Endogenous conversion of both 18:3ω3 and 18:2ω6 varies as a function of diet and age (Sauerwald et al. Pediatric Res, 1995) but there also is substantial variation within diet and age groups. We hypothesized that this latter variation is due to individual differences in conversion of the two fatty acids. To test this hypothesis, the fractional rates of conversion (FRC) of 18:3ω3 and 18:2ω6 and the fractional rates of incorporation(FRI) of 22:6ω3 and 20:4ω6 into plasma phospholipid (PL) were determined at 43 and 56 wk postmenstrual age in 11 preterm and 27 term infants fed formulas with 0.4%, 1% or 3.2% of fat as 18:3ω3 and correlations among FRC of 18:3ω3 and 18:2ω6 as well as FRI of 22:6ω3 and 20:4ω6 were determined by regression analysis. FRCs and FRIs were determined by a precursor/product compartmental model based on changes in enrichments of [M+18] fatty acids in plasma PL following administration of[U-13C]-18:3ω3 and -18:2ω6. The correlation coefficients(r) among FRCs and FRIs in 10 term infants on the same diet at 43 and 56 wk are shown: Table In all infants studied at both times, 55% of the variation in FRC of 18:2ω6 was explained by FRC of 18:3ω3 but only 12% of the variation in FRI of 20:4ω6 was explained by FRI of 22:6ω3. Although inherently obvious (ie, 18:3ω3 and 18:2ω6 are converted by the same desaturases and elongasés), this is the first demonstration of such a correlation. It indicates that not only diet and age but individual differences in activity of desaturases and elongases may be important determinants of ω3 andω6 fatty acid status during infancy.

Table 1
Full size table