Nitrogen metabolism

http://www.whatislife.com/reader2/Metabolism/pathway/nitrogen.html

The majority of useful nitrogen for animal metabolism comes from proteins in the form of reusable ammonia (NH3). Nitrogen is fixated in form of ammonia by microorganisms (see chapter on amino acid synthesis) and all 'higher' forms of life (eukaryotes) depend on this primordial source of nitrogen extracted from the air. The 'usefulness' of proteins depends on four distinct properties:

1. total amount of protein ingested 
2. digestibility of proteins 
3. amino acid composition of proteins 
4. total caloric intake

Digestibility and amino acid composition define the biological value of dietary proteins. Hair and skin keratin is non-digestible and useless as such. Pretreatment like heating can improve protein usefulness. Animal proteins are a better mix for our diet than plant proteins based on their amino acid composition. 
Nucleic acids, on the other hand, are not needed as dietary supplement. Excess nucleic acid in the diet is degraded and secreted and most nucleic acid synthesis in cells is provided by protein degradation (in form of amino acid precursors). Since all nitrogen containing compounds are dependent on protein supply, protein deficiency is one of the major nutritional problems in the world. This is specifically important for children and pregnant women, since the developing organism has a several fold higher need for proteins in the diet than the adult. Low protein intake results in lowered protein synthesis and thus in a lower supply of essential proteins involved in food digestion (proteases) and blood plasma transport (lipoproteins).
---Steve