Figure 1 summarizes the literature search and selection process. We identified 17,339 citations in the primary search, of which 162 were retrieved for full text evaluation after the initial screening of abstracts and titles. 12 studies were identified through manual examination of reference lists. Overall, a total of 66 analyses reported in 64 articles were included in our main analysis (Additional file 1: Appendix S4-9).
The included studies comprised of 25 prospective cohort studies, nine case-cohort studies, one nested case-cohort study, and 30 nested case-control studies. Twenty-five studies were conducted in Europe, 28 in the United States, seven in Asia, and five in Australia. Mean age of participants in the individual studies ranged from 41.1 to 80.5 years. For the measurements of omega-3 PUFA levels, 51 studies used gas chromatography (GC) analytic approach, 13 used gas-liquid chromatography, and one used GC-tandem mass spectrometry.
The Additional file 2: Appendix S1 shows the mean (standard deviation [SD]) proportion of each objective omega-3 PUFA relative to the total fatty acid contents in blood compartments or tissues. Fifty-eight studies were deemed to be of high quality, and the others judged as having a moderate quality (Additional file 1: Appendix S10-S11).
Omega-3 PUFA biomarkers and type 2 diabetes
Eleven studies comprised of 15,595 T2D incidence and 53,642 participants investigated the relationship between the concentrations of omega-3 PUFA biomarkers and T2D risk. Compared with participants in the lowest tertile, those in the highest tertile of ALA levels had a lower risk of T2D (RR,0.91;95%CI,0.83–0.99;Pheterogeneity=0.26; Fig. 2 and Additional file 2: Appendix S2). No significant associations were found for levels of EPA (RR,0.87;95% CI: 0.75-1.00;Pheterogeneity<0.001; Fig. 2), DPA (RR,0.87;95%CI,0.76-1.00;Pheterogeneity=0.001; Fig. 2), DHA (RR,0.92;95%CI,0.78–1.10;Pheterogeneity=0.001; Fig. 2) or the sum of EPA + DPA + DHA (RR,0.81;95%CI,0.60–1.09;Pheterogeneity=0.15; Fig. 2) with T2D risk (Additional file 2: Appendix S3-6 and Additional file 1: Appendix S12). The result of subgroup analyses did not substantially alter the association between level of omega-3 PUFA biomarkers and T2D risk. In dose-response analyses, a linear association was found between ALA biomarker and T2D risk (Additional file 2: Appendix S7). The RR of T2D for each 1-SD increase in concentrations of ALA was 0.91 (95%CI,0.83–0.99).
Omega-3 PUFA biomarkers and CVD
Association of omega-3 PUFA levels with total CVD was assessed in 12 studies, which included a total of 5,503 cases among 35,581 participants. When comparing the extreme tertiles, the risk of total CVD was significantly lower by 19% for EPA (RR,0.81;95%CI,0.72–0.91;Pheterogeneity=0.30; Fig. 2), 23% for DPA (RR,0.77;95%CI,0.69, 0.85;P heterogeneity=0.94; Fig. 2), 25% for DHA (RR,0.75;95%CI,0.65–0.87;Pheterogeneity=0.01; Fig. 2), and by 55% for the sum of EPA + DPA + DHA (RR,0.45;95%CI,0.27–0.74;Pheterogeneity=0.15; Fig. 2) (Additional file 3: Appendix S1-S4 and Additional file 1: Appendix S13). No association was observed between ALA and risk of CVD when comparing the highest with lowest categories (RR,1.09;95%CI,0.99–1.21;Pheterogeneity=0.65; Fig. 2 and Additional file 3: Appendix S5). Results from the dose-response analyses showed a significant linear decrease in the risk of CVD of individuals with increasing values of circulating EPA and DPA concentration, and per 1-SD increment was associated with 9% (RR,0.91;95%CI,0.86–0.96) and 11% (RR,0.89;95%CI,0.85–0.93) lower risk of CVD, respectively (Fig. 3). A potential nonlinear dose-response curve was detected for DHA-CVD association in that the CVD risk did not decrease until the DHA levels exceeded about 2% (Pnon−linearity=0.02; Fig. 3).
Omega-3 PUFA biomarkers and CHD
The association between omega-3 PUFA biomarker levels and CHD was evaluated in 13 studies, which consisted of 7,626 cases and 27,814 participants. The overall effect estimates of CHD comparing the top tertile compared with bottom tertile was 0.98 for ALA (95%CI,0.95–1.02;Pheterogeneity=0.89; Fig. 2), 0.85 for EPA (95%CI,0.77–0.95;Pheterogeneity=0.41; Fig. 2), 0.83 for DPA (95%CI,0.76–0.92;Pheterogeneity=0.84; Fig. 2), 0.70 for DHA (95%CI,0.58–0.84;Pheterogeneity=0.02) ; Fig. 2, and 0.67 for the sum of EPA + DPA + DHA (95%CI,0.47–0.96;Pheterogeneity=0.34; Fig. 2) (Additional file 4: Appendix S1-S5 and Additional file 1: Appendix S13). For the dose-response analyses, a linear association was observed for marine-derived omega-3 PUFA biomarkers and risk of CHD. For every 1-SD increase in levels of EPA, DPA, and DHA in circulating, the RR: of CHD decreased by 8% (RR,0.92;95% CI,0.88–0.97), 17% (RR,0.83;95%CI,0.74–1.06), and 16% (RR:0.84;95%CI,0.77–0.93), respectively (Fig. 3).
Omega-3 PUFA biomarkers and stroke
Twelve studies provided information on omega-3 PUFA levels and the subsequent risk of stroke, including a total of 7,036 events in 77,163 participants. The pooled estimate indicated that high DHA status was associated with a lower risk of stroke (RR,0.84;95%CI,0.72–0.99;Pheterogeneity=0.03; Fig. 2), while there was no significant association for biomarkers of ALA, EPA, DPA, or the sum of EPA + DPA + DHA (; Fig. 2, Additional file 5: Appendix S1-S5 and Additional file 1: Appendix S14). A linear relation was noted between DHA biomarker and stroke in the dose-response analysis (Additional file 5: Appendix S6), and the RR: was 0.91 (95%CI,0.83–0.99) for each 1-SD increment of DHA concentration in circulating.
Omega-3 PUFA biomarkers and cancer
Twenty-one studies were included in the analysis of omega-3 PUFA biomarker status and colorectal, breast, or prostate cancers. For colorectal cancer (n = 3), in comparison with the lowest category, the highest level category of DPA and DHA were associated with 24% (RR,0.76;95%CI,0.59–0.98;Pheterogeneity=0.87; Fig. 2) and 20% (RR,0.80;95%CI,0.65–0.99;Pheterogeneity=0.56; Fig. 2) reduced risk of colorectal cancer, respectively (Additional file 6: Appendix S1-S2). ALA and EPA biomarker had a non-significant association with incident colorectal cancer (Additional file 6: Appendix S3-S4). No association was observed between ALA, EPA, DPA, and DHA concentrations and incidence of breast cancer (n = 9) or prostate cancer (n = 9) (Fig. 2 and Additional file 6: Appendix S5-S12 and Additional file 1: Appendix S15).
Omega-3 PUFA biomarkers and total mortality
Eight studies investigated the relation of omega-3 PUFA biomarker levels with mortality with a total of 7,558 deaths from 23,484 participants. Pooled RR for the comparison of extreme tertiles was 0.80 for EPA (95%CI,0.71–0.90;Pheterogeneity=0.02; Fig. 2), 0.81 for DPA (95%CI,0.73–0.90;Pheterogeneity=0.59; Fig. 2), and 0.84 for DHA (95%CI,0.76–0.94;Pheterogeneity=0.07; Fig. 2) (Additional file 7: Appendix S1-S3 and Additional file 1: Appendix S16). Nonsignificant association was observed for ALA biomarker (RR,1.00;95%CI,0.93–1.08;Pheterogeneity=0.64; ; Fig. 2 and Additional file 7: Appendix S4).
Sensitivity analysis and publication bias
In sensitivity analyses omitting one study at a time from each analysis, the combined estimate did not substantially change for most omega-3 PUFA biomarkers, except for the studies that evaluated the association between DHA level and prostate cancer: the pooled RR (95% CI) was strengthened to 1.14 (95%CI,1.00-1.30) when the study by Chavarro et al. was removed . No indication of substantial publication bias was found for most outcomes with either Egger’s test or Begg’s test (P > 0.05 for both tests; Additional file 7: Appendix S5-S7).