Comparison of bolus versus fractionated oral applications of [C-13]-linoleic acid in humans

Background The endogenous conversion of linoleic acid into long-chain polyu nsaturated fatty acids is of potential importance for meeting substrate req uirements, particularly in young infants. After application of [C-13]-linol eic acid, we estimated its conversion to dihomo-gamma-linolenic and arachid onic acids from only two blood samples. Design Oral tracer doses were given to five healthy adults as a single bolu s. In four subjects the tracer was given in nine equal portions over 3 days . Concentration and C-13 content of fatty acids from serum phospholipids we re analysed by gas chromatography combustion isotope ratio-mass spectrometr y. Areas under the tracer-concentration curves were calculated, and fractio nal transfer and turnover rates estimated from compartmental models. Results The median fractional turnover of linoleic acid was 93.7% per day ( interquartile range 25.3) in the bolus group and 80.0% per day (6.3) in the fraction group (NS). Fractional conversion of linoleic to dihomo-gamma-lin olenic acid was 1.5% (0.9) vs. 2.1% (0.7) (bolus vs. fraction, P < 0.05), a nd fractional conversion of linoleic to arachidonic acid was 0.3% (0.3) vs. 0.6% (0.3) (bolus vs, fraction, NS). In the fraction group conversion was significantly higher based on areas under the curve. The ratio of tracer co ncentration in conversion products to linoleic acid 48 h after dosing corre lated very well (r greater than or equal to 0.94, P < 0.05) with the ratio of areas under the curve. Conclusions Using areas under the curve overestimates the conversion, becau se different residence times are not considered. Estimation of conversion i ntensity appears possible with only one blood sample obtained after tracer application.