Proteins are subject to constant breakdown and resynthesis. In the steady-state, rates of synthesis and breakdown are in balance (A). To any 100 g of exo-genously supplied food protein in the small intestine, about 70 g of protein is added into the intestinal tract from endogenous sources: secretions, enzymes, and sloughed-off cells. Protein digestion and absorption are highly efficient. Approximately 95 % are absorbed, only ~l0 g/d are lost through fecal matter. After complete hydrolysis of all pep-tides, and subtraction of those used by the intestinal mucosa, ~150 g free amino acids (AA) become available to the body each day.
Incoming free AA are first compartmentalized into various AA pools. Only a small amount is found in plasma, 7080 % of the free AA are found in skeletal muscle. Intracellular accumulation of free AA in the respective tissue constitutes a first controlled step in AA metabolism. Assuming a 10 g loss to feces, the daily exogenous protein supply is only ~90 g. Highly efficient AA recycling provides so many additional endogenous AA that the total protein turnover rate is as high as 300 g/d. For instance, of ~75 g of skeletal muscle protein that is synthesized and degraded, only ~10% is exchanged between skeletal muscle and plasma pool as free AA. Constant replacement of the intestinal mucosa, and synthesis and breakdown of plasma proteins and blood cells also contribute to the high protein turnover rate.
An assessment of protein turnover rates can be achieved by measuring the turnover rates of short-lived proteins. Plasma proteins with a high breakdown rate like prealbumin or retinol-binding protein (27% and 120% breakdown, respectively/day) are useful for purposes of detecting latent malnutrition. Nitrogen balance is more commonly used for this purpose. Nitrogen (N) is easy to determine: after oxidation of the organic matrix, the mass is converted to protein, using a factor. N balance is in homeostasis when the amount supplied by foods corresponds to that lost through urine, feces, and skin. Additional small amounts of N are lost through sweat, hair, menstrual blood, and semen.
Energy intake significantly influences N balance. Under reduced energy supply conditions, sufficient energy for protein metabolism is lacking. On the other hand, if protein supply is below a certain limit, nitrogen balance cannot be improved by high energy supply. Even with adequate energy supply, a multitude of diseases, protein energy malnutrition (PEM), or poor protein quality can nevertheless cause negative N balance. If adequate energy but no protein is supplied, daily N losses decrease to approximately 2-3 g/d within a few days. This daily loss persists even after prolonged protein-free diets. It can be deduced from this that a daily protein loss of 16-17 g is obligatory and has to be compensated in the long term to maintain body proteins and therewith life function.
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