Nonnutritive effects of glutamine

Inborn errors of proline metabolism

l-Proline concentration is primarily related to the balance of enzymatic activities of proline dehydrogenase [proline oxidase (POX)] and Delta-1-pyrroline-5-carboxylate (P5C) reductase. As a result, P5C plays a pivotal role in maintaining the concentration of proline in body fluids and inborn errors of P5C metabolism lead to disturbance of proline metabolism. Several inborn errors of proline metabolism have been described. Hyperprolinemia type I (HPI) is a result of a deficiency in POX. The POX gene (PRODH) is located on chromosome 22 (22q11.2) and this region is deleted in velo-cardio-facial syndrome, a congenital malformation syndrome. In addition, this gene locus is related to susceptibility to schizophrenia. The other type of hyperprolinemia is HPII. It is caused by a deficiency in P5C dehydrogenase activity. Hypoprolinemia, on the other hand, is found in the recently described deficiency of P5C synthetase. This enzyme defect leads to hyperammonemia associated with hypoornithinemia, hypocitrullinemia, and hypoargininemia other than hypoprolinemia. Hyperhydroxyprolinemia is an autosomal recessive inheritance disorder caused by the deficiency of hydroxyproline oxidase. There are no symptoms and it is believed to be a benign metabolic disorder. The deficiency of ornithine aminotransferase causes transient hyperammonemia during early infancy due to deficiency of ornithine in the urea cycle. In later life, gyrate atrophy of the retina occurs due to hyperornithinemia, a paradoxical phenomenon. Finally, prolidase deficiency is a rare autosomal recessive hereditary disease. Prolidase catalyzes hydrolysis of dipeptide or oligopeptide with a C-terminal proline or hydroxyproline and its deficiency can cause mental retardation and severe skin ulcers.

Clinical use of glutamine supplementation

Endogenous production of glutamine may become insufficient during critical illness. The shortage of glutamine is reflected as a decrease in plasma concentration, which is a prognostic factor for poor outcome in sepsis. Because glutamine is a precursor for nucleotide synthesis, rapidly dividing cells are most likely to suffer from a shortage. Therefore, exogenous glutamine supplementation is necessary. In particular, when i.v. nutrition is given, extra glutamine supplementation becomes critical, because most present formulations for i.v. use do not contain any glutamine for technical reasons. The major part of endogenously produced glutamine comes from skeletal muscle. For patients staying a long time in the intensive care unit (ICU), the muscle mass decreases rapidly, which leaves a tissue of diminishing size to maintain the export of glutamine. The metabolic and nutritional adaptation in long-staying ICU patients is poorly studied and is one of the fields that needs more scientific evidence for clinical recommendations. To date, there is evidence to support the clinical use of glutamine supplementation in critically ill patients, in hematology patients, and in oncology patients. Strong evidence is presently available for i.v. glutamine supplementation to critically ill patients on parenteral nutrition. This must be regarded as the standard of care. For patients on enteral nutrition, more evidence is needed. To guide administration of glutamine, there are good arguments to use measurement of plasma glutamine concentration for guidance. This will give an indication for treatment as well as proper dosing. Most patients will have a normalized plasma glutamine concentration by adding 20-25 g/24 h. Furthermore, there are no reported adverse or negative effects attributable to glutamine supplementation.

The 7th workshop on the assessment of adequate intake of dietary amino acids: summary of general discussion

Extensive discussion sessions were held at the end of each of the 2 d of the workshop. Through the course of the workshop, it became clear that there were different opinions on how to use uncertainty factors to obtain upper levels of intake from no observed adverse effect levels of a particular nutrient and that the selection of an appropriate uncertainty factor would be rather arbitrary. Much of the discussion centered around the potential for using metabolic limits, expressed as the level of intake at which the major pathway of metabolism may approach saturation and at which the amino acid is metabolized by alternative pathways, as a measurable early or surrogate marker for amino acid excess and possible toxicity. After extensive discussion on various conditions that would need to be satisfied for metabolic limits to be used as markers of excessive intake of amino acids, there was a general consensus that methods such as measuring oxidation limits are an attractive approach that merit future investigation. It was noted that there are many data on the clinical use of glutamine, whereas data for proline are very scarce. There was recognition that regardless of the available data, there is regulatory pressure for setting upper levels of intake for amino acids and that much more data are required.

Comparative aspects of tissue glutamine and proline metabolism

The cellular metabolism of glutamine and proline are closely interrelated, because they can be interconverted with glutamate and ornithine via the mitochondrial pathway involving pyrroline-5-carboxylate (P5C). In adults, glutamine and proline are converted via P5C to citrulline in the gut, then citrulline is converted to arginine in the kidney. In neonates, arginine is a semiindispensable amino acid and is synthesized from proline completely in the gut; because of low P5C synthase activity, glutamine is not an important precursor for neonatal arginine synthesis. Thus, splanchnic metabolism of glutamine and proline is important, because both amino acids serve as key precursors for arginine synthesis with some developmental differences. Studies investigating splanchnic extraction demonstrate that about two-thirds of dietary glutamine and almost all dietary glutamate are extracted on first pass and the vast majority is oxidized in the gut. This capacity to extract glutamine and glutamate appears to be very large, so diets high in glutamine or glutamate probably have little impact on circulating concentrations and consequent potential toxicity. In contrast, it appears that very little proline is extracted by the gut and liver, at least in the neonate, which may result in hyperprolinemia and potential toxicity. Therefore, the upper limits of safe dietary intake for glutamine and proline, and other amino acids, appear to be substantially different depending on the extent of first-pass splanchnic extraction and irreversible catabolism.

Nutrient risk assessment as a tool for providing scientific assessments to regulators

Regulatory officials world-wide are paying attention to the process for establishing the upper level of intake for nutrient substances. The rapidly expanding use of dietary supplements, fortified foods, and functional foods, coupled with increased trade in these products, has focused attention on ensuring their safety and on harmonizing standards internationally. The more traditional approaches, in which the regulators either provided no standards for upper levels of intake or developed standards based on some arbitrary multiple of the intake level known to provide an adequate amount of the nutrient, are recognized as outdated or inappropriate for the emerging issues. Preferred approaches are those that rely on the systematic scientific assessment of risk to determine the levels of intake below which no harm may occur. The scientific study of risk is playing an increased role in establishing the regulatory upper levels of "safe" nutrient intake. Risk assessment, as a component of risk analysis, offers a scientific basis for regulatory decision-making regarding the regulators' task associated with specifying safe upper levels of intake for nutrient substances. This article describes the key components of risk assessment as they are applied within the nutrition field. Although regulatory frameworks vary from country to country and all countries retain their right to determine their own level of protection, regulatory systems operate most effectively and are more likely to converge toward harmonization if they are informed by independent, organized, and scientific reviews that are conducted systematically in a transparent manner.

The metabolism of proline as microenvironmental stress substrate

Proline, a unique proteogenic secondary amino acid, has its own metabolic system with special features. Recent findings defining the regulation of this system led us to propose that proline is a stress substrate in the microenvironment of inflammation and tumorigenesis. The criteria for proline as a stress substrate are: 1) the enzymes utilizing proline respond to stress signaling; 2) there is a large, mobilizable pool of proline; and 3) the metabolism of proline serves special stress functions. Studies show that the proline-utilizing enzyme, proline oxidase (POX)/proline dehydrogenase (PRODH), responds to genotoxic, inflammatory, and nutrient stress. Proline as substrate is stored as collagen in extracellular matrix, connective tissue, and bone and it is rapidly released from this reservoir by the sequential action of matrix metalloproteinases, peptidases, and prolidase. Special functions include the use of proline by POX/PRODH to generate superoxide radicals that initiate apoptosis by intrinsic and extrinsic pathways. Under conditions of nutrient stress, proline is an energy source. It provides carbons for the tricarboxylic acid cycle and also participates in the proline cycle. The latter, catalyzed by mitochondrial POX and cytosolic pyrroline-5-carboxylate reductase, shuttles reducing potential from the pentose phosphate pathway into mitochondria to generate ATP and oxidizing potential to activate the cytosolic pentose phosphate pathway.

Dosing and efficacy of glutamine supplementation in human exercise and sport training

Some athletes can have high intakes of l-glutamine because of their high energy and protein intakes and also because they consume protein supplements, protein hydrolysates, and free amino acids. Prolonged exercise and periods of heavy training are associated with a decrease in the plasma glutamine concentration and this has been suggested to be a potential cause of the exercise-induced immune impairment and increased susceptibility to infection in athletes. However, several recent glutamine feeding intervention studies indicate that although the plasma glutamine concentration can be kept constant during and after prolonged strenuous exercise, the glutamine supplementation does not prevent the postexercise changes in several aspects of immune function. Although glutamine is essential for lymphocyte proliferation, the plasma glutamine concentration does not fall sufficiently low after exercise to compromise the rate of proliferation. Acute intakes of glutamine of approximately 20-30 g seem to be without ill effect in healthy adult humans and no harm was reported in 1 study in which athletes consumed 28 g glutamine every day for 14 d. Doses of up to 0.65 g/kg body mass of glutamine (in solution or as a suspension) have been reported to be tolerated by patients and did not result in abnormal plasma ammonia levels. However, the suggested reasons for taking glutamine supplements (support for immune system, increased glycogen synthesis, anticatabolic effect) have received little support from well-controlled scientific studies in healthy, well-nourished humans.

Proline precursors to sustain Mammalian collagen synthesis

Biochemically, one-third of the collagen molecule is composed of glycine. The next largest amino acid component is formed by proline (PRO) and hydroxyproline, which together comprise approximately 23% of the collagen molecule. The best method to support wound collagen biosynthesis is to provide adequate host nutrition, assuring adequate provision of calories and protein. However, despite adequate nutrition, clinically, there is a need to enhance collagen synthesis and research has focused on methods to enhance collagen precursor availability. PRO biosynthesis is related to both the citric acid cycle and the urea cycle. During the early phases of wound healing, wound fluid PRO levels are at least 50% higher than plasma levels, suggesting active import of PRO into the wound. Providing additional PRO in the diet to enhance PRO bioavailability for collagen biosynthesis does not result in increased collagen accumulation. Provision of other citric cycle precursors such as glutamine also does not enhance wound collagen synthesis. In looking at other PRO biosynthetic pathways, the arginine (ARG) --> ornithine (ORN) --> glutamic semialdehyde --> PRO pathway looks the most promising. ARG administration in quantities above those required for growth and reproduction results in a marked enhancement in wound collagen deposition. This effect is also shared by ORN, which cannot replace ARG for growth requirement but shares many of its biological and pharmacological activities. Several mechanisms have been postulated to explain the positive effect of ARG on wound healing, although none have been firmly proven. In conclusion, ARG and ORN supplementation are most effective in increasing collagen deposition, but whether this is accomplished by conversion to PRO is uncertain.

Expanded approach to tolerable upper intake guidelines for nutrients and bioactive substances

The original tolerable upper intake level (UL) method greatly improved the application of risk assessment to the evaluation of nutrient safety for humans, but a UL is only set where the data establish a hazard resulting from high intakes. Absence of a UL for those nutrients with no established hazard has been misinterpreted by regulators and resulted in overly restrictive policies. To prevent such misinterpretation, the observed safe level (OSL) was developed and defined as "the highest intake with convincing evidence of safety, even if there are no established adverse effects at any level." More recently, a FAO/WHO report gave a similar definition for the highest observed intake (HOI). Another disadvantage of the UL method is the application of arbitrary uncertainty factors (UF). An alternative to the traditional adjustment for uncertainty involves arranging the data in decreasing order of daily intake, followed by evaluation of each trial for quantity and quality of data. Studies are selected downward until no adverse effects are observed in a trial of sufficient quality to justify no further correction for uncertainty (i.e. selection of data that qualify for UF = 1). Thus, the no observed adverse effect level or OSL selected requires no further adjustment for uncertainty. For supplemental intakes of some vitamins, many bioactive substances, and most amino acids, no adverse effects that are clearly related to high intakes have been established, but where the dataset is sufficiently robust, application of the OSL-HOI technique can provide risk assessment values.

Glutamine metabolism and function in relation to proline synthesis and the safety of glutamine and proline supplementation

At normal intakes, dietary glutamine and glutamate are metabolized by the small intestine and essentially all glutamine within the body is synthesized de novo through the action of glutamine synthetase. The major sites of net glutamine synthesis are skeletal muscle, lung, and adipose tissue and, under some conditions, the liver. In addition to the small intestine, where glutamine is the major respiratory fuel, other sites of net glutamine utilization include the cells of the immune system, the kidneys, and the liver. The intestine expresses pyrroline 5-carboxylate (P5C) synthase, which means that proline is an end product of intestinal glutamine catabolism. Proline can also be synthesized from ornithine and the exact contribution of the 2 pathways is not certain. Infusion of proline i.v. to increase circulating concentrations is associated with increased proline oxidation and decreased proline synthesis. In contrast, conditions of proline insufficiency, after feeding low-proline diets or in response to high rates of proline catabolism in burn patients, do not result in increased proline synthesis. Glutamine supplementation is widespread and up to 0.57-0.75 g.kg(-1).d(-1) is well tolerated. Similarly, the only study of proline supplementation, in which patients with gyrate atrophy were given 488 mg.kg(-1).d(-1), reported no deleterious side effects. In the absence of controlled trials, it is currently not possible to estimate a safe upper limit for either of these 2 amino acids.

An approach to defining the upper safe limits of amino acid intake

The existing data on the safe upper limits of amino acid intake in humans is essentially observational; how much do individuals ingest and what side effects do they have? There are numerous studies in humans comparing the effects of high doses of amino acids given as protein bound vs. as free amino acids. These studies have shown that protein-bound amino acids have much less effect on plasma levels of the test amino acid, because protein intake stimulates protein synthesis as another sink for the increased amino acid intake. In practice, the highest amino acid intakes occur with free amino acid supplements that may be ingested by athletes who believe that the amino acids will benefit them in training and/or performance. Previously, in a piglet study, we were able to define the point at which maximal phenylalanine oxidation occurred, above which plasma phenylalanine concentration and body balance rose exponentially. We regard this value of maximal disposal (oxidation) of an amino acid as one metabolic marker of the upper limit of intake. Recently, others have demonstrated a similar maximal oxidation rate for leucine in rats. Based on these experimental data and the paucity of published human data in controlled experiments, we think that a systematic approach needs to be undertaken to define the maximal oxidation rate for all dietary indispensable amino acids and other amino acids that may be ingested in excess by humans. We believe that this will provide a rational basis to begin to define the upper limits of tolerance for dietary amino acids. However, some amino acids, such as threonine and methionine, will be more difficult to study, because they have more than 1 route of disposal or very complex metabolic regulation, in which case defining their upper limits will be more multifaceted.