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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 doi: https://doi.org/10.1136/bmj.f3706 (Published 27 June 2013) Cite this as: BMJ 2013;346:f3706

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

Meta-analytic delusions

The three different interventional subgroups described in this meta-analysis performed by Zheng J-S et al.: "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" show extremely different incidences of breast cancer.

• "Fish consumption and risk of breast cancer - Eleven studies from 11 independent cohorts reported an association between fish intake and risk of breast cancer, with 13 323 breast cancer events and 687 770 participants" means an incidence of 1.94 per cent.
• "Marine n-3 PUFA and risk of breast cancer - Seventeen articles from 16 independent cohort studies reported the association between marine n-3 PUFA and risk of breast cancer, involving 16 178 breast cancer events and 527 392 participants" means an incidence of 3.07 per cent.
• "ALA (alpha-linolenic acid), total n-3 PUFA, and risk of breast cancer - Twelve articles, involving 14 284 breast cancer events and 405 592 participants" means an incidence of 3.52 per cent.

Incidence of breast cancer in the marine n-3 PUFA group is 58.36 per cent higher than in the fish consumption group. In the ALA-group the incidence is 81.80 per cent higher than in the fish consumption group. Therefore the three different groups are not comparable.

The authors do not describe origin and dosage of the administered marine n-3 PUFA. They found no relevant "association with risk of breast cancer per 15 g/day increment of fish intake" which means only one ration of 450 g fish per month.

In their abstract the authors are declaring "Twenty six publications, including 20 905 cases of breast cancer and 883 585 participants from 21 independent prospective cohort studies were eligible. Eleven articles (13 323 breast cancer events and 687 770 participants) investigated fish intake, 17 articles investigated marine n-3 PUFA (16 178 breast cancer events and 527 392 participants), and 12 articles investigated alpha linolenic acid (14 284 breast cancer events and 405 592 participants)".

How is it possible to blow-up a meta-analysis from 21 eligible publications with 20 905 cases of breast cancer and 883 585 participants up to 40 articles with 43 785 breast cancer events with a study population of one-million-six-hundred-and-twenty-thousand-seven-hundred-and-fifty-four participants?

Sincerely Yours
Dr. med. Thomas G. Schätzler
Family Medicine Unit
24 Kleppingstr.
D 44135 Dortmund/Germany
th.g.schaetzler@gmx.de

Competing interests: No competing interests

20 July 2013
Thomas Georg Schaetzler
Family Medicine
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Medical Office
24 Kleppingstreet D Dortmund