One of the key functions of placental nutrient transport is to maintain the most appropriate balance of nutrients in the fetal circulation and the balance of nutrients transferred by the placenta may be as important as the overall transfer capacity in influencing the pattern of fetal growth. Nutrients such as the fatty acids and amino acids occur in many forms yet they are translocated across membranes by a relatively small number of transporter molecules. This nutritional 'bottleneck' results in competition for transfer and the possibility of pla-cental selectivity. An example of the resulting change in nutrient quality can be seen in the increase in the relative proportion of the essential to nonessential amino acids in the fetal circulation compared to the maternal circulation (Figure 4). The same is true of the fat-soluble vitamins where the relative concentration of the most biologically active form is increased in the fetal circulation. In the case of the fatty acids it is the long-chain poly-unsaturated fatty acids (LCPUFA) such as arachido-nic acid (20:4 n-6; AA) and docosahexaenoic acid (22:6 n-3; DHA) that perform most of the essential functions in the fetus. Although the overall concentration of the lipid classes are greatly reduced in the fetal circulation the critical LCPUFA make up a greater proportion of total fatty acid in the fetal circulation. In the case of the fatty acids the placenta has multiple mechanisms including preferential binding of LCPUFA by p-FABPpm, selective uptake by the syncytiotrophoblast, intracellular metabolic channeling of individual fatty acids, and selective export to the fetal circulation, which allow it to preferentially deliver DHA and AA to the fetal circulation.
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