Specific Fish Oils May Protect Against Breast Cancer

May Apply Only in Asia

Nick Mulcahy

June 27, 2013

The consumption of higher levels of dietary marine n-3 polyunsaturated fatty acids (PUFAs) is associated with a 14% lower risk for breast cancer than lower levels of consumption, according to a meta-analysis of 26 studies from Asia, Europe, and the United States.

Fish (especially salmon, tuna, and sardines) are the most abundant source of marine n-3 PUFAs, note the authors, led by Ju-Sheng Zheng, a PhD candidate in the Department of Food Science and Nutrition at Zhejiang University in Hangzhou, China. However, in this study, supplements were also considered dietary sources of these marine fatty acids.

Notably, no significant association was observed between overall fish intake and breast cancer.

In their study, published online June 27 in BMJ, the researchers cite various lab studies suggesting that n-3 PUFAs are the most promising subtypes of dietary fat "to inhibit or curtail carcinogenesis and reduce risk."

Previous literature on n-3 PUFAs and breast cancer is a mixed bag. Evidence from prospective studies suggests that "fish, the richest source of marine n-3 PUFA, shows inverse, null, or even positive associations with risk," Zheng and colleagues write.

So the team used , for the first time, all available prospective cohort studies to summarize the associations between the dietary intake of fish and n-3 PUFAs and incident breast cancer.

They found that marine n-3 PUFAs were associated with a 14% reduction in the risk for breast cancer (relative risk [RR] for highest vs lowest intake, 0.86). The relative risk remained similar whether marine n-3 PUFAs were measured as dietary intake (RR, 0.85) or as tissue biomarkers (RR, 0.86).

Subgroup analyses indicated that the regular intake of marine n-3 PUFAs is helpful. The risk for breast cancer was reduced by 5% with an increment of dietary marine n-3 PUFA intake of 0.1 g/day (RR, 0.95) or 0.1% energy/day (RR, 0.95). According to a press statement, this risk reduction can be achieved by eating 1 or 2 portions of oily fish per week.

Subgroup analyses also indicated that the inverse association between marine n-3 PUFAs and risk was more evident in studies that did not adjust for body mass index (RR, 0.74) than in studies that did (RR, 0.90).

Notably, no significant association was observed for exposure to alpha linolenic acid (ALA), which is contained in some nuts and in vegetable oils. About half of the studies in the meta-analysis contained data on ALA. Both ALA and n-3 PUFAs are types of omega-3 fatty acids.

The researchers offer a number of explanations for the "biologically plausible" anticarcinogenic effects of marine n-3 PUFAs, including the possibilty that marine n-3 PUFAs decrease the production of estrogen, thus reducing estrogen-stimulated cell growth.

Possibly Limited to Asians

The meta-analysis involved 26 publications, 20,905 cases of breast cancer, and 883,585 participants from 21 independent prospective cohort studies.

The results, however, could have a geographic limitation. Subgroup analyses indicated that the protective effect of marine n-3 PUFAs was more evident in Asian countries (RR, 0.69) than in Western countries (RR, 0.89).

Overall fish intake, which was not found to be associated with risk reduction, tended to be associated with a lower risk for breast cancer in Asian populations, the researchers report. "This could be because typical fish intake is much higher in Asian populations than in Western populations. Therefore fish intake in these Western populations might be too low to detect an expected protective effect," they write.

In North America and in some European countries, a "large proportion" of the intake of marine n-3 PUFAs "probably" comes from fish oil supplementation in the form of capsules, contributing to the different effects on risk, the researchers point out.

This, along with the fact that there are likely more pesticides in fish in the West, could explain the overall null association between fish intake (of all varieties) and risk for breast cancer, they note.

This study was funded by the National Natural Science Foundation of China, the PhD Programs Foundation of Ministry of Education of China, and the National Basic Research Program of China.

BMJ. Published online June 27, 2013. Abstract

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