Use of stable isotopes to study fatty acid and lipoprotein metabolism in man

Tracer studies have long been an important tool for lipid metabolism research. Recent advances and availability of high performance mass spectrometers (MS) and improved stable isotopically labeled tracers contribute to an increase in stable isotope tracer studies in humans. We briefly review recent studies and discuss advances in high sensitivity methods and applications. GC/MS analysis. Tracer studies with gas chromatography/mass spectrometry (GC/MS) usually rely on D labeling, where labeling with more that three D atoms shifts the analyte mass above that of the natural abundance envelope, and the MS monitors selected masses representing the isotopimers of interest. Recent examples are the work of Emken and coworkers, who investigated the desaturation of 18:0 and 16:0, and 18:2n-6 and 18:3n-3 elongation/desaturation, in adults, with oral doses of about 3 g of d2,4,6 fatty acids. They showed modest levels of 18:0 and 16:0 desaturation over 2 days and an influence of dietary 18:2n-6 on elongation. In premature infants, Salem, Uauy and coworkers recently have used d5-18:2n-6 and d5-18:3n-3 doses of 50-100 mg/kg body weight to show that infants as small as 1980 g and 32 weeks gestation elongate and desaturate both precursors within 24 h. Most fatty acid metabolites including 20:4n-6, 20:5n-3, and 22:6n-3 were easily detected in serum. High precision isotope ratio MS (IRMS). In 1992, we introduced a high sensitivity fatty acid tracer method based on [U-13C] tracers and GC-combustion-IRMS (GCC-IRMS). The combustion interface facilitates carbon-by-carbon tracer detection; with U-13C tracers all GC peaks are detected with highest precision. Rhee et al have quantified the desaturation of 18:0 and 16:0 in lipoproteins of adults using 30 mg oral doses (0.5 mg/kg). In the first 12 h, conversion of 18:0 to 18:1 was 7% in chylomicrons and 17% in VLDL, showing that both intestine and liver desaturate 18:0. Plasma conversion of 18:0 over 144 h was 14%, while that for 16:0 was 2%, showing that combined intestine and liver desaturation is minor compared with normal fluctuations in dietary levels. Carnielli applied GCC-IRMS to 8:0 elongation in very-low-birth-weight infants. Significant conversion products of 8:0 were 14:0 (5%), 16:0 (8%) but not 10:0 or 12:0.