Fuel metabolism dietary requirements

In order to survive, humans mnst meet two basic metabolic requirements we mnst be able to synthesize everything onr cells need that is not supplied by our diet, and we must be able to protect our internal environment from toxins and changing conditions in onr external environment. In order to meet these reqnirements, we metabolize onr dietary components through four basic types of pathways fuel oxidative pathways, fuel storage and mobilization pathways, biosynthetic pathways, and detoxification or waste disposal pathways. Cooperation between tissnes and responses to changes in onr external environment are commn-nicated thongh transport pathways and intercellnlar signaling pathways (Fig. 1.1). 

It is the continual catabolism of tissue proteins that creates the requirement for dietary protein. Although some of the amino acids released by breakdown of tissue proteins can be re-used, most are metabolized, by pathways which are discussed in section 9.3, yielding intermediates that can be used as metabolic fuels and for gluconeogenesis (section 5.7) and urea (section 9.3.1.4), which is excreted. This means that there is a need for dietary protein to replace losses even in an adult who is not growing. In addition, relatively large amounts of protein are lost from the body in mucus, enzymes and other proteins, which are secreted into the gastrointestinal tract and are not completely digested and reabsorbed. 

Current estimates of protein requirements are based on studies of the amount required to maintain nitrogen balance. If the intake is not adequate to replace the protein that has been broken down, then there is negative nitrogen balance — a greater output of nitrogen from the body than the dietary intake. Once the intake is adequate to meet requirements, nitrogen balance is restored. The proteins that have been broken down can be replaced, and any surplus intake of protein can be used as a metabolic fuel. 

An adult has a requirement for a dietary intake of protein because there is continual oxidation of amino acids as a source of metabolic fuel and for gluconeogenesis in the fasting state. In the fed state, amino acids in excess of immediate requirements for protein synthesis are oxidized. Overall, for an adult in nitrogen balance, the total amount of amino acids being metabolized will be equal to the total intake of amino acids in dietary proteins. 

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