Metabolism
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Why does an adult require so much protein in the diet?
It is obvious why a growing child has a relatively high requirement for dietary protein, since there is an increase in the total amount of protein in the body as the child grows.
What is less obvious is why an adult, whose total body protein content does not change, still has a relatively high requirement for dietary protein.
Under normal conditions a healthy adult has an intake of ~90 g of protein per
day, and excretes nitrogenous compounds in urine equivalent to 80 g of protein,
and loses nitrogenous compounds equivalent to 10 g of protein per day in faeces.
This is the state of nitrogen balance or nitrogen equilibrium
intake = output
This simple balance between intake and output hides the fact that some 200 g of protein per day is secreted into the intestinal lumen as digestive enzymes, shed intestinal mucosal cells and the proteins in mucus that protects the intestinal mucosa from the actions of digestive enzymes.
Total daily absorption of amino acids and dipeptides (which are hydrolysed to amino acids during absorption) from the gut is therefore about 280 g.
The amino acids are then used for synthesis of tissue proteins, and a variety of specialised metabolites.
Obviously, in a growing child, in pregnancy, and during recovery from protein losses, there is an increase in the total body protein content, and the output of nitrogenous compounds is less than the intake.
This is positive nitrogen balance - a gain in total body
protein
intake > output
The converse, loss of total body protein is negative nitrogen
balance
output > intake
The graph below shows daily nitrogen balance on a young
man who was fed different amounts of protein for a week at a time, starting
with his habitual intake of 80 g /day, when he was able to maintain nitrogen
balance. 
This was followed by a week with no protein intake, when his balance became very negative.
What conclusions can you draw from the very negative nitrogen balance with zero protein intake?
There is obviously continuing loss of protein, which is not being replaced. There are obviously obligatory losses of nitrogen from the body, meaning that there is a continuing need for a dietary intake of protein.
In the next weeks he was fed 30 g of protein /day for 1 week, then 40 g /day for the second week
What conclusions can you draw from these results?
30 g of protein /day is not enough to meet his requirements, and he is still in negative nitrogen balance, still losing protein from the body. However, at 40 g /day he is able to maintain nitrogen balance, suggesting that this is adequate to meet his requirements and permit replacement of obligatory losses.
He was then fed 100 g of protein /day for 2 weeks. Initially he was in positive nitrogen balance, then after the first week he returned to nitrogen equilibrium.