Metabolism
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Why does an adult require so much protein in the diet?
Key points from this exercise:
Under normal conditions in an adult, urinary excretion and faecal losses of nitrogenous compounds are equivalent to the dietary intake of nitrogenous compounds (mainly protein). This is the condition of nitrogen balance or equilibrium.
The total flux of protein through the gut is considerably greater than the dietary intake. The average dietary intake is ~ 90 g per day, but some 200 g of endogenous protein enters the intestinal lumen from shed intestinal mucosal cells and secreted proteins.
In growth or recovery from protein losses, the intake of nitrogenous compounds is greater than urinary and faecal losses. This is positive nitrogen balance - an increase in the total body content of protein.
If the dietary intake of protein is inadequate to meet requirements, and in pathological conditions that involve tissue protein loss, the urinary and faecal losses of nitrogenous compounds is greater than the intake. This is negative nitrogen balance.
Estimates of protein requirements are based on studies feeding varying levels of protein to determine the intake at which nitrogen balance can just be achieved.
There is continual turnover of proteins in the body, with catabolism and replacement synthesis, even when there is no change in the total body protein content - this is dynamic equilibrium. Different proteins turn over at very different rates; key regulatory enzymes have a half life of only a few hours.
The requirement is not just for protein, but for nine essential amino acids that cannot be synthesised in the body and must be provided in the diet.
In response to trauma and infection there is a considerable loss of protein from the body. This is partly the result of induction of key enzymes of gluconeogenesis and amino acid catabolism in response to cortisol, and partly due to the synthesis of acute phase proteins that contain disproportionately large amounts of essential amino acids. The resultant pool of amino acids depleted in one or more of the essential amino acids cannot be used for protein synthesis but will be catabolised.
Even in nitrogen balanced, there is cycling between positive balance after a meal and negative balance in the fasting state. In the fasting state the rate of protein synthesis falls, partly to conserve metabolic energy and partly because amino acids from protein catabolism are being used as substrates for gluconeogenesis. In the fed state the rate of protein synthesis increases, and this protein is replaced.
In the fed stated, amino acids in excess of immediate requirements for protein synthesis are catabolised; there is no storage of free amino acids in the body.
There is an increase on protein turnover, but not net protein synthesis, in response to a high protein meal. This leads to an increase in energy expenditure.
High-protein diets are effective for weight loss partly because of the increased energy expenditure associated with increased protein turnover, and partly because of the energy cost of gluconeogenesis from amino acids. For more on this see D.A.Bender (2012) British Journal of Nutrition 108: S113-121. Metabolism of “surplus” amino acids.