Estimation of arachidonic acid synthesis in full term neonates using natural variation of 13C content.

Neonates need arachidonic acid (AA) for their growing tissues, but it is unknown to what extent they can synthesize AA from dietary linoleic acid (LA). We studied infantile AA synthesis by using different natural 13C amounts in dietary fats. The diets of four infants ages 18 +/- 4 days (mean +/- SD) with newly diagnosed phenylketonuria were changed from breast milk or formula to a phenylalanine-free diet with corn oil as the only fat source. Compared with most dietary fats in Europe derived from C3 plants, corn oil LA has a higher 13C content. We determined the 13C content of LA and AA in 0.25-0.5 ml serum before and for 3-4 days after the diet change with gas chromatography combustion isotope ratio mass spectrometry (GC-IRMS). Baseline delta 13C values were -31.5 +/- 1.1% (mean +/- SD) for serum LA and -30.1 +/- 1.2% for AA. The corn oil diet induced changes of delta 13C values over baseline in LA of 8.9 +/- 1.0 on day 1 and 12.7 +/- 0.7 on day 4, respectively. The changes of AA delta 13C values were 0.5 +/- 0.7 and 2.7 +/- 0.7 on days 1 and 4. We conclude that reproducible detection of differences in delta 13C values of fatty acids in small samples of infantile serum, induced by dietary variation of natural 13C amounts, is feasible with GC-IRMS. Since the corn oil diet did not contain AA, the observed change of AA delta 13C shows active endogenous AA synthesis in full-term neonates.(ABSTRACT TRUNCATED AT 250 WORDS)