Nitrogen Balance Protein Turnover and Protein Synthesis

According to animal data, only about 80% of the nitrogen content of the fetus is found in protein; the rest is found in urea, ammonia, and free amino acids. Additional nitrogen requirements for urea excretion and for other possible nitrogen excretion products are not known for human fetuses.

Radioactive and stable isotopic tracers of selected amino acids, especially essential amino acids such as leucine and lysine, have been used to measure fetal protein synthesis, breakdown, and accretion. Limited human data is consistent with data in the fetal sheep, the only species studied in significant detail. Figure 4 shows results of experiments in fetal sheep over the second half of gestation, comparing fractional protein synthesis rates derived from tracer data and fractional body growth rates derived from body composition data. Whole body weight-specific protein turnover rate is higher in the early-gestation fetus primarily from increased rates of amino acid uptake from the placenta (exogenous entry of amino acids into the fetal circulation) and protein synthesis. These processes produce a 50% higher rate of net protein accretion in the mid-gestation fetus.

Mechanisms underlying the decrease in protein synthesis rate over gestation are not well understood, but they appear to be intrinsic to the fetus and not to a limitation of nutrient supply by the

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