Plasma and red blood cell fatty acid values as indexes of essential fatty acids in the developing organs of infants fed with milk or formulas.

The dietary requirement of n-6 and n-3 fatty acids for normal biochemical and functional development of the central nervous system (CNS) is an important, unresolved issue in infant nutrition. High levels of arachidonic acid (AA; 20:4n-6) and docosahexaenoic acid (DHA; 22:6n-3) are found in the CNS and are important to normal learning and visual function. Dietary fatty acids may be desaturated and elongated to AA and DHA, respectively, but may also be oxidized for energy. Synthesis of AA and DHA in the young infant, therefore, depends on adequate desaturase enzyme activity, as well as an adequate supply of dietary 18:2n-6, 18:3n-3, and energy. Levels of AA and DHA are lower in the plasma and red blood cell (RBC) lipids of infants fed formula rather than human milk and are not increased with increased formula 18:2n-6 or 18:3n-3 supply. The decline in AA and DHA in infants fed formula becomes evident in the order plasma phospholipid greater than RBC phosphatidylcholine greater than RBC phosphatidylethanolamine. As in infants, piglets fed formula rather than natural milk have lower plasma and RBC AA and DHA concentrations. Despite lower levels in the plasma and RBC, analyses of CNS lipids demonstrated adequate AA and DHA in piglets fed formula with greater than 7% kcal 18:2n-6 and greater than 0.3% kcal 18:3n-3. This finding suggests that circulating lipid fatty acids are not specific indexes of organ deficiency. The rapid decrease in circulating lipid AA and DHA concentrations experienced by premature infants during early postnatal parenteral and enteral nutrition, however, may be related to oxidation of 18:2n-6 and 18:3n-3, rather than equilibrium of circulating lipids with the dietary fatty acids. Arachidonic acid and DHA may be conditionally essential nutrients for these infants because of oxidation of 18:2n-6 and 18:3n-3 for energy during periods of negative energy balance.