For me, the availability of each Amino Acid in fasting blood RELATIVE to all others is more significant metabolically, than the absolute values of each AA (which are currently assessed individually to ascertain whether each falls within a said 'normal range' or not), as:
Consideration of all amino acids together and relative to each other, provides context, as well as strengths and weaknesses of the metabolome as a whole.
I think both might be important, depending on your biological or scientific question. The % abundance may be more signifficant for some metbolic disease (like phenylalanine ratio). However, the absolute concentration is very importantant in relation to the concentration of other metabolites or the value of the hematocrit.
John,
We have conducted multiple metabolomic studies using both % abundance and absolute readings. What we found is that they give different sets of information. We also found that using the urinary ratios with creatinine also may hide a considerable amount of information especially when there is a chronic disease which is associated with alterations in creatine and creatinine such as seen in some renal diseases. the absolute level usually indicate the change in the free amino acid in a sample and in urine samples may indicate an aminoaciduria response as seen with inflammation or renal tubular acidosis situations. When you evaluate the % abundance in those very same samples you can see the variation in metabolites indicate certain underlying variations which are uniform across the group and these appear to indicate the underlying genetic or inflammatory mediated changes induced. If we used the accepted creatinine ratio analysis than this tended to hide all the changes we could identify using the other analyses.
The summary conclusion is that you really should analyses both and use the findings to indicate the actual mechanisms
regards
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