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- Materials and Methods
Components of the Japanese diet that might contribute to the relatively low breast cancer incidence in Japanese women have not been clarified in detail. To evaluate associations between broad dietary patterns and breast cancer risk in a Japanese population, the authors conducted a case–control study using data from the hospital-based epidemiologic research program at Aichi Cancer Center (HERPACC). Factor analysis (principal components) was used to derive food patterns based on the 31 food variables and allowed designation of four major dietary patterns: prudent, fatty, Japanese and salty. In total, 1885 breast cancer cases were included and 22 333 female non-cancer patients were recruited as the control group. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were determined using multiple logistic regression analysis. After adjusting for potential confounders, there were no clear associations between the fatty, Japanese or salty dietary patterns and overall breast cancer risk. In contrast, an inverse association was evident for the prudent dietary. Women in the highest quartile of the prudent dietary pattern scores, had a 27% decreased risk of breast cancer compared with those in the lowest (95% CI: 0.63–0.84, p for trend < 0.0001). In addition, for women with a body mass index ≥25, the highest quartile of the fatty factor score was associated with a 58% increment in breast cancer risk compared to the lowest quartile, with a significant linear trend (P = 0.027). The authors found the prudent dietary pattern to be negatively associated with breast cancer risk. In addition, the fatty and Japanese patterns may increase breast cancer risk among obese women. (Cancer Sci 2007; 98: 1431–1438)
Although the incidence of female breast cancer in most Asian countries is much lower than that in Western countries,(1) there has been a marked increase in recent years. The age-adjusted incidence per 100 000 women (standardized on the world population) in Japan (27.9, Osaka Prefecture) is approximately one-quarter of that in the USA (103.9, Los Angeles county). The age distribution of breast cancer in Japanese women is very different from that in Western countries,(1) that is, the drop in the age-specific incidence curve for breast cancer follows behind the average age of menopause, even though the age-dependent elevation of risk in premenopausal women is somewhat similar.
Much of the international variation is due to differences in established reproductive risk factors, but diet might also contribute and provide a potentially modifiable target for prevention. A case–control study the authors’ conducted detected a statistically inverse association between the risk of breast cancer and intake of soybean products or isoflavones,(2) and a cohort study based on public health center in Japan found frequent miso soup and isoflavone consumption to be associated with a reduced risk of breast cancer.(3) The authors also reported a significant decrease in postmenopausal breast cancer risk was observed for increasing intake for fish.(4) A Japan Collaborative Cohort (JACC) study has found that an intake of fish fat or long-chain n-3 fatty acids was associated with a decreased risk of breast cancer among a Japanese population.(5) However, any components of the Japanese diet that might underlie the relatively low breast cancer incidence observed in Japan have not been clarified in detail.
A number of previous epidemiologic studies have indicated that individual foods or nutrients, such as vegetables, fruits, fish, and soybean products, are possibly associated with the risk of breast cancer, although no particular dietary factor has shown strong or consistent results. Although most recent debate on the relationship between diet and breast cancer risk has been concentrated on specific nutrients and/or food, overall dietary patterns may be of greatest importance for primary prevention recommendations. People eat meals consisting of a variety of foods with complex combinations of nutrients, so that broad dietary patterns reflect many simultaneous dietary exposures. Factor analysis is a statistical tool that is increasingly being used to determine such patterns, although the results obtained so far have been inconsistent regarding risk of breast cancer.(6–9) This might be because all of the studies hitherto have been performed with Western populations at high risk. The same approach with Japanese women experiencing a low risk of breast cancer may provide additional insights.
In the present study, the authors therefore identified major dietary patterns among Japanese women and examined their association with breast cancer using data from the hospital-based epidemiologic research program at Aichi Cancer Center (HERPACC).
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- Materials and Methods
Table 1 shows factor loadings, which are equivalent to simple correlations between the food items and the dietary patterns. A positive loading indicates that the food item is positively associated with the dietary pattern, and a negative loading indicates an inverse association with the dietary pattern. Pattern 1 was labeled prudent because it reflected the correlated intakes of foods commonly thought to be healthy, such as vegetables and fruit, soybean curd, fish, milk and lower intake of fatty and salty foods. Pattern 2 was labeled fatty because it reflected the correlated intakes of meat, ham, sausage, and other fatty foods. Pattern 3 was labeled Japanese because it reflected the correlated intake of a typical Japanese diet, such as cooked rice for breakfast and miso soup and low intake of coffee. Pattern 4, which we called salty, is characterized by a high intake of salty foods, such as pickles, salty/dried fish.
Table 1. Factor-loading matrix for the four major dietary patterns identified using factor analysis
|Food items||Factor I||Factor II||Factor III||Factor IV|
|Green leafy vegetables||65||9||3||2|
|Soy bean curd (tofu)||46||6||21||8|
|Japanese green tea||14||0||10||11|
|Ham or sausage||3||52||–18||10|
|Preference for fatty food||–22||32||–4||19|
|Volume of meals||–5||14||7||11|
|Type of breakfast||4||–1||60||21|
|Miso (soy bean paste) soup||24||2||36||26|
|Salted vegetables (asazuke)||11||4||5||50|
|Preference for salty food||–24||11||–3||40|
|Variance explained by each factor||3.0118||1.9858||1.3988||1.2420|
Characteristics according to quartiles of dietary pattern scores in control group are presented in Table 2. Age was positively associated with the prudent and Japanese dietary patterns, and inversely with the salty pattern. Women with high prudent and Japanese pattern scores were less likely to be smokers and drinkers, with a lower prevalence of nulliparous women. Women with a high prudent pattern score were more physically active, but this did not extend to the Japanese pattern. Women who scored high on the fatty and salty pattern exercised less and were more likely to be smokers and drinkers than those who scored low on these patterns.
Table 2. Characteristics according to quartiles of dietary pattern scores in control group
| ||Dietary pattern||Quartile of dietary pattern score|
|1 (lowest)||2||3||4 (highest)|
|Age (mean ± SD, years)||Prudent||51.4 ± 8.8||52.0 ± 8.9||52.6 ± 8.9||54.5 ± 9.3|
|Fatty||56.3 ± 9.3||53.8 ± 9.0||51.3 ± 8.5||49.2 ± 7.8|
|Japanese||50.3 ± 8.1||51.7 ± 8.8||52.9 ± 9.1||55.7 ± 9.3|
|Salty||53.1 ± 9.4||52.1 ± 8.9||52.3 ± 8.9||53.1 ± 9.0|
|Body mass index (mean ± SD, kg/m2)||Prudent||22.6 ± 3.0||22.4 ± 2.9||22.3 ± 2.8||22.2 ± 2.8|
|Fatty||22.4 ± 3.0||22.3 ± 2.8||22.3 ± 2.8||22.5 ± 2.9|
|Japanese||22.0 ± 2.7||22.3 ± 2.8||22.5 ± 2.9||22.6 ± 3.1|
|Salty||21.9 ± 2.8||22.3 ± 2.8||22.4 ± 2.9||22.8 ± 3.0|
|Current smoker (%)||Prudent||18.2||10.5|| 7.0|| 4.5|
|Fatty|| 9.8|| 8.8|| 9.9||11.7|
|Japanese||15.3||11.4|| 8.7|| 4.8|
|Salty|| 8.5|| 9.7||10.3||11.7|
|Current drinker (%)||Prudent||33.1||27.8||24.3||18.6|
|Exercise ≥2 times/week (%)||Prudent||11.8||16.1||18.5||24.1|
|Age at menarche (mean ± SD, years)||Prudent||14.0 ± 1.7||13.9 ± 1.7||13.9 ± 1.7||14.0 ± 1.8|
|Fatty||14.4 ± 1.8||14.0 ± 1.7||13.8 ± 1.6||13.6 ± 1.6|
|Japanese||13.7 ± 1.6||13.9 ± 1.6||14.0 ± 1.7||14.3 ± 1.8|
|Salty||13.8 ± 1.7||13.8 ± 1.6||14.0 ± 1.7||14.2 ± 1.8|
|Nulliparous women (%)||Prudent||13.7|| 8.3|| 7.6|| 8.1|
|Fatty||14.7||10.2|| 7.8|| 5.0|
|Japanese||11.0|| 9.8|| 8.3|| 8.6|
|Salty||12.4||10.0|| 8.3|| 7.0|
|Postmenopausal women (%)||Prudent||47.4||50.4||52.6||60.1|
|Family history of breast cancer (%)†||Prudent|| 4.0|| 4.1|| 4.3|| 4.1|
|Fatty|| 4.1|| 4.2|| 4.0|| 4.2|
|Japanese|| 4.4|| 4.4|| 4.0|| 3.7|
|Salty|| 4.3|| 4.0|| 3.6|| 4.5|
|Proportion of selected food items (%)|
| Carrot (daily)||Prudent|| 1.1|| 6.2||18.1||59.1|
| Pork (≥3 times/week)||Prudent||10.5||14.7||17.7||22.4|
|Fatty|| 0.6|| 3.3||10.9||48.4|
| Salted vegetables (asazuke) (≥3 times/week)||Prudent||20.3||25.7||28.9||32.7|
| Miso (soy bean paste) soup (daily)||Prudent||45.8||58.2||66.0||75.1|
| Proportion of cooked rice intake for breakfast||Prudent||42.3||44.9||46.0||47.5|
After adjusting for potential confounders, there were no clear associations between the fatty, Japanese or salty dietary patterns and overall breast cancer risk (Table 3). In contrast, an inverse association was evident for the prudent dietary pattern in the multivariate-adjusted risk factor mode. The association was strongest among women 50 years of age or older. Older women in the highest quartile of the prudent dietary pattern had a 34% decreased risk of breast cancer as compared with those in the lowest (95% CI: 0.55–0.79, P for trend < 0.0001). Among younger women, aged 40–49 years, the corresponding estimate was a statistically significant 21% decreased risk (P for trend = 0.06). The other three dietary patterns were not associated with breast cancer risk among younger or older women, and results in the subgroup analyses were similar to the overall results for these patterns (Table 3).
Table 3. Odds ratios (ORs) and 95% confidence intervals (95%CIs) of breast cancer according to quartiles of factor scores for each dietary pattern
|Dietary pattern||Quartiles of factor score||P for trend|
|1 (low)||2||3||4 (high)|
| Cases/controls||565/5584||479/5584||450/5582||391/5583|| |
| Multivariate OR (95% CI)||1.00 (ref)||0.89 (0.78–1.01)||0.85 (0.74–0.97)||0.73 (0.63–0.84)||<0.0001|
| Cases/controls||477/5578||519/5591||443/5578||446/5586|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.12 (0.98–1.28)||0.97 (0.84–1.11)||0.99 (0.85–1.14)|| 0.414|
| Cases/controls||440/5582||507/5588||483/5580||455/5583|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.16 (1.01–1.33)||1.10 (0.96–1.27)||1.04 (0.90–1.20)|| 0.823|
| Cases/controls||472/5583||502/5584||466/5581||445/5585|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.08 (0.95–1.24)||1.04 (0.91–1.19)||1.04 (0.90–1.19)|| 0.761|
|Age 40–49 years|
| Cases/controls||222/2480||195/2472||204/2474||166/2478|| |
| Multivariate OR (95% CI)||1.00 (ref)||0.92 (0.75–1.13)||0.95 (0.77–1.17)||0.79 (0.63–0.98)|| 0.060|
| Cases/controls||211/2477||198/2474||175/2476||203/2477|| |
| Multivariate OR (95% CI)||1.00 (ref)||0.96 (0.78–1.18)||0.84 (0.68–1.04)||0.99 (0.80–1.22)|| 0.646|
| Cases/controls||180/2477||223/2477||202/2475||182/2475|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.31 (1.07–1.62)||1.20 (0.97–1.48)||1.10 (0.88–1.38)|| 0.594|
| Cases/controls||195/2475||219/2480||188/2476||185/2473|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.18 (0.96–1.45)||1.07 (0.86–1.32)||1.09 (0.88–1.36)|| 0.627|
|Ages 50–79 years|
| Cases/controls||369/3107||249/3109||254/3101||226/3112|| |
| Multivariate OR (95% CI)||1.00 (ref)||0.70 (0.59–0.84)||0.73 (0.62–0.87)||0.66 (0.55–0.79)||<0.0001|
| Cases/controls||278/3107||284/3116||270/3097||266/3109|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.05 (0.88–1.26)||0.99 (0.83–1.18)||0.99 (0.82–1.19)|| 0.726|
| Cases/controls||263/3109||299/3107||260/3111||276/3102|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.13 (0.94–1.34)||1.00 (0.83–1.20)||1.05 (0.88–1.27)|| 0.922|
| Cases/controls||270/3109||289/3107||287/3103||252/3110|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.06 (0.89–1.26)||1.08 (0.90–1.29)||1.00 (0.83–1.20)|| 0.971|
Results of multivariate OR considering two categories of BMI (<25 and ≥25), are shown in Table 4. For women with BMI <25, the highest quartile of the prudent factor score was associated with a 30% reduction in breast cancer risk compared to the lowest quartile, with a significant linear trend (P < 0.001), whereas the corresponding estimate for women with BMI ≥25 was a statistically non-significant 23% decreased risk (P for trend = 0.138). For women with BMI ≥25, those with highest factor scores for fatty and Japanese dietary patterns had a significantly higher OR for breast cancer. The OR in the top quartile of the fatty factor scores was 1.58 (95% CI: 1.14–2.19, P for trend = 0.027) with reference to the lowest quartile. The corresponding estimate for the Japanese dietary pattern was 1.45 (95% CI: 1.05–1.99, P for trend = 0.031). The risk with the salty dietary pattern did not differ between leaner and more obese women.
Table 4. Odds ratios (ORs) and 95% confidence intervals (95% CIs) of breast cancer according to quartiles of factor scores for each dietary pattern, stratified by BMI
|Dietary patterns||Quartiles of factor score||P for trend|
|1 (low)||2||3||4 (high)|
| Cases/controls||463/4669||373/4674||344/4667||314/4668|| |
| Multivariate OR (95% CI)||1.00 (ref)||0.83 (0.72–0.96)||0.78 (0.67–0.90)||0.70 (0.60–0.82)||<0.0001|
| Cases/controls||389/4672||407/4663||351/4675||347/4668|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.06 (0.91–1.23)||0.91 (0.78–1.06)||0.90 (0.76–1.05)|| 0.060|
| Cases/controls||348/4665||501/5531||307/3811||338/4671|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.26 (1.08–1.46)||1.13 (0.97–1.32)||1.02 (0.86–1.20)|| 0.826|
| Cases/controls||376/4671||399/4669||374/4665||345/4673|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.09 (0.94–1.26)||1.07 (0.92–1.25)||1.05 (0.89–1.22)|| 0.628|
| Cases/controls||100/878||101/877||97/877||80/877|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.02 (0.76–1.38)||1.00 (0.74–1.35)||0.77 (0.56–1.07)|| 0.138|
| Cases/controls||87/877||108/877||83/880||100/875|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.39 (1.02–1.89)||1.13 (0.82–1.57)||1.58 (1.14–2.19)|| 0.027|
| Cases/controls||75/877||96/876||96/878||111/878|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.26 (0.91–1.74)||1.28 (0.93–1.77)||1.45 (1.05–1.99)|| 0.031|
| Cases/controls||89/878||101/878||96/875||92/878|| |
| Multivariate OR (95% CI)||1.00 (ref)||1.22 (0.90–1.66)||1.21 (0.88–1.65)||1.19 (0.87–1.64)|| 0.308|
For the prudent pattern, the multivariate OR was 0.68 (95% CI: 0.58–0.79, P for trend < 0.001) comparing top to bottom quartiles among non-smokers. Among current smokers, the corresponding estimate was a statistically non-significant 19% decreased risk (95% CI: 0.53–1.23, P for trend = 0.313; data not shown). For the other three dietary patterns, there were no differences between results for subsets (non-smoker, smoker) and the groups overall. Results stratified by drinking status did not differ from the main analysis (data not shown).
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- Materials and Methods
Four major dietary patterns were discerned in the population of Japanese women and it was found that the prudent pattern was negatively associated with breast cancer risk, particularly among relatively lean women above 50 years of age. In addition, the fatty and Japanese dietary patterns may increase breast cancer risk among obese women.
Weaknesses of the present study were that the dietary questionnaire did not include portion size and has not been validated, so it is not possible to estimate the intake of nutrients. The food items in the questionnaire were selected out of major food groups. The authors have confirmed that these food items were a major contributor in each food group among Japanese in the Tokai area, from which subjects were recruited.(18) A strength is that the number of cases was relatively large. The Japan Public Health Center-based prospective study identified the healthy, traditional and Western dietary patterns; they reported that the traditional and the Western dietary patterns were positively associated with colon cancer risk in women.(19) Another study of dietary pattern among Japanese populations also observed the Japanese dietary pattern; however, there was no reported association between Japanese dietary pattern and colorectal adenomas.(20) The present study is the first to evaluate the association between dietary patterns and breast cancer risk using factor analysis in Japanese women.
A previously published study of an Italian population, focused on the relationship between diet and breast cancer using factor analysis,(21) and showed a ‘salad vegetable pattern’, characterized by high consumption of raw vegetables, had a significant protective effect against breast cancer. Regarding epidemiological results, a meta-analysis confirmed the protect effect of the intake of vegetables against breast cancer risk from 26 published studies;(22) meanwhile pooled prospective studies failed to detect any protective effect.(23) The authors’ previous studies using HERPACC data also generated clear evidence that the risk of breast cancer in Japanese women is markedly decreased by a high intake of fruit and green–yellow vegetables (green leafy vegetables, carrots, pumpkin).(4,12) This effect may be related, in part, to dietary fiber and antioxidant vitamins because of their biochemical and physiological effects. Green vegetables are rich in folic acid and folate deficiency can reduce the availability of S-adenosylmethionine for DNA methylation and thereby influence gene expression.
Fish consumption among Japanese is much higher than in Western countries. An international ecologic study revealed that Japanese consume 6.21% of total energy as fish, while the figure for US whites is only 0.74%.(24) Evidence of the anticarcinogenic effects of fish n-3 polyunsaturated fatty acids on breast cancer has been provided by animal studies,(25–27) but the results from epidemiological studies that have examined the association between fish consumption and breast cancer risk in humans are inconsistent. Several case–control studies,(28–30) including the authors’ previous study,(4) have suggested the protective effects of fish intake against breast cancer. In a Norwegian cohort frequent consumption of poached fish was associated with a decreased risk of breast cancer.(31) A cohort study showed an inverse association between intake of fish fat and long-chain n-3 fatty acids and breast cancer risk among Japanese women.(5) Other cohort studies, however, have found either no clear association,(32–34) or actually an increased risk.(35,36) Mechanisms might include inhibition of cyclooxygenase and p21 gene expression and up-regulation of p53 gene expression.(37,38) Furthermore, the beneficial effect of fish intake may better capture the synergistic interactions of food compounds in the prudent dietary pattern in the present study.
Japanese frequently consume various forms of soybean products, including tofu (soybean curd), so that the diet is likely to be much richer in isoflavones than in the Western world. Soybeans in fact constitute a unique dietary source of a group of phytochemicals and several natural anticarcinogens, such as protease inhibitors, phytates, phytosterols, saponins and lignans. Furthermore, soybeans are an excellent source of dietary fiber and micronutrients, particularly calcium. The results are in line with the inverse association between intake of soybean products and breast cancer risk suggested from ecologic/cross-sectional studies,(39) and also from analytical investigations. Thus case–control studies have found that soybean food intake was associated with a decreased risk of breast cancer among premenopausal Singaporean women,(40) and both pre- and postmenopausal Asian–American women,(41) although a Chinese case–control study failed to detect any protective effects of soybean food.(42) Cohort studies among Japanese,(3,43) Japanese–American,(44) and Caucasian–American women,(45) have provided some evidence that soybean products may reduce the risk of breast cancer. The authors have already reported a reduction in breast cancer risk with high soybean consumption among Japanese women.(2) The prudent dietary pattern identified in the present study implies an adequate intake of vitamins, dietary fiber, and a variety of biologically active compounds, all of which may contribute to the protective effect against breast cancer.
In the authors’ previous study a risk reduction with high intake of milk among premenopausal and postmenopausal Japanese women was observed.(4,12) Experiments have suggested favorable effects of lactic acid-forming bacteria on the intestinal microflora and the immune system.(46) Some components of milk, for example, calcium and vitamin D, might play roles as anticarcinogenic agents. Calcium has been proposed to reduce fat-induced cell proliferation by maintaining intracellular calcium concentrations.(47,48) Vitamin D modulates calcium metabolism and has calcium-independent antiproliferative actions.(48) The prudent dietary pattern is loaded by a high intake of vegetables, fruit, fish, soybean curd and milk. The combination of these compounds may reduce the breast cancer risk among Japanese women.
The fact that the protective effect of the prudent dietary pattern was more pronounced among women 50 years of age or older is in line with the greater intake of the component food items among older women.(49) For example, women 50 years of age or older consume 324.0 g per day of vegetables on average, while women 30–49 years of age consume 256.3 g. Average intakes of fruits are 161.6 g and 87.5 g, respectively.
From subgroup analysis the protective effect of the prudent dietary pattern was more prominent among women with BMI <25. Many studies have examined the relationship between anthropometric indices and breast cancer, and negative correlations among premenopausal women and positive correlations among postmenopausal women have been the predominant finding.(50–52) The authors also confirmed the association of early adult body size and subsequent weight gain with the development of breast cancer among Japanese women.(13) Women with a higher factor score of the prudent dietary pattern who tried to put health messages into practice by the adoption of appropriate behaviors were likely to be conscious of a feeling of health and to control their weight. Therefore, the finding that protective effects were strongest among lean women is in line with expectation. In addition, obese women may over-report their vegetable intake.(53) An Italian study,(21) and a Breast Cancer Detection Demonstration Project follow-up cohort study in the USA,(54) showed reduced risks associated with salad vegetable and traditional southern patterns were more pronounced among women with a BMI <25, but protective effects were no longer present in overweight women. A possible explanation of the lack of the protective effect among obese women could be that adipocyte production of estrogen in obese postmenopausal women may mask a diet–breast cancer association.(55,56)
We found no overall association between our fatty and Japanese pattern and the risk of breast cancer, although a positive link was evident among overweight women. When obese women (BMI ≥25) were stratified by menopausal status, only among postmenopausal women were the fatty and Japanese patterns associated with an increased breast cancer risk (data not shown). The present results are in agreement with previous evidence,(57) that obesity is an important risk factor among postmenopausal women. Because these dietary patterns are associated with high energy intake, these dietary patterns might accelerate the risk of breast cancer among obese women due to extra-ovarian estrogen production.
There are certain potential limitations in the present study that should be considered. One methodological issue is possible bias due to use of hospital-based non-cancer patients as referents. It is possible that there are discrepancies in patient characteristics compared with the general population, but outpatients in Japan, in general, visit hospitals directly when they have symptoms and/or some anxiety about their health. This situation is very different from that in the USA, where people visit local general clinics first, and are then referred to hospitals that function as secondary and/or specific facilities for further medical treatment. At the ACCH, incident cancer cases were found to comprise only 16% of all new female outpatients. Among randomly sampled non-cancer outpatients (n = 1000) in 1988–1989, only 34% were found to have specific diseases, the most common being benign tumors and/or non-neoplastic polyps (13.1%), mastitis (7.5%), digestive disease (4.1%), or benign gynecologic disease (4.1%).(58) The authors have compared lifestyle characteristics between outpatients in ACCH and 1231 persons from the general population randomly selected from the Nagoya electoral roll, and confirmed that they are not substantially different.(59) In women aged 50–59 years, the percentages of daily drinker of miso soup were 59.7% and 60.5% among community residents and first-visit outpatients of ACCH, respectively. The corresponding percentages of daily fruit intake were 46.0% and 52.8%, respectively. The percentages of everyday intake of green tea were not markedly different between community residents (87.3%) and first-visit outpatients (82.9%). It was concluded that, with due consideration of age, sex, and season in the analysis, it is feasible to use non-cancer outpatients as referents in epidemiological studies. The present study was free of response information bias to the questionnaire because all data were collected prior to diagnoses. Eligible referents were not matched, because the authors’ previous study showed that a large number based on outpatients gives a steadier estimate.(15)
Furthermore, there are some limitations due to applying factor analysis. Factor analysis involves decisions that can be called subjective or arbitrary, decision that can have some impact on both the results and their interpretation.(16,60) The selection and grouping of foods for analysis, the methods of rotation, and the manner in which the factors are ultimately labeled, depend heavily on the subjectivity of investigators. Statistical analyses have a key role in nutritional epidemiology to confirm associations between dietary factors and breast cancer risk. The associations between dietary factors and breast cancer have not been clarified in detail.(61) The reasons include methodological limitations,(62) therefore we need to continually develop useful and new techniques. Factor analysis, although not commonly applied hitherto, appears to be a promising approach for the investigation of breast cancer with reference to diet, and useful for the combination of information on many dietary variables. The present study, to the authors’ knowledge, is the largest case–control investigation carried out so far on the topic, and identified a dietary pattern that is negatively associated with breast cancer risk, namely the prudent dietary pattern.
In conclusion, the results indicate that a dietary pattern characterized by frequent consumption of vegetables, fruits, fish, soybean curd and low fat intake is associated with a reduced risk of breast cancer in Japanese women. The authors’ finding that fatty and Japanese type diets may elevate the risk of breast cancer among obese women suggested the possibility of prevention of breast cancer by low fat and calorie intake plus successful weight control.