Protein Metabolism

This one I'm bumping for myself, but perhaps some folks might find it interesting.  The red bolding is new.

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Original Publication:  4/17/10

Looking for some info on protein, I came across this link:

http://www.nutrition-partner.com/index.cfm?C27B8FF7FABF494CAA480EF21E0D228D

The page focuses on changes in dietary requirements and/or the impact on the body of malnutrition, disease and stress, but I found this site through the following graphic on protein metabolism:

Something in the numbers doesn't add up here, but I was surprised to see how much protein is "recycled" in our guts.  Perhaps the total protein synthesis is balanced by catabolism/turnover and that is the missing number?  Not sure.  Here is the quoted text referring to this diagram (my apologies for the messed up formatting).  

Protein in the body is not static; protein synthesis and breakdown is constantly taking place. However, the total body protein pool in a healthy adult is constant. Synthesis of protein from endogenous and exogenous amino acids is equal to degradation and external losses. Some proteins have a long lifetime, such as muscle protein and plasma albumin, while others have a high turnover rate. Muscle protein constitutes up to approximately 50% of total body protein, but contributes only approximately 30% of the protein turnover in the body, because in visceral and other organs, protein turnover rates are several times higher than in muscle tissue.  Protein metabolism is dependent on a vast number of endogenous mediators. These mediators define the balance between anabolic and catabolic processes. Insulin is the major anabolic hormone and also has an important role in amino acid and protein homeostasis. During injury and stress two major alterations in insulin are noted: a catecholamine-mediated suppression of insulin release and an insulin resistance, leading to a release of skeletal muscle amino acid for gluconeogenesis and, at the same time, a decreased utilisation of glucose by insulin-dependent tissues. This mechanism provides glucose to the insulin-dependent tissues that are important for survival and the healing of injury, such as the central nervous system (CNS), immune system and red blood cells. Other hormones, like glucagon and the catecholamines, control or counteract the effects of insulin and are more or less proteolytic; the exact role of catecholamines is still under discussion, however. In order to define protein requirements, an overview of protein metabolism is necessary. By determining the renewal of proteins susceptible to measurement, such as plasma, muscle and digestive secretion proteins, it has been possible to estimate the daily turnover in proteins. Considerable recycling of endogenous amino acids seems to occur, the quantity amounting to twice the daily intake.   Hence, normal protein metabolism incorporates about 100 g of dietary amino acids and over 200 g of endogenous amino acids daily. Allowance must therefore be made for increased losses of endogenous proteins when assessing patients´ protein intake.

So fully 2/3rds of our daily protein metabolism comes from proteins in our body -- the amino acid pool in our cells and/or breakdown of tissues.  Another factoid:  Structural proteins have about a 6 month half life, while hormones/peptides/etc. can have half-lives of a matter of minutes.  However hormones etc. constitute a very small amount of protein weight wise.