(04-01-06) Fish Consumption Inversely Associated with Breast Cancer in Japanese Women

Evidence that total dietary fat and various types of fat contribute to the risk of developing breast cancer remains contradictory. Fat intake in most western countries is usually greater than 30% of total energy so that groups with low fat intake are difficult to identify in these countries. A recent meta-analysis of 45 cohort and case-control studies reported an 11% increase in breast cancer risk in the 14 cohort studies and a 14% increased risk in the 31 case-control studies between women with the highest and lowest total fat intakes. Increased risks associated with saturated fat and meat from all studies were 19% and 17%, respectively. However, not all studies have found a positive association with total fat intake.

On the positive side, several, but not all, studies have reported a lower risk of breast cancer associated with higher intakes of fish or omega-3 polyunsaturated fatty acids (n-3 PUFA). There has been at least one report of increased breast cancer incidence in postmenopausal women as fish consumption rose.

Asian populations have lower rates of most cancers compared with western societies and diet is frequently cited for its protective effect. Compared with western diets, Asian diets are usually substantially lower in total fat and higher in seafood. In this report from the Japan Collaborative Cohort Study for the Evaluation of Cancer Risk of over 110,000 participants, 26,291 women aged 40 to 79 at enrolment, who had a baseline food frequency questionnaire and no history of breast cancer, met the selection criteria. Incidence of cancer was determined from population-based cancer registries and a systematic review of death certificates over an average of follow- 7.6 years. During the study 2.7% of participants were lost to follow-up. Food and nutrient intakes were determined from food frequency questionnaires and adjusted for total energy intake and different types of dietary fat on the basis of validated 3-day weighed food records collected over one year.

Relative risk of developing breast cancer was calculated for all women and separately for those who were postmenopausal at baseline. The sample was divided into quartiles according to total and fish fat intake and the analysis adjusted for 12 potentially confounding variables. As a percent of total energy, total fat ranged from less than 18.4% to more than 24.6% and fish fat from less than 1.4% to more than 3.3%. Mean fish fat intake differed 4-fold between the highest and lowest intake groups.

After an average of 7.6 years there were 129 cases of breast cancer, but incidence was not associated with total fat, animal or vegetable fat intake in multivariate adjusted analysis. However, there was a 40% decrease in breast cancer relative risk in the highest quartile of fish fat intake (3.27% energy or more) compared with the lowest consumption (1.41% energy or less) (RR=0.56, 95% CI= 0.33-0.92, P=0.04). In terms of long-chain n-3 PUFAs, the difference in risk between the highest and lowest quartiles was 50% (RR=0.50, 95% CI=0.30-0.85, P=0.02). When the analysis included only women who were postmenopausal at baseline, there was a 40% to 50% reduction in breast cancer risk with fish fat or n-3 LC-PUFAs, respectively, but these relative risks did not reach statistical significance.

The study reported several other interesting associations. In the total cohort, breast cancer risk was significantly lower in women who ate green leafy vegetables almost every day (RR=0.76 compared with women who ate vegetables twice a week or less) and tended to be lower as saturated fat intake increased. However, the confidence interval for the relative risk of 0.76 was 0.50 to 1.15, which indicates that the relative risk was not statistically significant. In postmenopausal women, increasing vegetable fat consumption was associated with increased breast cancer risk (RR=2.08, P for trend=0.04), but this association was not seen in the total cohort. A positive association with n-6 PUFAs and breast cancer was reported previously in Swedish women, but the association was negative in postmenopausal women in Finland.

This study confirms previous reports that total fat intake is not associated with risk of breast cancer in a population consuming much less fat than most western countries. Further, it confirmed the inverse association between n-3 long-chain (LC) PUFAs from fish and incidence of breast cancer. However, this association was not statistically significant in postmenopausal women. In Japan, breast cancer predominates in premenopausal women. The question that cannot be answered from this study is whether n-3 LC-PUFAs are protective when total dietary fat consumption is low.
Wakai K, Tamakoshi K, Date C, Fukui M, Suzuki S, Lin Y, Niwa Y, Nishio K, Yatsuya H, Kondo T, Tokudome S, Yamamoto A, Toyoshima H, Tamakoshi A; JACC Study Group. Dietary intakes of fat and fatty acids and risk of breast cancer: a prospective study in Japan.

Cancer Sci. 2005;96:590-599.

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