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physicochemical measurements ought to offer an opportunity to make significant progress in nutritional evaluation. Therefore, this possibility has continued to be explored, and in this short review paper, particular attention will be given to presenting preliminary results from recent studies in this laboratory involving simultaneous use of stable isotope tracers and of imaging techniques involving positron emission tomography (PET).

To establish a reasonable framework for the discussion to follow, a general organization of body protein and amino acid metabolism is depicted in Figure 10-1; this includes some examples of the types of biochemical measures used to assess the status of the different metabolic-organ systems indicated. In the present context of a consideration of emerging technologies, however, the working hypothesis is that, by accumulating reliable data on the quantitative rates of protein anabolism and catabolism in various regions and organs of the body, practical new diagnostic tools can be developed for assessment of protein nutritional status in individuals under various pathophysiological states. Hence, the goal has been to refine, apply, and further develop noninvasive methods for determination of (1) protein turnover at the whole body and organ-tissue levels and (2) turnover of specific proteins in vivo. The principal analytical methods that have been exploited by this laboratory to date are those of isotope ratio and selected ion monitoring mass spectrometry and, more recently, positron emission tomography imaging.

FIGURE 10-1 A simplified organization of protein and amino acid metabolism, with an indication of the major systems involved and measures used to assess the status of these systems. CHI, creatine height index; Urea N, urea nitrogen; OH-Proline, hydroxyproline. SOURCE: Young et al. (1990) © J. Nutr. (120:1496–1502), American Society for Nutritional Sciences. Figure is based on G. Arroyave.

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