Re: Intake of fish and marine n-3 polyunsaturated fatty acids and risk of breast cancer: meta-analysis of data from 21 independent prospective cohort studies
I read the recent paper by Zheng et al. with interest. As the authors state, fish is an important source of n-3 polyunsaturated fatty acids (PUFA). However, not all fish species are good sources of n-3 PUFA: while 100 g of raw wild Atlantic salmon gives 2.5 g PUFA, 100 g of raw Atlantic cod gives only 0.2 g of PUFA (http://ndb.nal.usda.gov/). Failing to acknowledge these differences and comparing total fish intake with biomarkers of PUFA, therefore, is like comparing apples and oranges.
The authors adjusted their results for age, BMI, total energy intake, and education, but were unable to adjust for important breast cancer risk factors such as reproductive history, use of exogenous hormones, and alcohol consumption. Still they claim their results are solid and robust evidence for an inverse association between n-3 PUFA and risk of breast cancer.
Most analyses of cancer risk include only first incident cancer cases. The authors chose to include one study of breast cancer recurrence and new breast cancers in women already diagnosed with breast cancer, in addition to the studies only considering first incident cases. A more serious mistake was the pseudoreplication resulting from including findings by Stripp  from the Danish part of the European Prospective Investigation into Cancer and nutrition (EPIC) both as an independent paper, and as part of the paper by Engeset covering the entire EPIC-study.
The authors claim that their findings have important public health implications, without further specification. Given the methodological limitations of this research, I am not convinced. However, the dietary supplement industry seems to be (e.g. http://newhope360.com/supplements/omega-3-fatty-acids-breast-cancer?cid=...).
1. Zheng JS, Hu XJ, Zhao YM, Yang J, Li D. Intake of fish and marine n-3 polyunsaturated fatty acids and risk of breast cancer: meta-analysis of data from 21 independent prospective cohort studies. Bmj 2013;346:f3706 doi: 10.1136/bmj.f3706.
2. Stripp C, Overvad K, Christensen J, et al. Fish intake is positively associated with breast cancer incidence rate. J Nutr 2003;133(11):3664-9
3. Engeset D, Alsaker E, Lund E, et al. Fish consumption and breast cancer risk. The European Prospective Investigation into Cancer and Nutrition (EPIC). Int J Cancer 2006;119(1):175-82 doi: 10.1002/ijc.21819.
Competing interests: No competing interests