![]() ![]() Most of this nitrogen will be funneled into a common pathway leading to its excretion as urea. Once amino acids are produced by protein degradation, the next step involves the removal of the amino nitrogen from the carbon skeleton. High quality protein meals are therefore important for convalescence after surgery or severe burns or infection. The metabolic response to infection or injury results in a massive mobilization of skeletal protein and large increases in urea excretion. Negative nitrogen balance is also associated with fever, septicemia and trauma. Muscle breakdown will allow an individual to survive for about two months without food. Continued deprivation results in increased breakdown of muscle. Initially, much of the lost protein comes from the liver, pancreas and gut. Now more nitrogen is excreted than is ingested resulting in negative nitrogen balance. On the other hand, starvation or inadequate protein nutrition lead to degradation of body protein to provide amino acids for synthesis of critical proteins, other nitrogenous substances, or as a source of energy. However, when body protein is accumulating such as during childhood or pregnancy or in muscle mass due to exercise, the body retains more of the protein nitrogen than it excretes as urinary nitrogen, resulting in a positive nitrogen balance. In a healthy adult with a good diet, the amount of nitrogen consumed usually approximates the amount excreted as urea. Small amounts of nitrogen are excreted as ammonia, uric acid and creatinine (uric acid comes from nucleotides and creatinine from muscle metabolism). This correlation is important clinically in monitoring protein metabolism. The levels of urea reflect protein intake (proteins contain about 16% nitrogen). ![]() Nearly all of this nitrogen is excreted in urine as urea. The metabolism of dietary protein and the continuous turnover of body protein and other nitrogenous substances leads to elimination of unused nitrogen. The use of valuable protein for energy requirements can be greatly reduced by including carbohydrate in the diet. In the absence of other fuels, dietary protein is also degraded for energy. Regrettably, excess amino acids do not make for more muscle but rather are converted to carbohydrate or fat - use it or lose it! If protein intake is restricted, tissue proteins are degraded to provide amino acids for the synthesis of more needed proteins. The amounts of individual proteins are determined primarily by their functions. Unlike carbohydrates and lipids that have major storage forms as glycogen and triglycerides, there are no specialized reserve stores for amino acids or proteins. The free amino acid pool is metabolically very active - more than 95% of the amino acids in plasma are replaced every 10 minutes. This observation led to the concept of a dynamic equilibrium not only between amino acids and proteins, but also between individual amino acids. In one of the first "tracer" studies with 15N-labeled amino acids, Schoenheimer and his colleagues showed that the nitrogen from one amino acid was redistributed to other amino acids. ![]()
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