Protein & Amino acid requirements
المؤلف:
Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil
المصدر:
Harpers Illustrated Biochemistry
الجزء والصفحة:
32nd edition.p533-534
2025-12-11
28
Protein Requirements Can Be Determined by Measuring Nitrogen Balance
The state of protein nutrition can be determined by measuring the dietary intake and output of nitrogenous compounds from the body. Although nucleic acids also contain nitro gen, protein is the major dietary source of nitrogen and measurement of total nitrogen intake gives a good estimate of protein intake (mg N × 6.25 = mg protein, as N is 16% of most proteins). The output of N from the body is mainly in urea and smaller quantities of other compounds in urine, undigested protein (including digestive enzymes and shed intestinal mucosal cells) in feces; significant amounts may also be lost in sweat and shed skin. The difference between intake and output of nitrogenous compounds is known as nitrogen balance. Three states can be defined. In a healthy adult, there is nitrogen equilibrium; intake equals output, and there is no change in the total body content of protein. In a growing child, a pregnant woman, or a person in recovery from protein loss, the excretion of nitrogenous compounds is less than the dietary intake and there is net retention of nitrogen in the body as protein—positive nitrogen balance. In response to trauma or infection, or if the intake of protein is inadequate to meet requirements, there is net loss of protein nitrogen from the body—negative nitrogen balance. Except when replacing protein losses, nitrogen equilibrium can be maintained at any level of protein intake above requirements. A high intake of protein does not lead to positive nitrogen balance; although it increases the rate of protein synthesis, it also increases the rate of protein catabolism, so that nitrogen equilibrium is maintained, albeit with a higher rate of protein turnover. Both protein synthesis and catabolism are ATP expensive, and this increased rate of protein turnover explains the increased diet-induced thermogenesis seen in people consuming a high-protein diet.
The continual catabolism of tissue proteins creates the requirement for dietary protein, even in an adult who is not growing; although some of the amino acids released can be reutilized, much is used for gluconeogenesis in the fasting state. Nitrogen balance studies show that the average daily requirement is 0.66 g of protein per kg body weight (giving a reference intake of 0.825 g of protein/kg body weight, allowing for individual variation); ~55 g/d, or 8 to 9% of energy intake. Average intakes of protein in developed countries are of the order of 80 to 100 g/d, that is, 14 to 15% of energy intake. Because body protein increases in growing children, they have a proportionally greater requirement than adults and should be in positive nitrogen balance. Even so, the need is relatively small compared with the requirement for protein turnover. In some countries, protein intake is inadequate to meet these requirements, resulting in stunting of growth. There is little or no evidence that athletes and body builders require large amounts of protein; simply consuming more of a normal diet providing about 14% of energy from protein will provide more than enough protein for increased muscle protein synthesis— the main requirement is for an increased energy intake to permit increased protein synthesis.
There Is a Loss of Body Protein in Response to Trauma & Infection
One of the metabolic reactions to a major trauma, such as a burn, a broken limb, or surgery, is an increase in the net catabolism of tissue proteins, both in response to cytokines and glucocorticoid hormones, and as a result of excessive utilization of threonine and cysteine in the synthesis of acute-phase proteins. As much as 6 to 7% of the total body protein may be lost over 10 days. Prolonged bed rest results in considerable loss of protein because of atrophy of muscles. Protein catabolism may be increased in response to cytokines, and without the stimulus of exercise it is not com pletely replaced. Lost protein is replaced during convalescence, when there is positive nitrogen balance. Again, as in the case of athletes, a normal diet is adequate to permit this replacement protein synthesis.
The Requirement Is Not Just for Protein, but for Specific Amino Acids
Not all proteins are nutritionally equivalent. More of some is needed to maintain nitrogen balance than others because different proteins contain different amounts of the various amino acids. The body’s requirement is for amino acids in the correct proportions to replace tissue 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 precursors— cysteine from methionine and tyrosine from phenylalanine. The dietary intakes of cysteine and tyrosine 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. If one of these amino acids is omitted from the diet, nitrogen balance can still be maintained. However, only three amino acids, alanine, aspartate, and glutamate, can be considered to be truly dispensable; they are synthesized by transamination of common metabolic intermediates (pyruvate, oxaloacetate, and ketoglutarate, respectively). The remaining amino acids are considered as nonessential, but under some circumstances the requirement may outstrip the capacity for their synthesis.
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