Protein turnover is the continual synthesis and breakdown of protein in the body. This is a core process in biology and many talents have attempted to measure this in the whole body .
The use of radio labelled amino acids requires a model system to enable calculations.
Q = I + B + N = S + M + C
Q is the flux of the amino acid, I the dietary intake, plus the input from protein breakdown B and N is the input from de novo synthesis. The flux is equal to the incorporation of the amino acid into body protein S, oxidation and other forms of metabolism M. There is small additional loss from the gastrointestinal tract as faeces
( 1 g N / day ) , skin ( 20 mg N / kg body weight ) and non measurable urinary uric acid ( 3 % of urinary nitrogen). Q can be calculated with the flux based on NH3 ( Q A ) or on urea (Q u ).
In human studies stable isotopes are used eg 13 C and 15 N. The amino acid chosen is usually leucine as this has one catabolic pathway and is predominantly metabolised in muscle. The amino acid can be given orally or more usually intravenously for 2 hours to achieve a plateau enrichment of the plasma . If leucine is used, its muscle metabolic product a-keto isocaproic acid (KIC) is used for measurement, being a better indicator of intracellular leucine enrichment. Such methodology requires
1. that a steady state plasma concentration of tracer is achieved
2. the dose of tracer has no consequences for the metabolism of the tracer.
3. the labelled and unlabelled amino acids are metabolised identically
4. no significant recyling of the isotope occurs
5. the enrichment in plasma is representative of that at the site of protein synthesis.

In a review Duggleby and Waterlow ( British Journal of Nutrition 2005, vol 94, pp 141-153 ) have analysed an cheap alternative , the end product approach using labelled glycine. This method has merit for population results.
This method relies on a two-pool model , and that the two products of the same precursor have the same labelled activity. The other assumption is that the activity of the end product reflects the amino acid-N mixture taken up into protein. The end products of ammonia or urea may not give the same result.
In the complex of all the proteins in the body and their varied rate of turnover this is a basic assumption, and is reflected in the range of inexplicable results.
If nitrogen flux is measured in the same subject over a period of time using a range of amino acid precursors then the measure of protein synthesis ( mg protein /kg per hour) ranges from 70 to 1038 ) and between oral and intravenous. However closer results are obtained from the individual amino acids. Few studies have been made on the same subject using different labelled amino acids at the same time.
In many respects this is an unresolved and demanding area.
Review Duggleby and Waterlow (British Journal of Nutrition 2005, vol 94, pp 141-153)

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