we were unable to account for unknown and hidden sources of soy proteins (ie, isoflavones; 3-5), we expect that our intake estimates are actually underestimates and that—because misclassification is likely to be nondifferential in this prospective study—our risk estimates are consequently attenuated.
Messina suggests that Western soy consumers are different from nonconsumers in many aspects that could serve as potential confounders. We previously studied soy intake in 10 European countries and found that participants who reported habitual consumption of soy did differ from nonreporters in the percentage of energy consumed from carbohydrates (higher in the soy consumers), the intake of fruit and vegetables (higher), age (lower), and BMI (lower) (6). The estimated daily isoflavone intake of soy consumers was in the same range as in traditional Asian diets (15–30 mg/d; 6).
In our recent publication, isoflavone intake ranged from 0.01 to 52.2 mg/d, but only 116 of the 15 555 participants (0.7%) had daily isoflavone intakes 
10.0 mg/d (1). Therefore, confounding by a selected subgroup of high soy consumers (or, rather, high isoflavone consumers) is unlikely. Furthermore, we did adjust our risk estimates for energy intake, age, weight, and height but refrained from adjusting for intake of fruit and vegetables—the main sources of lignans and partial sources of isoflavones—to avoid overadjustment.
Messina refers to the beneficial effects of isoflavones in relation to bone density and cardiac diseases, as seen in several clinical trials. As stated before, our main goal was to study habitual consumption of phytoestrogens. Still, it seems only fair to mention that the findings for higher doses of isoflavones may be just as spurious as the findings for lower doses, at least with regard to bone mineral density (7, 8) and blood lipid concentrations and cardiovascular disease morbidity (9-11).
Messina states that the amount of isoflavones needed to exert beneficial effects may vary according to the disease in question; thus, one cannot assume that the level of isoflavone exposure needed for skeletal and coronary benefits is the same as that needed for the prevention of breast cancer. Furthermore, Messina states that it is possible that the amount of isoflavones needed for health benefits when consumed over the course of a lifetime may be lower than that needed to produce benefits in short-term clinical trials. This, indeed, was our rationale for assessing the habitual consumption of both isoflavones and lignans and their association with breast cancer risk.
Last, Messina presents an intriguing hypothesis that early isoflavone exposure is protective against breast cancer. We mentioned this hypothesis in our discussion and suggested that the same idea may be applied to early lignan exposure, and that early lignan exposure may be more likely in a population exposed to a Western diet, which is richer in lignan sources than in isoflavone sources.
Messina concludes that extreme caution is needed when making pronouncements about the possible health effects of soy on the basis of Western epidemiologic studies involving non-Asian participants. We conducted our research with the utmost professional caution and studied the effect not only of isoflavones but also of lignans, which are more widespread in Western diets. We believe that our findings, which were mostly null, are accurate and very much relevant to other Western populations with low habitual intakes of isoflavones and lignans.
REFERENCES
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