Amino acids and proteins - new definitions and requirements, hormonal interactions, methodological advances and pitfalls (1999)

In this issue we are pleased to feature papers by a distinguished group of clinically oriented investigators who provide comprehensive information directly relevant to protein anabolic and catabolic response in man. It is becoming clear that the estimated daily requirements, essentiality and metabolism of certain amino acids during both sickness and health needs to be re-evaluated. Several of the reviews cover this important aspect of clinical nutrition investigation, the results of which will be relevant for such diverse areas as public health planning for prevention of malnutrition in the developing world to the administration of specialized amino acid formulations in hospitalized patients. The role of classical nitrogen and tracer balance techniques to determine amino acid needs in humans and the utility of different isotopic dilution methods to determine whole body and tissue protein metabolism are discussed in depth. Amino acid metabolism and therapy during sepsis and liver dysfunction remain important areas of controversy. This issue of Current Opinion in Clinical Nutrition and Metabolic Care includes in-depth summaries of data on molecular aspects of counter-regulatory hormone-cytokine interactions in mediating protein catabolism during sepsis and on amino acid and ammonia metabolism during hepatic failure and encephalopathy. Specific amino acid precursor substrates, including glutathione and α-ketoglutarate (α-KG), appear to exert beneficial effects on whole body protein anabolism, cell proliferation and tissue function and are receiving increased attention as possible therapeutic agents. Anabolic hormones including growth hormone and insulin-like growth factor-I (IGF-I) enhance utilization of nutrients for anabolism and improve wound healing when given endogenously. Several papers in this section concern the efficacy and safety of selected amino acid-related substrates and hormones with discussion of the relevant data derived from both animal models and recent clinical trials. The elegant review by Hasselgren and Fischer covers a critical area of metabolism, namely the role and mechanisms of counter-regulatory hormones, in particular glucocorticoids, and cytokines, in mediating skeletal muscle amino acid proteolysis during sepsis and other catabolic states (pp. 9-14). New information relevant to clinical amino acid therapy is provided on the interactive effects of these hormones at the molecular level. For example, the role of glucocorticoids and cytokines in muscle glutamine synthesis, cellular transport and export are covered in depth; these data should facilitate studies designed to assess mechanisms of glutamine anabolic actions during administration of this amino acid in catabolic patients. In the first issue of this journal, several authors touched upon the possible role of growth factors in human protein metabolism. At the time of submission, an association between growth hormone administration and increased mortality was reported to regulatory agencies from a large multicenter European study in critically ill patients. In the footnote, we requested a critical review concerning this finding; however, data from this trial currently remain unpublished. The review by Wilmore provides critical and straightforward information and suggestions on the controversial topic of anabolic hormone therapy, using recent clinical studies of growth hormone administration as a primary case example (pp. 15-21). Previous clinical trials with recombinant peptides and anabolic steroids, including growth hormone, IGF-I, testosterone and oxandrolone, have suggested the possible clinical utility of these agents to enhance protein anabolism [1-3]. These hormones variously enhance nitrogen retention and skeletal muscle function and exert tissue specific effects (e.g. wound healing and intestinal absorptive capacity) in animal models and in some clinical settings. The overall lesson from this article is that clinical investigations into the use of anabolic hormones must also take into account the patients' underlying nutritional and clinical status, physical activity, concomitant medications and other important variables that regulate tissue sensitivity and thus the effectiveness of these agents. As discussed by Wilmore, for optimal effects, anabolic agents must be administered in a milieu that includes provision of all necessary amino acids and micronutrients, optimal target-tissue oxygenation and absence of end-stage organ dysfunction. Standardization of these factors needs to be incorporated into clinical trials, the design and performance of which should ideally include investigators with specialized training in clinical nutrition and metabolism. Although growth factors and anabolic steroids clearly increase net body protein anabolism, the value of this effect and the risk/benefit ratio of these agents in clinical care remain unclear. For example, growth hormone may exert deleterious effects on splanchnic blood flow and oxygen consumption in the setting of sepsis [4,5]. Anabolic agents may also inhibit skeletal muscle release and thus systemic provision of key amino acids such as glutamine in severely catabolic patients [6]. Further, certain of the metabolic and clinical effects of growth hormone, IGF-I and anabolic steroids are conflicting in available clinical reports [7-9]. Several double-blind studies are currently being conducted that will define the metabolic and clinical usefulness of these agents in catabolic and malnourished patients. Additional data on adverse effects of growth hormone and other agents in patient subgroups such as the critically ill and the elderly will be forthcoming in the near future as several large blinded studies are completed and analyzed. Two excellent reviews highlight the critical questions related to assessment and interpretation of protein turnover. The information from Reeds and Davis (pp. 23-28) concerning applicability, advantages, drawbacks and pitfalls of isotopic dilution are of great value for all scientists dealing with protein synthesis and breakdown. In the past, it was generally accepted that measurement of whole body protein turnover truly reflects changes in protein synthesis and degradation in muscle. Deutz et al. (pp. 29-32) emphasize the discrepancy between muscle and whole body turnover. It is obvious that the muscle protein pool turns over slowly compared to other tissues like gut, liver, skin and kidney, contributing appreciably to whole body fractional rate of protein synthesis. On the other hand, it should be remembered that there are numerous protein fractions in muscle tissue which may possess different turnover rates. Few of these have been measured individually in man. At present, the question is how much these fractions contribute to the fractional synthesis rate of mixed muscle protein. Indeed, there are claims that high turnover enzymes may contribute not less than 30% to the total muscle protein. The interesting approach outlined in this review will certainly be the scope of future discussion and evaluations until the proposed concepts find practical application. Undoubtedly, the efficacy of providing substrates to the injured, immunocompromised and/or malnourished host has caused a re-awakening of interest in the clinical application of dietary intervention in the treatment and prevention of disease [10]. As described by Cynober, clinical application of αKG and αKG-containing salts open fascinating new perspectives for the treatment of patients suffering from catabolic illness (pp. 33-37). Ornithine αKG, a true new substrate, exerts beneficial effects such as improvement of gut morphology and function, reduction of trauma-induced immunosuppression and anti-catabolic action on protein metabolism. There are numerous underlying mechanisms proposed which might account for observed beneficial effects of αKG including glutamine formation, arginine, proline, polyamine generation, increase in growth hormone and insulin secretion etc. A principal question is whether αKG is a true precursor for glutamine. According to textbooks, αKG is the precursor of glutamic acid while its precursor function for glutamine is limited. Accordingly, several reports are demonstrating that the in vivo transformation from glutamic acid to glutamine is restricted, corresponding to not more than 5-6% [11]. Certainly, labeled αKG might be recovered in tissue glutamine, yet this finding does not reflect the extent of transformation in a quantitative manner. Since glutamic acid is known to be poorly transported across the cell membrane, it can be concluded that αKG or related compounds cannot replace glutamine in clinical nutrition unless αKG is directly taken up by cells and converted first to glutamic acid and subsequently to glutamine. This theoretical pathway, however, has not yet been established. Young and Borgonha propose a new set of amino acid requirement values for healthy adults (pp. 39-45). These recommendations are based on the use of 13C tracer experiments considering predicted obligatory oxidative losses [12]. Tentative estimates on these techniques are much higher than earlier assessment of the requirements based on the classical nitrogen balance approach. At present, an essential amino acid has been variously described as conditionally essential, indispensable, functional or acquired indispensable. It has been proposed that regardless of the definition used, a final judgment of the usefulness of an essential amino acid will be on grounds of clinical and nutritional efficacy [13]. According to a more general position, as outlined in the present review, a possible useful direction might put more emphasis on metabolic control and its regulation of tissue and organ function and nutritional status. This definition offers suggestions as to how certain metabolic characteristics shared by amino acids might be used to differentiate the various nutritionally significant amino acids. This would mean that the dietary essentiality of a given amino acid is dependent on the ratio of supply to demand; the distinction between ‘essential’ and ‘nonessential’ largly disappears because it is dependent on conditions [10,14]. Indeed, the notions as highlighted in this review are of utmost importance and may serve as future guidelines for adequate nutritional support in stress and disease. Holm et al.'s scholarly review (pp. 47-53) offers an interesting approach by recalling old knowledge in reflection of new discoveries and stating controversies which remain to be clarified. A highly relevant topic relates to the deficiency of sulphydryl amino acids and glutathione which, as described, is probably due to hepatic dysfunction. In this respect it is pertinent to note that the above compounds are potent antioxidants and cellular mediators. Cysteine enhances cytotoxic T-cell activity. A high glutamate/cysteine ratio is associated with a low proportion of T-helper cells. Reduced glutathione and cysteine inhibit the expression of the nuclear transcription factor in stimulated T-cell lines. This observation might provide an interesting approach in the treatment of viral disease since the transcription factor enhances human virus mRNA expression. Stimulatory effects of tumor necrosis factor, induced by free radicals, on virus replication in monocytes can be inhibited by S-containing antioxidants. These basic studies indicate that treatment of inflammatory diseases and viral infections with sulphydryl antioxidants may be beneficial, and powerful arguments have been advanced in favor of such treatment. Clinical studies using this strategy are not yet available (for references cf [15]). The question should be raised whether modulation of sulphydryl compounds could be beneficial in the treatment of liver disease. As described in the review by Holm et al., inactivation of methionine adenosyltransferase due to free radical generation occurs in hepatic dysfunction and this is prevented by sulphydryl compounds and glutamine. It is known that glutamine supports glutathione synthesis [16,17]. A crucial part of the review is devoted to brain function as influenced by ammonia, glutamic acid and glutamine. It is well accepted that cerebral net retention of ammonia is associated with an increase of glutamine as well as a net release of glutamine which is accompanied by cell swelling. In this context it is important to note that hepatic encephalopathy is not necessarily linked with hyperammonemia but is rather coupled with the impairment of overall hepatic function. Consequently, the glutamine response might be due to an increased passage of ammonia across the blood-brain barrier. Finally, the strong influence of hyperammonemia on branched-chain amino acid (BCAA) metabolism should be mentioned. Since the plasma concentrations of BCAA are dependent on portal systemic shunting and not on hepatic failure, the BCAA/aromatic ratio cannot be accepted as an index of hepatic function. Future studies should be directed to identify and evaluate nutritional factors which participate in the progress leading to overt hepatic encephalopathy. The papers presented in this section of Protein Metabolism and Therapy serve as important references relevant to protein administration and amino acid handling and requirements in man. Studies on administration of novel combinations and doses of amino acids and peptide substrates in health and disease states are being increasingly performed. These investigations, coupled with new knowledge on mechanisms of action and regulation of amino acids, will inform dietary guidelines and specialized nutrition therapy in the future. The use of growth factors and anabolic steroids which enhance the efficiency of amino acid utilization for protein accrual and tissue-specific functions is also being studied extensively, both alone and in combination with specific nutrients such as glutamine. Additional, well-designed blinded clinical trials are needed to define the safety and cost-effectiveness of these approaches in clinical care.