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Am J Obstet Gynecol. 1975 Oct 15;123(4):356-63.

Fetal ingestion and metabolism of amniotic fluid protein.


Fetal swallowing, digestion, and utilization of amniotic fluid protein were studied in near-term rhesus monkeys in which 35S-protein, synthesized biologically from 35S-methionine, was injected into the amniotic sac. The half-time of the injected protein in amniotic fluid was 1.1 days, a figure consistent with findings of others that fetal swallowing represents the principal mechanism of clearance of amniotic fluid protein. On delivery at timed intervals up to 7 days after injection, evidence of progressive proteolysis along the fetal alimentary tract was found. The amnio acids liberated from protein hydrolysis were apparently utilized in protein synthesis in the gut wall as well as absorbed in fetal plasma, where they equilibrated rapidly with maternal plasma and amniotic fluid. Maximal amino acid radioactivity in these three compartments occurred 3 days after injection and was followed 1 day later by maximal fetal plasma protein radioactivity. Incorporation into protein of amino acids absorbed after hydrolysis was found in fetal lung, liver, skeletal muscle, and brain. The results indicate that ingested amniotic fluid protein undergoes proteolysis in the fetal alimentary tract and the amino acids thus made available are utilized in protein synthesis by the developing fetus. While this mechanism can provide only a relatively minor proportion (estimated at 10 to 15 per cent) of fetal nitrogen requirements, it may represent an important aspect physiologically by preparing the fetus for extrauterine nutrition. Moreover, the use of intra-amniotic nutrition as a mode of fetal therapy, while speculative at present, offers possibilities for the furture.

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