Submitted to: American Journal of Physiology - Endocrinology and Metabolism
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/15/2003
Publication Date: 8/1/2003
Citation: Sunehag AL, Haymond MW. Maternal Protein Homeostasis and Milk Protein Synthesis During Feeding and Fasting in Humans. Am J Physiol Endocrinol Metab 2003 Interpretive Summary: The present studies, to our knowledge, are the first to explore the impact of lactation per se on maternal amino acid and protein metabolism and to measure the sources of amino acids for milk protein synthesis using stable isotope dilution methodologies. Despite some small differences in the plasma concentration of urea, lactation per se had little impact on maternal amino acid and protein metabolism. We have reported that plasma glucose was the primary but not only source of milk lactose. Similarly, we found that over the course of 8 to 12 h of isotope infusion, the leucine enrichment in milk proteins was always less than that of the plasma leucine and KIC enrichments. These data strongly suggest that a portion of the amino acids for milk protein synthesis is derived from some source (presumably a protein source) other than the plasma free amino acid pool. In addition, we demonstrated a striking difference in the fractional rate of albumin synthesis in the fed study condition between the two groups of women, suggesting that the plasma albumin pool may, in fact, be a significant contributor to the unlabeled amino acid pool contributing to milk protein synthesis.
Technical Abstract: Little is known about amino acid (aa) and protein metabolism in lactating women. We hypothesized: 1) aa sources other than the plasma acid pool provide substrate for milk protein synthesis in humans; and 2) if albumin was one such source, then albumin fractional synthesis rate (FSR) is higher in the lactating women. To test these hypotheses, 6 healthy exclusively breastfeeding women (27±3 y; BMI 26±2 kg/m2) between 6 wks and 3 months postpartum, and 6 healthy non-lactating women (28±2 y; BMI 22±1 kg/m2) were studied twice, in random order, during 22 h fasting or 10 h of continuous feeding with a mixed nutrient drink. Protein metabolism was determined using [1-13C]leucine and [15N2]urea. In both the fed and fasted states, a significant portion of milk protein (20±5 and 31±6%, respectively) was derived from sources other than the plasma free aa pool. A 70% higher (p<0.02) FSR of albumin was observed in lactating women during feeding suggesting that albumin is a likely source of aa for milk protein synthesis. We conclude that plasma free amino acids contribute only 70-80% of the substrate for milk protein synthesis in humans and that albumin may be a significant source of amino acids for the remainder.