Protein nitrogen balance

Donovan, S.M., Atkinson, S.A and Lonnerdal, B. 1986 Total nitrogen and non-protein nitrogen balance in preterm infants fed preterm human milk. In Jensen, R.G. and Neville, M.C., eds.. Human Lactation 2. Maternal and environmental factors. New York, Plenum Press 603-610. 

Protein Requirements Can Be Determined by Measuring Nitrogen Balance... 

Not all proteins are nutritionally equivalent. Mote of some than of others is needed to maintain nittogen balance because different proteins contain different amounts of the various amino acids. The body s requirement is for specific amino acids in the correct proportions to replace the body proteins. The amino acids can be divided into two groups essential and nonessential. There are nine essential or indispensable amino acids, which cannot be synthesized in the body histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. If one of these is lacking or inadequate, then—regardless of the total intake of protein—it will not be possible to maintain nitrogen balance since there will not be enough of that amino acid for protein synthesis. 

Two amino acids—cysteine and tyrosine—can be synthesized in the body, but only from essential amino acid ptecutsots (cysteine from methionine and tyrosine from phenylalanine). The dietary intakes of cysteine and tytosine thus affect the requirements for methionine and phenylalanine. The remaining 11 amino acids in proteins are considered to be nonessential or dispensable, since they can be synthesized as long as there is enough total protein in the diet—ie, if one of these amino acids is omitted from the diet, nitrogen balance can stiU be maintained. Howevet, only three amino acids—alanine, aspartate, and glutamate—can be considered to be truly dispensable they ate synthesized from common metabolic intetmediates (pyruvate, ox-... 

O Parenteral nutrition (PN), also called total parenteral nutrition (TPN), is the intravenous administration of fluids, macronutrients, electrolytes, vitamins, and trace elements for the purpose of weight maintenance or gain, to preserve or replete lean body mass and visceral proteins, and to support anabolism and nitrogen balance when the oral/enteral route is not feasible or adequate. 

Specific nitrogen goals are positive nitrogen balance or nitrogen equilibrium and improvement in serum concentration of protein markers (e.g., transferrin or prealbumin). 

Although these experiments showed growth was possible using casein hydrolysate, Rose also demonstrated that when the amino acid mixture was used rather than the intact protein, additional calories had to be provided as fat plus carbohydrate, if nitrogen balance was to be maintained. It was later shown that the carbohydrate was needed to protect the free amino acids from oxidation in the intestinal epithelium in the course of absorption. Further, amino acids are poorly tolerated by mouth, causing vomiting and/or diarrhea. After World War II attempts to feed very emaciated prisoners in concentration camps with protein hydrolysates were unsuccessful. It was then recognized that osmotic effects from the amino acids were responsible for the unpleasant consequences. 

Amino acids are not only important for protein synthesis but also serve as precursors for hormones, coenzymes, alkaloids, cell wall polymers, porphyrins, antibiotics, nucleotides, pigments, and neurotransmit-ters. ° A deficiency of one amino acid may result in a negative nitrogen balance (Figure 9). ... 

Nitrogen balance is determined by how well the amount of dietary nitrogen-based compounds (principally proteins) matches the nitrogen needs of the body. 

---