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
Dear Dr. Schätzler,
Thanks for your questions and suggestions. We calculated the summary sample size carefully in our article (1). There was some overlapping studies within the different meta-analysis as you have indicated. For example, four studies (2-4) reported both fish and marine n-3 PUFA intake and breast cancer risk. All the articles reporting ALA and breast cancer risk had data for marine n-3 PUFA and the risk of breast cancer. Therefore, after removing those duplicated, we have included twenty six publications, including 20 905 cases of breast cancer and 883 585 participants from 21 independent prospective cohort studies, as described in the abstract (1).
In addition to prospective cohort studies, we have included eight nested case-control studies and two case-cohort studies. Therefore, the incidence of breast cancer cannot be calculated in the way Dr Schätzler has described. There are no comparability issues for these groups, as the study design and inclusion criteria are the same for these groups.
1. 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. Zheng JS, Hu XJ, Zhao YM, Yang J, Li D. BMJ 2013;346:f3706.
2. Wakai K, Tamakoshi K, Date C, Fukui M, Suzuki S, Lin Y, et al. Dietary intakes of fat and fatty acids and risk of breast cancer: a prospective study in Japan. Cancer Sci2005;96:590-9.
3. Gago-Dominguez M, Yuan JM, Sun CL, Lee HP, Yu MC. Opposing effects of dietary n-3 and n-6 fatty acids on mammary carcinogenesis: the Singapore Chinese Health Study. Br J Cancer2003;89:1686-92.
4. Cho E, Spiegelman D, Hunter DJ, Chen WY, Stampfer MJ, Colditz GA, et al. Premenopausal fat intake and risk of breast cancer. J Natl Cancer Inst2003;95:1079-85.
5. Folsom AR, Demissie Z. Fish intake, marine omega-3 fatty acids, and mortality in a cohort of postmenopausal women. Am J Epidemiol2004;160:1005-10.
Competing interests: No competing interests