Relation between stable isotope ratios in human red blood cells and hair: Implications for using the nitrogen isotope ratio of hair as a biomarker of eicosapentaenoic acid and docosahexaenoic acid

Background: The nitrogen isotope ratio (expressed as δ¹⁵N) of red blood cells (RBCs) is highly correlated with the RBC long-chain ω-3 (n-3) fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in Yup'ik Eskimos. Because δ¹⁵N can also be measured in hair samples, it could provide a noninvasive, retrospective biomarker for EPA and DHA intakes. Objectives: We investigated the agreement between δ¹⁵N in hair and RBCs and then evaluated the relations between hair δ¹⁵N and RBC EPA and DHA. We also assessed the agreement in carbon isotope ratios (δ¹³C) between hair and RBCs, because δ¹³C has been proposed as a dietary biomarker in other populations. Design: We assessed relations between hair and RBC δ¹⁵N and δ¹³C in a community-based sample of 144 Yup'ik Eskimos and examined the correlations between δ¹⁵N and RBC EPA and DHA in a subset of these participants (n = 44). Results: We showed a 1:1 relation with good agreement between hair and RBC δ¹⁵N (r = 0.91) and δ¹³C (r = 0.87). Hair isotope ratios were greater than RBC ratios by 1.5‰ for δ¹⁵N and by 2.3‰ for δ¹³C. There were strong correlations between hair δ¹⁵N and RBC EPA and DHA (r = 0.83 and 0.84, respectively). Conclusions: These results support the use of hair δ¹⁵N values as a biomarker of EPA and DHA intakes. Because hair collection is noninvasive and the samples require no special processing, studies of EPA and DHA intakes in large populations could use biomarkers rather than self-reports to assess these fatty acids.