Experimental models showed consistently a modulation of carcinogenesis by omega 3 polyunsaturated fatty acids (ω3 PUFA). Fish intake is often described as part of a beneficial dietary pattern. However, observational epidemiological studies on the relationship between ω3 PUFA reported conflicting results. The objective of this systematic review is to determine whether there exists any progress in the evaluation of the causal relationship between dietary ω3 PUFA and cancers since the previous FAO/OMS expert consultation and whether it is possible to propose preventive and/or adjuvant therapeutic recommendations. Prospective and case-control observational studies published since 2007 and meeting validity criteria were considered together with RCT. Experimental studies are mentioned to provide for biological plausibility. When evaluating the level of evidence, a portfolio approach was used, weighted by a hierarchy giving higher importance to prospective studies followed by RCT if any. There is a probable level of evidence that ALA per se is neither a risk factor nor a beneficial factor with regards to cancers. Observational studies on colorectal, prostate and breast cancers only provided limited evidence suggesting a possible role of LC-ω3PUFA in cancer prevention because insufficient homogeneity of the observations. Explanation for heterogeneity might be the inherent difficulties associated with epidemiology (confounding and dietary pattern context, measurement error, level of intake, genetic polymorphism). The role of LC-ω3PUFA as adjuvant, might be considered of possible use, in view of the latest RCT on lung cancers even if RCT on other cancers still need to be undertaken.

In order to assess nutritional adequacy, valid estimates of nutrient intake are required. Specifically, the EURopean micronutrient RECommendations Aligned (EURRECA) Network of Excellence needs clear guidelines for assessing the validity of reported micronutrient intakes and n-3 fatty acid (FA) intakes. The aim of the present study was to review the validity of methods used to measure the usual n-3 FA intake of a population. A systematic literature search was conducted for studies validating the methodology used for measuring the dietary intake of n-3 FA. The quality of the validation studies and the quality of the different dietary assessment methods were assessed using scoring systems developed by EURRECA. Fourteen papers, describing twenty studies, were identified for inclusion. According to the score system developed by EURRECA, all the studies were ranked as average, except two that were ranked as poor. The correlation coefficients between FA in subcutaneous fat and dietary intake of n-3 FA from four FFQ, one weighed record and one 24-h recall ranged between 0·40 and 0·60. Correlations between intake of n-3 FA from five FFQ, one dietary history and three weighed records and blood lipids were similar to the ones observed for subcutaneous fat. The summarised quality of the n-3 FA estimates derived from the FFQ was judged as good or acceptable according to the EURRECA scoring system. The literature describes subcutaneous fat as the best reference method, and the studies where this was used had moderate correlation coefficients and no dietary intake method was superior to any other.