Is the omega-3 index a valid marker of intestinal membrane phospholipid EPA+DHA content?

Despite numerous studies investigating n-3 long chain polyunsaturated fatty acid (LCPUFA) supplementation and inflammatory bowel diseases (IBD), the extent to which dietary n-3 LCPUFAs incorporate in gastrointestinal (GI) tissues and correlate with red blood cell (RBC) n-3 LCPUFA content is unknown. In this study, mice were fed three diets with increasing percent of energy (%en) derived from eicosapentaenoic acid (EPA)+docosahexaenoic acid (DHA). Dietary levels reflected recommended intakes of fish/fish oil by the American Heart Association. We analyzed the FA composition of phospholipids extracted from RBCs, plasma, and GI tissues. We observed that the 0.1%en EPA+DHA diet was sufficient to significantly increase the omega-3 index (RBC EPA+DHA) after 5 week feeding. The baseline EPA levels were 0.2-0.6% across all tissues increasing to 1.6-4.3% in the highest EPA+DHA diet; these changes resulted in absolute increases of 1.4-3.9% EPA across tissues. The baseline DHA levels were 2.2-5.9% across all tissues increasing to 5.8-10.5% in the highest EPA+DHA diet; these changes resulted in absolute increases of 3.2-5.7% DHA across tissues. These increases in EPA and DHA across all tissues resulted in strong (r>0.91) and significant (P<0.001) linear correlations between the omega-3 index and plasma/GI tissue EPA+DHA content, suggesting that the omega-3 index reflects the relative amounts of EPA+DHA in GI tissues. These data demonstrate that the GI tissues are highly responsive to dietary LCPUFA supplementation and that the omega-3 index can serve as a valid biomarker for assessing dietary EPA+DHA incorporation into GI tissues.