Nitrogen balance and protein requirements

There is only a small pool of free amino acids in the body, in equilibrium with proteins that are being catabolized and synthesized. A small proportion of the amino acid pool is used for synthesis of a wide variety of specialized metabolites (including hormones and neurotransmitters, purines and pyrimidines). An amount of amino acids equivalent to that absorbed is oxidized, with the carbon skeletons being used for gluconeogenesis (sections 5.7 and 9.3.2) or as metabolic fuels, and the nitrogen being excreted mainly as urea (section 9.3.1.4). 

The state of protein nutrition, and the overall state of body protein metabolism, can be determined by measuring the dietary intake of nitrogenous compounds and 

The output of N from the body is largely in the urine and faeces, but significant amounts may also be lost in sweat and shed skin cells — and in longer-term studies the growth of hair and nails must be taken into account. Obviously, any loss of blood or tissue will also involve a loss of protein. Although the intake of nitrogenous compounds is mainly protein, the output is mainly urea (section 9-3.1.4), though small amounts of a number of other products of amino acid metabolism are also excreted, as shown in Table 9.1. 

The difference between intake and output of nitrogenous compounds is known as nitrogen balance. Three states can be defined  

Ammonium 340-1200 mg 20-70 mmol Depends on the state of acid-base balance 

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