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10
Combined Stable Isotope-Positron Emission Tomography for In Vivo Assessment of Protein Metabolism

V. R. Young,1Y-M. Yu, H. Hsu, J. W. Babich, N. Alpert, R. G. Tompkins, and A. J. Fischman

INTRODUCTION

The improved biochemical assessment of protein nutritional status and determination of quantitative dietary needs to maintain homeostasis and function in individuals, especially those who may be subjected to significant stress as in military combat situations, remain important research challenges for nutritional physiologists and biochemists. The advantages and limitations of approaches that have been taken so far to assess protein nutritional status have been discussed previously (Young et al., 1990). In that review it was proposed that an expanded application of stable isotope tracer techniques and of sophisticated

1  

V. R. Young, Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge, MA 02139; Shriners Burns Institute and Departments of Surgery and Radiology, Massachusetts General Hospital, Boston, MA 02114

The authors' studies were supported by grants from the Shriners Hospitals for Crippled Children and the National Institutes of Health (GM 21700, DK 15856, T 32-GM 07035, and T 32-CA 09362).



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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability 10 Combined Stable Isotope-Positron Emission Tomography for In Vivo Assessment of Protein Metabolism V. R. Young,1 Y-M. Yu, H. Hsu, J. W. Babich, N. Alpert, R. G. Tompkins, and A. J. Fischman INTRODUCTION The improved biochemical assessment of protein nutritional status and determination of quantitative dietary needs to maintain homeostasis and function in individuals, especially those who may be subjected to significant stress as in military combat situations, remain important research challenges for nutritional physiologists and biochemists. The advantages and limitations of approaches that have been taken so far to assess protein nutritional status have been discussed previously (Young et al., 1990). In that review it was proposed that an expanded application of stable isotope tracer techniques and of sophisticated 1   V. R. Young, Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge, MA 02139; Shriners Burns Institute and Departments of Surgery and Radiology, Massachusetts General Hospital, Boston, MA 02114 The authors' studies were supported by grants from the Shriners Hospitals for Crippled Children and the National Institutes of Health (GM 21700, DK 15856, T 32-GM 07035, and T 32-CA 09362).

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability 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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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability Because there has been substantial discussion previously, as well as elsewhere in these workshop proceedings, on the use of stable isotope tracer techniques in protein nutritional metabolic research (Halliday and Rennie, 1982; Matthews and Bier, 1983; Waterlow et al., 1978; Young et al., 1991), the major focus of attention in this paper will be with respect to PET. This is a technique familiar to many concerned with clinical nuclear medicine, but it is the authors' position that emission computed tomography has not yet been embraced sufficiently or understood as an experimental tool for nutritional metabolic research. Hence, it will first be indicated why it is an exciting prospect to use PET, and then a general account will be presented of the physical principles involved in PET and methods used for imaging. For this purpose, summary tables and figures will be used to illustrate major points and then discussion will turn briefly to some actual or potential applications of these techniques in nutritional research before a summary and a series of conclusions are drawn. ISOTOPE TRACER ESTIMATES OF PROTEIN AND AMINO ACID TURNOVER IN VIVO Stable Isotope Techniques The advantages and disadvantages of use of stable isotope tracers for nutritional metabolic studies have been discussed by Bier in these proceedings (see Chapter 8) and also elsewhere (Matthews and Bier, 1983). However from the present perspective, it is pertinent to emphasize that these tracers have been applied extensively in studies of whole body protein (nitrogen) and amino acid kinetics in human subjects. A widely used model for estimation of whole body rates of protein turnover is the ''two-pool plasma precursor model" (Figure 10-2); this might involve, for example, use of [1-13C]leucine as tracer, with the rates then being based on isotopic data obtained with blood and expired air samples, following intravenous and/or oral administration of amino acids labeled with 13C or 15N and/or 2H (Garlick and McNurlan, 1994; Waterlow, 1995). In addition, 15N-end product methods have been applied for determination of rates of whole body protein synthesis, most often by giving [15N]glycine as tracer and then measuring the isotopic abundance of nitrogen in urinary urea and ammonia (Grove and Jackson 1995; Waterlow 1995). This latter approach is sufficiently safe and noninvasive to be applied in studies involving infants, pregnant women, and repeated measurements in other individuals or patients where blood sampling and/or venous puncture could not be justified or where it is not practical to do so. In both instances (precursor method and end-product method), whole body measurements of protein synthesis and breakdown rates are obtained. These parameters have been found to be quite valuable for enhancing understanding of human protein metabolism and nutrition, and it is clear that they represent a major investigative advance over information gained from the whole-body nitrogen balance method. However, they do not give any informa-

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability FIGURE 10-2 The two-pool model that is used widely to calculate rates of whole body protein synthesis and degradation (turnover) from data obtained with stable isotope (e.g., 15N or 13C) labeled amino acids. This diagram indicates that the turnover of whole body proteins is the sum of those occurring in the different organs and tissues. In separate or simultaneous studies, along with stable i sotope tracers, rates in these specific regions might be determined from arteriovenous (A-V) difference studies, by measurement of incorporation of labeled tracers into and their loss from proteins and/or by positron emission tomography, using a short-lived radionuclide such as 13N. SOURCE: Adapted from Garlick and McNurlan (1994). tion about the quantitative contribution made by the major organs and tissues that collectively contribute to the status of whole-body protein balance and its integrity, nor do the parameters elucidate the changes occurring within those organs and tissues, possibly in different directions. Attempts are made to emphasize this point in Figure 10-2, which attempts to convey the need to dissect the kinetic parameters of whole body protein turnover into its major components, particularly by developing estimates of protein synthesis and breakdown in the principal protein-active organs such as liver, muscle, and the intestines. Indeed, various approaches have been used to examine the dynamic status of protein and amino acid metabolism within individual tissues and organs, and these basically involve (1) arteriovenous (A-V) balance determinations across an organ (Tessari et al., 1995; Yu et al., 1990) or limb (Biolo et al., 1995; Cheng et al., 1985), while infusing a labeled amino acid such as 2H-phenylalanine or [15N, 13C]leucine, or (2) a direct measurement of the incorporation of a labeled amino acid into tissue protein by taking a biopsy of the specific tissue (e.g., Essen et al., 1992; Nair et al. 1988, 1995; Welle et al., 1993; Yarasheski et al.,

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability 1992, 1993). Although analytical developments now allow these types of studies to be conducted with much smaller tissue samples than were required previously (Calder et al., 1992), both of these major approaches are invasive, which limits their broad and convenient application in human nutritional-metabolic research. Finally, in relation to an understanding of the integration of amino acid metabolism in vivo and how stressful conditions might compromise metabolism and homeostasis, it would be of considerable interest to develop a detailed picture of the quantitative metabolic fate of the individual amino acids within and among various organs. As illustrated in Figure 10-3, there is an organ specificity or orientation to the metabolism of the individual amino acids (e.g., Munro 1983; Young and El-Khoury, 1995), and the nutritional status of the individual will depend in part on how well the roles of the organs involved and their interactions with other organs are maintained. These could well be affected by dietary, hormonal, and other factors, including infection and physical and psychological stressors. Furthermore, the status of protein and amino acid metabolism within discrete areas of an organ, including the brain (Williams et al., 1994), also might be affected by such factors. It is within this metabolic framework that investigators in this laboratory have turned recently to an exploitation of PET for assisting in the inquiry into human protein metabolism under different conditions of health and disease. Therefore, the principles and some of the practical issues involved in PET and its application to nutritional research will now be FIGURE 10-3 A simplified outline of the organ fate and origin of various amino acids. SOURCE: Adapted from Munro (1983).

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability considered in brief. This is done to help the informed reader, who may nevertheless be unfamiliar with PET, to perhaps better appreciate both its advantages and limitations. The discussion is, therefore, somewhat superficial, but consultation of the references will serve to elaborate upon the points made here. Positron Emission Tomography PET is a technique for measuring the concentration of a positron-emitting radio isotope, within a three-dimensional body, via external detection of the radiation emerging from the isotope. Positron-emitting radio nuclides of carbon, nitrogen, oxygen, and fluorine can be prepared readily, and because these elements are present in compounds that are relevant to the study of in vivo aspects of amino acid-protein metabolism, PET offers an opportunity to enhance current investigations of specific organ and tissue protein metabolism by the noninvasive means that will be discussed below. The basic principles of PET and measurement techniques are as follows, while the reader is referred to more extensive discussions for further detail (Christian, 1994; Daghighian et al., 1990; Fowler and Wolf, 1986; Hoffman and Phelps, 1986; Links, 1994; Ter-Pogossian et al., 1980). Positron Decay and Coincidence Detection of Radiation Annihilation Proton-rich nuclides decay to a more stable isotope by two possible decay processes: (1) by electron capture and (2) by positron decay, which is the particular emission of interest in this paper. In this latter process, a proton (p) is converted to a neutron (n), a positron (a positive electron, ß+), a neutrino (v), and energy, as follows: The emitted positron, after traveling a short distance, combines with an electron in the surroundings, resulting in an annihilation interaction.2 This latter event converts all of the mass of the positron and electron into electromagnetic energy, which results in the emission of two 0.511-MeV photons (gamma rays) (Christian, 1994). These two gamma rays travel in very nearly opposite directions (Figure 10-4), penetrate the surrounding tissue, and are recorded outside of the subject by a circular array of detectors, which permit measurement of both the quantity and the location of the positron-emitting radio isotope (Daghighian et al., 1990; Links, 1994). Because the two annihilation photons are created si- 2   In an annihilation interaction, the rest mass of the electron-positron pair disappears and is replaced by the two 511-KeV photons, in accord with conservation of energy and momentum.

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability FIGURE 10-4 Annihilation of positron and electron produces two 0.511-MeV gamma photons. υ, neutrino; , antineutrino. SOURCE: P. E. Christian in Nuclear Medicine, Technology and Techniques, 3d ed., D. R. Bernier, P. E. Christian, and J. K. Langan, eds., St. Louis, 1994, Mosby. multaneously, as well as being emitted at 180° angles to each other, the electronics of the detectors are arranged so as to count only coincident events in the opposite detectors (i.e., within a 5 billionth of a second). This, then, is the principle of annihilation coincidence detection (Figure 10-5) (Hoffman and Phelps, 1986). Gamma rays originating outside of the space between the two detectors interact with only one of the detectors per annihilation, and these rays are not counted, also as illustrated in Figure 10-5. It might be appreciated, therefore, that the accuracy of the tissue localization of activity and the detection efficiency depend on the size and cross-sectional geometry of the detectors and the detector materials used in their construction. These factors influence the cost of the equipment used in measurements, with scanners generally now costing over $2 million. Additional details concerning detector systems and image reconstruction are given elsewhere (Daghighian et al., 1990; Links, 1994), but it should be evident that PET can be used to image, in a quantitatively functional context, in vivo aspects of tissue metabolism, blood flow, and receptor occupancy, for example. However, PET cannot distinguish among different chemical (metabolic) species since it only measures radioactivity concentration. This is a major limitation of this technique, where, for example, specific meta-

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability FIGURE 10-5 Schematic diagram of the principle of annihilation coincidence detection. SOURCE: Hoffman and Phelps, "Positron emission tomography: Principles and quantitation" in Positron Emission Tomography and Autoradiography: Principles and Applications for the Brain, 1986, pp. 237–286, reprinted with permission of Lippincott-Raven Publishers. bolic interconversions are of particular interest in addition to where they are occurring within body tissues and organs. Nuclide Production and Radiotracer Synthesis While a large number of positron-emitting radionuclides exist, the physical properties of some positron (ß+)-emitters are summarized in Table 10-1. Additionally for comparison, information also is listed here about the more traditional radioisotopes, 14C and 3H. The half lives of the four positron emitters given in this table range from 2 minutes (15O) to 110 minutes (18F), and these physical properties offer some flexibility in tailoring the design of metabolic PET studies. For example, it is possible to use H215O to measure blood flow in a particular tissue or organ and then, after a short while, to give 18F-deoxyglucose TABLE 10-1 Physical Properties of 11C, 15O, 13N, 18F, 3H, and 14C Nuclide Half Life Decay Mode 11C 20.4 min ß+ 15O 2.07 min ß+ 13N 9.96 min ß+ 18F 109.7 min ß+ 3H 12.35 yr ß- 14C 5,730 yr ß-

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability as a tracer to measure the glucose metabolic rate in the same organ. These nuclides are produced within a cyclotron whose design varies, and this determines cost and production capability (Fowler and Wolf, 1986). However, this aspect of PET-related technology will not be detailed further here. With respect to carbon-11 (11C), it is produced by 11B (p,n)11C and 14N (p,a)11C nuclear reactions (Table 10-2), with 11C-labeled carbon monoxide (11CO), carbon dioxide (11CO2), and cyanide (11CN) being used as precursors for radiotracer synthesis (Table 10-3). 13N is also cyclotron produced, generally via the 16O (p,a)13N nuclear reaction, which is performed using a water target to yield a [13N]nitrate ion (Quaim et al., 1993). 13N-ammonia can be produced directly by deuteron bombardment of methane and then, following purification, can be used for the synthesis of 13N-labeled amino acids or other compounds using both biosynthetic and synthetic strategies (Fowler and Wolf, 1986). Some examples of the types of 11C-, 13N-, and 18F-labeled compounds that can be made and could, perhaps, be used as probes of protein amino acid and substrate metabolism in vivo are listed in Table 10-3. This table was generated from data presented in the extensive review about positron-emitter tracers by Fowler and Wolf (1986). APPLICATIONS OF PET Although PET has not yet found extensive use in nutritional or metabolic research, its spatial resolution and quantitative features allow quantification of metabolic parameters in volumes of tissue as small as 1.0 cm3. Thus, PET techniques have been validated for measurement of regional blood flow, blood volume, pH, and oxygen utilization (Huang et al., 1986) and applied in studies of glucose metabolism in the brain (Phelps et al., 1986), for measuring blood flow (Huang et al., 1983; Raichle et al., 1983), glucose metabolism and tricarboxylic acid cycle activity in the myocardium (Huang and Phelps, 1986; Schelbert and Schwaiger, 1986), and regional glucose uptake (leg, arm, and heart) (Nautila et al., 1993). Only limited studies using PET have been conducted on amino acid metabolism in peripheral tissues (e.g., Hawkins et al., 1989; Planas et al., 1992), TABLE 10-2 Production of Positron-Emitting Radionuclides Nuclide Production 11C 14N(p,a )11C 15O 14N(a,n)15O 13N 16O(p,a )13N 18F 18O(p,n)18F

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability TABLE 10-3 Some Positron-Labeled Compounds that Have Been Synthesized for Biomedical Studies Compound Labeled Precursor [11C] Amino Acids   L-[1-11C]phenylalanine H[11C]N L-[1-11C]alanine [11C]O2 L-[methyl-11C]methionine [11C]O2 L-[1-11C]leucine H[11C]N 18F-Labeled Compounds   2-deoxyl-2[18F]fluoro-D glucose [18F]F2 3-deoxy-3-[18F]fluoro-D glucose H[18F]anhydrous 13N-Labeled Compounds   L-[13N]asparagine [13N]H3 L-[13N]leucine [13N]H3 L-[13N]methionine [13N]H3   SOURCE: Adapted from Fowler and Wolf (1986). although this technique is particularly attractive because in principle it is possible to make sequential, site-specific, time-dependent physiological and biochemical measurements within the same subject. More importantly, due to its relatively noninvasive nature, PET measurements can be made routine for application in experiments with human subjects. L-methyl-[11C]methionine (11C-Met) has been found to be a useful tracer with PET for evaluation of amino acid kinetics in vivo (e.g., Stalnacke et al., 1982) and for detection of tumors (Bustany et al., 1986; Kubota et al., 1985; LaFrance et al., 1987; Mosskin et al., 1987). Therefore, and because it was possible to prepare 11C-Met without a considerable synthetic or analytical research developmental effort (Figure 10-6), this laboratory has explored recently whether and how 11C-Met might be used to estimate the rate of mixed protein synthesis in skeletal muscle. The major reasons for doing this and a summary of the initial results are given below.

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability FIGURE 10-6 A general outline of one route of synthesis of L-[11C-methyl]methionine. Further details given in Fowler and Wolf (1986) and Meyer et al. (1993). Muscle Protein Synthesis as Measured with PET Importance of Skeletal Muscle The maintenance of lean body mass is critical for continued cell and organ function, whether this relates to the physical performance of the individual or the capacity to withstand a stressful condition, such as systemic infection. A major contributor to the total lean mass of the body is the skeletal musculature. This accounts for about 40 percent of body weight (Forbes, 1987, 171) and approximately 52 percent of the combined protein in the muscle and nonmuscle mass of the body (Cohn et al., 1980). Hence, it might be predicted that changes in the size and metabolic status of the skeletal musculature would have significance for the well-being and functional capacity of the individual. For example, in reference to energy metabolism, the skeletal musculature (1) is the principal site of facultative thermogenesis in response to excess carbohydrate (Astrup and Christensen, 1992), (2) acts as a buffer to maintain glucose homeostasis during postprandial energy-yielding substrate storage (Taylor et al., 1993), (3) is a major determinant of daily basal energy expenditure (Zurlo et al., 1990), and (4) accounts (Tzankoff and Norris, 1977) for a large proportion, if not all, of the decline in basal or resting energy expenditure during aging (McGandy et al., 1966; Vaughan et al., 1991; Young, 1992). Further, the skeletal musculature plays a key role in the regulation, maintenance, and integration of whole-body nitrogen homeostasis (Young, 1970). An example of this latter point is depicted in Figure 10-7, which shows that branched chain amino acids, glutamine, and arginine metabolism interact via the active participation of the skeletal musculature, intestines, kidney, and liver. The point to be emphasized here is that there are metabolically significant interactions and signaling among these major anatomic regions, in amino acid and protein metabolism. Hence, it is important to

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability tations of the PET methodology include: (1) an exposure of the subject to ionizing radiation, (2) inability to make a direct estimate of the specific metabolic pathways followed by the tracer, and (3) in comparison with the A-V difference method, PET does not provide a measure of protein breakdown. The second comment is that the potential importance of the influence of blood flow rate and substrate delivery on metabolism emerges from these data. Thus, from the present stable isotope and leg weight data, the rate of protein synthesis in the total hind limb of these experimental animals was found to be equivalent to 7.3 ± 1.7 (mean ± SD) g·day-1. This compares with limited published values of about 3.2 g·day-1 (Barrett et al., 1987) and 8.6 to 11.5 g·day-1 (Biolo et al., 1992) for the hind limbs of dogs of comparable body weight. The authors believe that these differences might well be related to the blood flow rate across the limb during the study. In the study described here, PET measurements required that the leg of the dog be immobile. Hence, the dogs were heavily sedated and the hind limb restrained. These are procedures that could well have affected regional blood flow and the status of protein and amino acid metabolism in the limb. Recent reviews (Clark et al., 1995; Elia, 1995) emphasize the importance of blood flow on oxygen and substrate delivery in affecting metabolism in tissues and organs. In the present study, mean hind limb blood flow rate was 91 ml·min-1, whereas it was about 228 to 240 ml·min-1 in studies by Biolo et al. (1992, 1994) and 33 ml·min-1 in that of Barrett et al. (1987). Thus, when differences in blood flow rates are taken into consideration, the rate of limb protein synthesis measured in the present study appears to be similar to these other published values. Of further relevance to these workshop proceedings is the fact that cold exposure induces changes in blood flow to the skin and extremities, and high-altitude exposure can result in reduced blood flow in peripheral tissues (IOM, 1996). As noted above, blood flow can be accurately measured using oxygen-15 [15O]water, and due to the short half life of 15O, this tracer permits blood flow measurements to be made in conjunction with other metabolic studies involving positron emitter-labeled compounds in physiological studies. A major limitation, of course, is that such blood flow studies have to be carried out in the physical proximity of the cyclotron. Third, in this study, high levels of 11C-Met utilization by bone marrow also were noted (Hsu et al., 1996). This finding emphasizes the further potential value of PET in that it allows a simultaneous, discrete examination of the metabolic activity of different tissues within a particular organ. Finally, in view of the authors' interest in the regulation of muscle protein mass and the metabolic basis that underlies changes in physical performance, it ought to be recognized that the PET approach described briefly above does not directly lead to an estimate of the rate of muscle protein breakdown. However, it would be entirely feasible to combine PET with one of the stable isotope protocols described elsewhere in this volume. The model that is proposed is the one described by Rathmacher et al. (1995), involving a bolus dose of L-3[2H3-

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability methyl]histidine given into the vein, followed by compartment modeling of the plasma tracer data. This is based on the principle that 3-methylhistidine, a component of actin and myosin, is a marker of muscle protein breakdown (Young and Munro, 1978). The model described by Rathmacher and coworkers (1995) is presented in Figure 10-10; for the present purpose it is shown to illustrate yet another example of the research value to be derived from a combined stable isotope-PET approach for in vivo assessment of protein metabolism in human subjects. AUTHORS' SUMMARY AND CONCLUSIONS This paper has attempted to illustrate the potential, as well as the limitations, of PET as an investigative technique in nutritional research. Within the past 10 years, major improvements in the production and handling of short-lived isotopes have occurred (Stocklin and Pike, 1993), and this has gone in step with advances in the design and use of instrumentation for radiation detection and tomography. Now, more than a hundred PET groups have been established worldwide. It might be anticipated, therefore, that there will be an expanded use of PET in nutritional-metabolic research. However, at the same time, it is recognized that PET is not likely to become widely available to the entire nutri- FIGURE 10-10 Schematic of a three-compartment model to analyze the kinetics of distribution, metabolism, and de novo production of 3-methylhistidine (3-MH) in human subjects. M1, M2, and M3 represent the mass of 3-MH in compartments 1, 2, and 3, respectively. L2,1, L1,2, L2,3, and L3,2 are fractional transfer rate coefficients of 3-MH within the system. The tracer, 3-[2H3-methyl]-methylhistidine (D33-MH), is injected into compartment 1. Sampling is performed from compartment 1(V). De novo production of 3-MH occurs in compartment 3. SOURCE: Adapted from Rathmacher et al. (1995).

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability tional research community, even in the United States, and so it will probably remain a valuable technique at a limited number of academic research centers during the foreseeable future. Finally, in preparing this paper the authors were asked to address a number of questions that relate to the technology discussed, and these, together with their brief answers, are presented in Table 10-6. As pointed out by Faulkner et al. (1991), the technology associated with position imaging is extremely complex; a successful PET clinical research program requires a highly skilled team of scientists and technologists working in well-equipped, specialized laboratories and involves a significant annual financial expenditure. Nevertheless, the authors believe that it is entirely reasonable to suggest that, where the necessary mixture of technical expertise, collaborative interest and physical facilities exist, or where there is an obvious institutional commitment, PET must be considered among the exciting new and emerging technologies for nutritional and metabolic research. TABLE 10-6 PET in Reference to Emerging Technologies: Some Questions and Answers Will the technology be a significant improvement over current technologies? There is no alternative to a noninvasive ''view" of the distribution of human amino acid metabolism in vivo. How likely is the technology to mature sufficiently for practical use, and if so, how soon will it be available? It is available now but only in selected medical-academic centers. Cost-benefit ratio? High cost: $2 million for camera, $2 million for cyclotron. Benefit: Probably also very high, but who knows? What about Small Business Innovative Research? Certainly in the area of tracer development. Practicality? It is demanding and complex but exciting! Multidisciplinary. Field-testing scenarios? Not at all.

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Emerging Technologies for Nutrition Research: Potential for Assessing Military Performance Capability DONALD McCORMICK: I am thinking of cases where you have a carbon and a nitrogen, and you generate an azide that is very toxic. VERNON YOUNG: Oh, but the actual amount is so small. I thought what you were going to do was to ask me what the radiation load was, which I think is a very fair question. Well, I will tell you, since you did not ask. (Laughter) I understand that in studies of this kind the radiation dose is roughly somewhere between a CAT scan and a barium enema. It is certainly in that range. ARTHUR ANDERSON: I am intrigued by the bone marrow observations that you made. Have you gotten any preliminary evidence now about whether the protein synthesis is related to cell division versus antibody production? VERNON YOUNG: Oh, no. That is a darned good question. What this [PET] actually does is that it gives you rates of biochemical processes, but it does not tell you what that process is. So this particular approach does have significant limitations, but I think the combination of approaches is extremely powerful. DENNIS BIER: I do not want to ask a question, but earlier Vernon said he only had one regret, that he was getting on a plane. He was too embarrassed to tell you that, really, his other real regret is that as a young person and a British citizen, he was not a member of the United States military. (Laughter) VERNON YOUNG: Nor was I member of the British military. (Laughter)

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