Affiliations of authors: Cancer Information and Epidemiology Division, National Cancer Center Research Institute, Tokyo, Japan (SY, TS); Epidemiology and Biostatistics Division, National Cancer Center Research Institute, East, Kashiwa, Japan (MK, ST); Department of Health Care and Nutrition, Showagakuin Junior College, Ichikawa, Japan (MK); National Institute of Health and Nutrition, Tokyo (SS).
Correspondence to: Seiichiro Yamamoto, PhD, National Cancer Center Research Institute, Tsukiji, Chuo-ku, Cancer Information and Epidemiology Division, 5-1-1, Tsukiji, Chuo-ku, Tokyo, Japan 104-0045 (e-mail: siyamamo{at}activemail.jp)
In our article (1), we showed an association between breast cancer risk and intake of miso soup and isoflavones in a prospective cohort study of Japanese women. Furthermore, the association between breast cancer risk and soyfood and isoflavone intake was stronger for postmenopausal women than for premenopausal women.
From their cross-sectional analysis of their cohort, Fujimaki and Hayashi report a negative association between breast cancer history and miso soup intake, no association with natto intake, and a positive association with tofu intake. They speculated that the isoflavones included in miso soup and natto may be more anticarcinogenic because they are aglucones, which are more potent and more rapidly absorbed than -glucoside-conjugated isoflavones, such as those found in tofu. Their interpretation is plausible based on that theory and on previous in vivo and in vitro studies, and it is an important to note that the effects of isoflavones may vary among soyfoods and those effects should be assessed in humans. Their resultsthat an inverse association was not observed for soyfoods but was observed for miso soupare consistent with our results, especially if the majority of women with a history of breast cancer had breast cancer before menopause. However, we showed an inverse association between relative risk estimates for both miso soup and soyfoods and breast cancer, although that for soyfoods was not statistically significant. Because the items in our baseline questionnaire were too broad to investigate differences associated with specific soyfoods such as miso soup, natto, and tofu, we think the differences in our results between miso soup and soyfoods may arise from variations in exposure measurements rather than from variations in the effects of soyfoods. Possible differences associated with various soyfoods should be investigated using a more precise questionnaire (2).
Fujimaki and Hayashi suggested that their different results for miso soup and tofu may be due to the increase in tofu use for symptoms of menopause. However, the study they cite for increased use of dietary soy was conducted among American women (3). Such extrapolation may not necessarily apply to Japanese women because alternative therapies for symptoms of menopause can vary over time and among countries and ethnic groups. We know of no studies that examined the increase of soyfood use for symptoms of menopause in Japan. Our data did not show increased isoflavone intake for women of perimenopausal age but did show a linear increase according to age [Table 2 of (1)]. Their data show a positive association between tofu intake and breast cancer history but do not necessarily show increased tofu intake for women aged 5060 years. In addition, results from the study by Fujimaki and Hayashi came from a cross-sectional analysis of their cohort, not from a prospective follow-up. Cross-sectional analyses may sometimes lead to erroneous cause-and-effect relationships because they analyze the exposure and the endpoint data collected simultaneously. For example, Fujimaki and Hayashi show that women who have a history of breast cancer consume more miso soup, but this observation may reflect the fact that the women changed their eating habits after they were diagnosed with breast cancer. Consequently, we might observe a spurious association implying that tofu intake increased the risk of breast cancer. To examine the possible increase of soyfood intake during the perimenopausal period, data for soyfood intake must be collected for pre-, peri-, and postmenopausal periods. In addition, the sensitivity of cancer diagnoses by self-report is low among Japanese women, although the sensitivity for breast cancer history is higher than that for other cancers (4). Therefore, results from cross-sectional studies, especially those that include self-reported cancer as an endpoint, should not be used in etiologic studies examining the association of cancer with diet.
REFERENCES
1 Yamamoto S, Sobue T, Kobayashi M, Sasaki S, Tsugane S; Japan Public Health Center-Based Prospective Study on Cancer and Cardiovascular Diseases Group. Soy, isoflavones, and breast cancer risk in Japan. J Natl Cancer Inst 2003;95:90613.
2 Yamamoto S, Sobue T, Sasaki S, Kobayashi M, Arai Y, Uehara M, et al. Validity and reproducibility of a self-administered food-frequency questionnaire to assess isoflavone intake in a Japanese population in comparison with dietary records and blood and urine isoflavones. J Nutr 2001;131:27417.
3 Newton KM, Buist DS, Keenan NL, Anderson LA, LaCroix AZ. Use of alternative therapies for menopause symptoms: results of a population-based survey. Obstet Gynecol 2002;100:1825.
4 Yoshinaga A, Sasaki S, Tsugane S; JPHC Study Group (Japan Public Health Center). Sensitivity of self-reports of cancer in a population-based prospective study: JPHC Study Cohort I for the JPHC Study Group. Clin Epidemiol2001;54:7416.[CrossRef][ISI][Medline]
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