Compound-specific isotope analysis resolves the dietary origin of docosahexaenoic acid in the mouse brain

DHA (22:6n-3) may be derived from two dietary sources, preformed dietary DHA or through synthesis from alpha-linolenic acid (ALA; 18:3n-3). However, conventional methods cannot distinguish between DHA derived from either source without the use of costly labeled tracers. In the present study, we demonstrate the proof-of-concept that compound-specific isotope analysis (CSIA) by GC-isotope ratio mass spectrometry (IRMS) can differentiate between sources of brain DHA based on differences in natural C-13 enrichment. Mice were fed diets containing either purified ALA or DHA as the sole n-3 PUFA. Extracted lipids were analyzed by CSIA for natural abundance C-13 enrichment. Brain DHA from DHA-fed mice was significantly more enriched (-23.32 parts per thousand to -21.92 parts per thousand) compared with mice on the ALA diet (-28.25 parts per thousand to -27.49 parts per thousand). The measured C-13 enrichment of brain DHA closely resembled the dietary n-3 PUFA source, -21.86 parts per thousand and -28.22 parts per thousand for DHA and ALA, respectively. The dietary effect on DHA C-13 enrichment was similar in liver and blood fractions. Our results demonstrate the effectiveness of CSIA, at natural C-13 enrichment, to differentiate between the incorporation of preformed or synthesized DHA into the brain and other tissues without the need for tracers.