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United States Department of Agriculture

Agricultural Research Service

Title: Maternal Protein Homeostasis and Milk Protein Synthesis During Feeding and Fasting in Humans

Authors
item Sunehag, Agneta
item Haymond, Morey

Submitted to: American Journal of Physiology - Endocrinology and Metabolism
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 8, 2003
Publication Date: August 1, 2003
Citation: Sunehag, A.L., Haymond, M.W. 2003. Maternal protein homeostasis and milk protein synthesis during feeding and fasting in humans. American Journal of Physiology Endocrinology and Metabolism. 285(2):E420-E426.

Interpretive Summary: Not much is known about amino acid (aa) and protein metabolism in lactating women. As a test, 6 healthy exclusively breastfeeding women between 6 wks and 3 months postpartum, and 6 healthy non-lactating women were studied twice, in random order, during 22 h fasting or 10 h of continuous feeding with a mixed nutrient drink. In both the fed and fasted states, a significant portion of milk protein was derived from sources other than the plasma free aa pool. A 70% higher 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.

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.

Last Modified: 7/22/2014
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