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Labile Protein Reserves and Protein Turnover¹

Departments of Nutritional Sciences, and Meat and Animal Science,University of Wisconsin, Madison 53706

ABSTRACT

The concept of a labile protein reserve is based on the relatively slow establishment of a new equilibrium in the rate of nitrogen excretion after an abrupt change in dietary supply. The evidence reviewed shows that a majority of this nitrogen is derived from or deposited in skeletal muscle proteins. The rates of synthesis and degradation of total body protein are rapid in large animals (man and swine) and are correlated with heat production. The rate of protein synthesis in skeletal muscle greatly exceeds the rate of growth and is sensitive to nutritional status. The rate exceeds the rate of degradation in response to the ingestion of an adequate diet so that tissue proteins are accumulated, but it decreases below the rate of degradation under conditions of deprivation. In this latter state, proteins of skeletal muscle supply amino acids for energy or for the synthesis of other more essential proteins, e.g., milk proteins during lactation. Thus, we conclude that the labile protein reserve is a product of the normal, dynamic metabolism of protein.

¹ Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison and United States Public Health Service Grant AM 14704. Presented in Symposium: Hormonal relationships to nutritional physiology at 71st Annual Meeting American Dairy Science Association, Raleigh, NC.

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