Abstract

OBJECTIVES:

• To review all prospective studies which published information about total fish, or specific types of fish, in relation with total cancer risk, disease progression and mortality/survival.
• To define the amount of consumption found to be related with possible effects on the different cancer end points.
• To define possible effect modification by confounders.

DATA SOURCE: The Pubmed database was searched (No start date - Dec 13, 2010) for relevant articles using the keyword fish, omega 3, polyunsaturated fat, or seafood, combined with prospective, cohort, follow-up, or longitudinal and a fair amount of other keywords. The exact search term is described Here.
Prospective studies published in the English language were included. Reference lists were searched for additional articles.
RESULTS: 19 articles were found which provided information about 16 different cohorts. Of these, 0 articles were excluded.
Results are described when any evidence for an association - as defined in the Methods - was found. In addition, data about total fish, fatty fish, and lean fish is described.

• Cancer risk: Data about total fish consumption was provided by 14 cohorts, including 62,967 cases. Few effects were found. No evidence was found for an association between total fish consumption and total cancer risk (RR = 1.00).
  Data about fatty fish consumption was provided by one cohort. No association was found. And no data was found about lean fish.
• Advanced stage/metastatic disease risk or disease progression: No data was found.
• Cancer mortality: Data about total fish consumption was provided by 12 cohorts, including 24,543 cases. Few effects were found. No evidence was found for an association between total fish consumption and total cancer mortality (RR = 1.00).
  No data was found about fatty fish or lean fish.

CONCLUSION: For total fish consumption few effects were found at any level of consumption, and the average RR's showed no evidence for any effect at all. No evidence was found for an association between total fish consumption and total cancer risk or total cancer mortality. Little data was available about fatty fish, and no data was available about lean fish intake. Hardly any data was available about other specific types of fish, and inconclusive/no evidence was found for any association.

Methods.

Defining fish consumption: "Total fish" included data about fish consumption with or without seafood/shellfish, or without using a definition for fish. Preferably, the definition excluded seafood/shellfish.
Data about "omega-3 fatty acids" was included as a surrogate for fish consumption, if these fatty acids reflected (primarily) non-supplemental dietary intakes; were defined as "marine omega-3 fatty acids", "omega-3 fatty acids from fish (oil)", or "dietary fish oil"; and did not include omega-3 from other dietary sources.
Preferably, fish consumption was chosen over consumption of marine omega-3 fatty acids.
Data from the author "Hirayama T.": Dr. Hirayama examined the effects of a small amount of food groups in relation to a large amount of mortality end points in a Japanese cohort of very large size. An extended review of his work was published as a book in 1990 (Hirayama T [10]). Data about this cohort is seldom included in current systematic reviews about the relations mentioned. Dr. Hirayama published a lot of articles stating that vegetables and meats were related to several disease end points, adjusted for age and sex. However, the book included one page showing effects after multivariate analysis including cigarette smoking, meat, green-yellow vegetables, and alcohol. This analysis showed that a large amount of previously published effects completely changed when these variables were taken into account.
Since a) Dr. Hirayama himself only published sex-, and age-adjusted results in the English language, while results following multivariate analysis often were completely different b) the results were published as a book and not in a peer-reviewed journal, and c) Dr. Hirayama was the only researcher examining this cohort, results from his cohort are debatable. Results will be presented including effects from his work, but his work will not be included when the evidence for a possible effect is judged.

Total fish and total cancer.

Total cancer risk: Data about total fish consumption and total cancer risk was provided by 14 cohorts, including 62,967 cases. A significant protective effect was found among men in one cohort of very large size (Hirayama T [3]), and a significantly increased risk was found among men in one cohort of small size, but no adjustments were made for any possible confounders (Ikeda M [1]). The average RR could be calculated from 12 cohorts: RR = 0.94. Excluding the debatable results from Hirayama T [3] eliminated any effect: RR = 1.00.
Advanced stage/metastatic disease risk or disease progression: No data was found.
Total cancer mortality: Data about total fish consumption and total cancer mortality was provided by 12 cohorts, including 24,543 cases. Significant effects were identical to the ones for the analysis of cancer risk. The average RR could be calculated from 10 cohorts: RR = 0.83. Again, excluding the debatable results from Hirayama T [3] eliminated any effect: RR = 1.00.

Inclusion of intermediate levels of consumption.
(Non)significant effects at any level of consumption were as follows:

• Hirayama T [3]: Significant protective for men at any level of consumption, and for women at consumption 1-3 times/wk. Nonsignificant for women at other levels of consumption.
• Gillum RF [7]: Significant protective for white man at consumption once/week.

Effect modification: No effect modification was found by omega-6 fay intake (Virtanen JK [14]), BMI, or income (Tomasallo C [16]).
Subjects with prevalant disease: No data was found.

Conclusion: Very few effects were found at any level of consumption, and the average RR's showed zero effect. No evidence was found for an association between fish consumption and total cancer risk, or total cancer mortality.