The use of stable isotopes in metabolic investigation

Valuing the Diversity of Research Methods to Advance Nutrition Science

The ASN Board of Directors appointed the Nutrition Research Task Force to develop a report on scientific methods used in nutrition science to advance discovery, interpretation, and application of knowledge in the field. The genesis of this report was growing concern about the tone of discourse among nutrition professionals and the implications of acrimony on the productive study and translation of nutrition science. Too often, honest differences of opinion are cast as conflicts instead of areas of needed collaboration. Recognition of the value (and limitations) of contributions from well-executed nutrition science derived from the various approaches used in the discipline, as well as appreciation of how their layering will yield the strongest evidence base, will provide a basis for greater productivity and impact. Greater collaborative efforts within the field of nutrition science will require an understanding that each method or approach has a place and function that should be valued and used together to create the nutrition evidence base. Precision nutrition was identified as an important emerging nutrition topic by the preponderance of task force members, and this theme was adopted for the report because it lent itself to integration of many approaches in nutrition science. Although the primary audience for this report is nutrition researchers and other nutrition professionals, a secondary aim is to develop a document useful for the various audiences that translate nutrition research, including journalists, clinicians, and policymakers. The intent is to promote accurate, transparent, verifiable evidence-based communication about nutrition science. This will facilitate reasoned interpretation and application of emerging findings and, thereby, improve understanding and trust in nutrition science and appropriate characterization, development, and adoption of recommendations.

Measurements of Gluconeogenesis and Glycogenolysis: A Methodological Review

Gluconeogenesis is a complex metabolic process that involves multiple enzymatic steps regulated by myriad factors, including substrate concentrations, the redox state, activation and inhibition of specific enzyme steps, and hormonal modulation. At present, the most widely accepted technique to determine gluconeogenesis is by measuring the incorporation of deuterium from the body water pool into newly formed glucose. However, several techniques using radioactive and stable-labeled isotopes have been used to quantitate the contribution and regulation of gluconeogenesis in humans. Each method has its advantages, methodological assumptions, and set of propagated errors. In this review, we examine the strengths and weaknesses of the most commonly used stable isotopes methods to measure gluconeogenesis in vivo. We discuss the advantages and limitations of each method and summarize the applicability of these measurements in understanding normal and pathophysiological conditions.

Parallel labeling experiments and metabolic flux analysis: Past, present and future methodologies

Radioactive and stable isotopes have been applied for decades to elucidate metabolic pathways and quantify carbon flow in cellular systems using mass and isotope balancing approaches. Isotope-labeling experiments can be conducted as a single tracer experiment, or as parallel labeling experiments. In the latter case, several experiments are performed under identical conditions except for the choice of substrate labeling. In this review, we highlight robust approaches for probing metabolism and addressing metabolically related questions though parallel labeling experiments. In the first part, we provide a brief historical perspective on parallel labeling experiments, from the early metabolic studies when radioisotopes were predominant to present-day applications based on stable-isotopes. We also elaborate on important technical and theoretical advances that have facilitated the transition from radioisotopes to stable-isotopes. In the second part of the review, we focus on parallel labeling experiments for (13)C-metabolic flux analysis ((13)C-MFA). Parallel experiments offer several advantages that include: tailoring experiments to resolve specific fluxes with high precision; reducing the length of labeling experiments by introducing multiple entry-points of isotopes; validating biochemical network models; and improving the performance of (13)C-MFA in systems where the number of measurements is limited. We conclude by discussing some challenges facing the use of parallel labeling experiments for (13)C-MFA and highlight the need to address issues related to biological variability, data integration, and rational tracer selection.

Testosterone metabolic clearance and production rates determined by stable isotope dilution/tandem mass spectrometry in normal men: influence of ethnicity and age

The metabolic clearance rate (MCRT) and production rate (PRT) of testosterone (T) were measured using constant infusion of trideuterated (d3) T and quantitating serum d3T by liquid chromatography-tandem mass spectrometry (LC-MS-MS). Serum unlabeled T (d0T) was measured by LC-MS-MS, and serum total T (d3T + d0T) was measured by RIA. Mean MCRR (measured by LC-MS-MS) in young white men (1272 +/- 168 liters/d) was not significantly different from young Asian men (1070 +/- 166 liters/d). Mean PRT was also not significantly different between the two ethnic groups (whites, 9.11 +/- 1.11 mg/d; Asians, 7.22 +/- 1.15 mg/d; P = 0.19 using d0T data). Both the mean MCRR (812 +/- 64 liters/d; P < 0.01) and the PRT (3.88 +/- 0.27 mg/d; P < 0.001) were significantly lower in middle-aged white men when compared with their younger counterparts. The mean MCRR and PRR calculated using serum total T or d0T data showed a diurnal variation, with levels at midday significantly higher than those measured in the evening in the young (MCRT, P < 0.01; PRT, P < 0.001) and to a lesser extent in the older men (MCRT, P < 0.05; PRT, P < 0.05 using total T and P < 0.001 using d0T data). We conclude that using LC-MS-MS to detect d3T in serum after constant infusion of stable isotope-labeled T allows the measurements of MCRT and PRT, which can be used to study androgen metabolism repeatedly after physiological or pharmacological interventions.

Liquid chromatography-tandem mass spectrometry assay for human serum testosterone and trideuterated testosterone

A liquid chromatography tandem mass spectrometry assay for serum testosterone (T) and trideuterated testosterone (d(3)T) was developed in order to support clinical research studies that determine the pharmacokinetics, production rate, and clearance of testosterone by administration of trideuterated testosterone. After adding 19-nortestosterone as the internal standard (I.S.), sodium acetate buffer, and ether, to a serum aliquot, the mixture was shaken and centrifuged, and the ether was dried. The extract was reconstituted in methanol and 15 microl was injected into a liquid chromatograph equipped with an autosampler and Applied Biosystems-Sciex API 300 triple quadrupole mass spectrometer operated in the positive ion mode. T, d(3)T, and I.S. were monitored with transitions m/z 289 to m/z 97, m/z 292 to m/z 97, and m/z 275 to m/z 109, respectively. The two calibration curves were linear over the entire measurement range of 0-20 ng/ml for T and 0-2.0 ng/ml for d(3)T. The LOQs for T and d(3)T were 0.5 ng/ml and 0.05 ng/ml. The recoveries for T and d(3)T were 91.5 and 96.4%. For T at 1.25 ng/ml and 4.0 ng/ml, the intra-day precision (RSD, %) was 3.9 and 4.3% and intra-day accuracy 0.01 and 4.5%, respectively. The inter-day precision at these levels was 5.3 and 5.4% and inter-day accuracy was 1.9 and 0.3%. For d(3)T at 0.125 ng/ml and 0.4 ng/ml, the intra-day precision (RSD, %) was 2.8 and 8.3% and intra-day accuracy was 1.8 and 5.6%. The inter-day precision at these levels was 10.0 and 7.6% and inter-day accuracy was 5.7 and 3.4%. The concentrations of T in the 38 healthy subjects ranged from 2.5 to 14.0 ng/ml (mean 6.2 ng/ml).

Stable isotope approaches, applications, and issues related to polyunsaturated fatty acid metabolism studies

The use of stable isotope tracers for investigating fatty acid metabolism in human subjects has increased substantially over the last decade. Advances in analytical instrumentation, commercial availability of labeled substrates, and safety considerations are major reasons for this increased use of stable isotope tracers. Several experimental design options are available for using either deuterium or carbon-13 as tracers for fatty acid and lipid studies. Options include feeding a pulse dose of labeled fat or a mixture containing two or more labeled fats. Multiple doses of the labeled fat can be fed at timed intervals to increase enrichments. Administration by injection or continuous intravenous infusion is an alternative. Another option is to use diets containing foods from plants that have slightly higher natural carbon-13 enrichment. Each basic experimental design has its specific strengths, and the best choice of experimental design depends on the study objectives. Stable isotope studies have been used to address a variety of questions related to unsaturated fatty acid metabolism in humans. Examples are provided that illustrate the use of stable isotopes to investigate oxidation of docosahexaenoic acid, desaturation of linoleic and linolenic acids in infants and adults, incorporation of long-chain n-6 and n-3 fatty acids, bioequivalency of linolenic acid in primates, 13C nuclear magnetic resonance spectra of arachidonic acid in living rat brain, and effect of triacylglycerol structure on absorption. Radioisotope and stable isotope tracer studies in animals and humans are responsible for much of our understanding of fatty acid and lipid metabolism. However, tracer studies have limitations, and there are some unresolved issues associated with isotope studies. Examples of unresolved issues are quantification of isotope data, validity of in vivo fatty acid metabolite results, kinetic modeling, subject variability, and use of blood lipid data as a reflection of tissue lipid metabolism. Resolving these issues, developing novel methodology, and applying stable isotope tracer methods to questions related to PUFA metabolism are broad areas of interesting and challenging research opportunities.

1999 Jonathan E. Rhoads lecture. Isotopic metaprobes, nutrition, and the roads ahead

The 1999 Jonathan E. Rhoads lecture, delivered by Vernon R. Young at the annual meeting of American Society for Parenteral and Enteral Nutrition (A.S.P.E.N.), San Diego, February 2, 1999, with the printed version coauthored with Alfred M. Ajami, is concerned with the application of isotopic probes and how, in particular, they may be used as diagnostic tools to enhance the role of nutrition in the comprehensive medical management of the patient. Following a brief review of the early uses of stable isotopes in metabolic research we consider the present and possible future application of stable isotope probes. The concept of a "gateway" enzyme in a discrete biochemical pathway and how the flow of substrate through this step might be assessed by giving a "metaprobe" is developed. The specific and desirable structural requirements of the metaprobe are considered. A number of examples are given that further exploit the concepts of "underground" metabolism and of metabolic "hijackers." It is our view that we are on the verge of a new era where, for the many pragmatic and exciting reasons discussed, stable isotope probes will find and increasing use in the practice of clinical medicine and in the preventive and public health areas.

The Rudolf Schoenheimer Centenary Lecture. Isotopes in nutrition research

The present lecture begins with a brief overview of the professional and scientific journey taken by Rudolf Schoenheimer, before turning to a discussion of the power of isotopic tracers in nutrition research. Schoenheimer's remarkable contributions to the study of intermediary metabolism and the turnover of body constituents, based initially on compounds tagged with 2H and later with 15N, spanned a mere decade. It is difficult, however, to overestimate the enormous impact of Schoenheimer's research on the evolution of biological science. After a relative hiatus, following Schoenheimer's death in 1941, in the use of stable nuclides as tracers in metabolism and nutrition, especially in human subjects, there is now an expanded and exciting range of techniques, experimental protocols and stable-isotope tracer compounds that are helping to probe the dynamic aspects of the metabolism of the major energy-yielding substrates, amino acids and other N-containing compounds, vitamins and mineral elements in human subjects. Various aspects of the contemporary applications of these tracers in nutrition research are covered in the present lecture.

The use of stable isotope techniques for nutritional and metabolic research in paediatrics

Stable isotope methods are increasingly used in paediatrics for clinical diagnosis and research due to marked improvements in analytical technologies and better availability of suitable tracers. The safety of stable isotopes is of major importance for use in children. Large amounts of deuterium well above the doses applied under clinical conditions may induce adverse effects. In contrast, heavier stable isotopes such as 13C, 15N or 18O do not induce adverse effects even at the highest enrichments obtained, and they are safe. Breath tests with measurements of 13CO2 enrichment after oral application of a tracer can reliably evaluate helicobacter pylori infection and gastric emptying kinetics. Combined with an estimation of total CO2 production, 13CO2 breath tests allow estimation of the absorption and oxidation of 13C-labelled substrates, such as medium- and long-chain triglycerides, and demonstrates the beneficial effect of carnitine supplements on fat oxidation in primary carnitine deficiency. The study of metabolic processes may require the sampling of blood for isotopic analyses of metabolites of the applied tracer. Gas chromatography-isotope ratio mass spectrometry can detect tracer in individual components from small plasma samples. The high precision enabled us to utilize the small differences in natural 13C-enrichment between dietary fats to study fatty acid turnover in term infants, in whom the dietary fat source was switched to corn oil with a slightly higher 13C-content. With this approach we demonstrated active conversion of linoleic into arachidonic acid. We also applied biotechnologically produced, U-13C labelled linoleic and alpha-linolenic acids to infants and detected the conversion of these essential fatty acids to their longer chain polyunsaturated derivatives, with an apparent change of conversion activity with age. Moreover, it has become possible to measure tissue protein synthesis from small biopsy samples obtained in situ without surgery, such as forceps biopsies of rectal tumors. These few examples of recent developments demonstrate the great clinical and scientific potential of stable isotope methods in future paediatric applications.

Protocol development for biological tracer studies

Improved instrumentation and the increased availability of labeled compounds have democratized the application of isotope-dilution (tracer) methodology in nutrient metabolism. Still, the most challenging aspects of tracer experimentation reside in the steps that precede the measurement of an isotopically labeled tracer, i.e. the design of a suitably labeled tracer and its isolation and purification from complex biological matrices. Construction of useful mathematical models of nutrient dynamics require methodologies that guarantee that the integrity of the tracer is maintained across the entire sampling and analyte isolation protocol. The ability to provide accurate and reliable data highlights a need for analytical chemists to play a central role in these studies. In this regard, examples and discussion of issues relevant to stable-isotope experimentation are provided.

Determination of testosterone production rates in men and women using stable isotope/dilution and mass spectrometry

Production rates of testosterone were determined in healthy men and women during the follicular phase of their menstrual cycle using the stable isotope dilution technique and analysis by gas chromatography/mass spectrometry. In an initial series of experiments, 0.07 mg/h (men) or 0.01 mg/h (women) 1,2-d-testosterone was infused for 36 h. After an equilibration period of 12 h, blood samples were obtained at 20-min intervals throughout 24 h. In men, no diurnal rhythmicity of testosterone production was observed, whereas in women, testosterone production rates were largest from 0400-1200 h. In a second series of experiments, the infused dose of 1,2-d-testosterone was reduced to 0.015 mg/h (men) and 0.0001 mg/h (women), respectively. Blood samples were obtained at 20-min intervals during the last 12 h (0800-2000 h) of the observation period. In accordance with results obtained by others using radioactive tracers, estimated production rates were 147 +/- 31 microg/h (3.7 +/- 2.2 mg/day in men) and 1.8 +/- 0.6 microg/h (0.04 +/- 0.01 [corrected] mg/day in women). To reduce both the duration of the experiment and the number of samples to be processed, we subsequently demonstrated that similar production rates may be obtained when the equilibration period before blood sampling is reduced to 6 h and the period of blood sampling is reduced to 4 h. This protocol is suitable for clinical use in a routine setting to obtain analytically correct estimates of testosterone production in vivo.

15N-tracer studies in formula-fed preterm infants: the role of glycine supply in protein turnover

In preterm infants, protein-turnover rates obtained by [15N]glycine as a tracer are known to be overestimated. This may reflect the insufficient supply of dietary glycine. In this randomized study, the influence of a glycine-rich diet on whole body protein turnover rates in eight male preterm infants (29-32 weeks, 1,200-2,540 g birthweight) using the 15N-tracer technique on days 21 and 28 of life was investigated to evaluate the necessity of supplementing preterm infant formulas with proteins rich in glycine. Before and during the study, the infants were alternately fed with a commercial available preterm infant formula (I, 2% protein, 40 mg glycine/dl) and a variety of this formulation with glycine-rich proteins (II, 2% protein, 130 mg glycine/dl). The protein-turnover rates were computed after 15N-single-pulse labeling with the help of the three-compartment model (TCM) and the urinary ammonia end-product method (AEPM). The tracer used was [15N]glycine (dosage: 2 mg 15N/kg). For the determination of 15N-excess-excretion kinetics, fractionated urine specimens were collected over a 36-h period. The protein-turnover rate calculated by TCM was 8.8 +/- 1.6 g/kg/day (formula I) and 7.7 +/- 2.0 g/kg/day (formula II); using AEPM, the rate was 8.7 +/- 2.5 g/kg/day and 7.5 +/- 1.5 g/kg/day, respectively. We conclude that the presaturation of the precursor pool by an adequate glycine intake minimizes drawbacks that may arise when using [15N]-glycine as a tracer in preterm infants, and a protein concentration of 2%, as in formula I, and consequently, a 170% glycine content when compared with the same human milk volume, meets the glycine requirement.

Hypoglycaemia: principles of diagnosis and treatment in children

The diagnostic approach to children with suspected hypoglycaemia is facilitated by the fact that two clinically distinct groups exist with little overlap: 1. Hypoglycaemias characterized by a fuel deficit only. 2. Hypoglycaemias in which the fuel deficit is overshadowed by symptoms of intoxication. This differentiation is used when taking the medical history, performing the physical examination, and planning the laboratory investigations. The latter may encompass tolerance tests, a fasting test or the use of a non-radioactive glucose isotope. Protocols for such tests are given.

Protein and amino acid metabolism in cancer cachexia: investigative techniques and therapeutic interventions

Cancer cachexia is a complex syndrome characterized primarily by diminished nutrient intake and progressive tissue depletion that is manifest clinically as anorexia and host weight loss. The gradual loss of host protein stores is central to this process. This review outlines the techniques that have been used to evaluate human amino acid metabolism, their application in patients with cancer cachexia, and possible therapeutic interventions designed to overcome alterations in host protein and amino acid metabolism associated with malignant cachexia. The techniques of nitrogen balance and 3-methylhistidine excretion provide indirect estimates of overall nitrogen metabolism and skeletal muscle myofibrillar protein breakdown. Measurement of circulating amino acid concentrations, particularly when combined with assessment of arterial-venous differences and regional amino acid balance allows for investigation of interorgan amino acid metabolism. One of the most significant advances in in vivo amino acid metabolic research has been the development of labeled amino acid tracer studies to evaluate whole body and regional amino acid kinetics. The use of stable and unstable amino acid isotopes in these techniques is reviewed in detail. Virtually all of these techniques have now been employed in the evaluation of human cancer cachexia. The results of studies evaluating amino acid concentrations, regional amino acid balance, and 3-methylhistidine excretion are summarized. The use of regional and whole body kinetic studies in cancer cachexia are reviewed extensively. Most investigators have observed increased rates of whole body protein turnover, synthesis, and catabolism in both weight-stable and weight-losing cancer patients. Some studies have suggested a relationship between the extent of disease and the degree of aberration in amino acid kinetic parameters. Investigators have attempted to reverse some of these alterations by provision of substrate (nutritional support) or administration of specific pharmacologic or anabolic agents such as hydrazine sulfate, insulin, growth hormone, and beta-2 agonists. The role of total parenteral nutrition (TPN) in cancer and its effects on protein and amino acid kinetics and tumor growth are addressed. The possible benefits of specific amino acid nutritional formulations with increased branched chain amino acids, arginine, and glutamine are reviewed. Although many of these approaches appear promising, significant impact on clinically definable parameters remains to be demonstrated. A better understanding of the underlying protein catabolic mechanisms of cancer cachexia will likely lead to more effective therapies to reverse the protein calorie malnutrition associated with cancer cachexia.

Isolation of aminoacyl-tRNA and its labeling with stable-isotope tracers: Use in studies of human tissue protein synthesis

We isolated aminoacyl-tRNA (60-70% yield) from human and rat tissues and measured, by GC/MS, its labeling in vivo by [15N]- and [13C]leucine. Tracer dilution artifacts seemed unlikely since, after infusion of L-[1-13C,15N]leucine into rats, (i) muscle leucyl-tRNA labeling exceeded tissue free leucine labeling, (ii) values were largely unaffected by storing over 5 min at 22 degrees C, and (iii) L-[2,4,5-methyl-13C]leucine was not incorporated into leucyl-tRNA during homogenization. Leucyl-tRNA labeling in liver and muscle suggested charging from extra- and intracellular pools: e.g., after infusing L-[1-13C,15N]leucine, rat muscle tissue free leucine 13C labeling (8.97 +/- 0.30 atom % excess) exceeded that by 15N (3.37 +/- 0.33 atom % excess), and both were significantly lower (P less than 0.02) than venous plasma (13C, 12.1 +/- 1.8; 15N, 5.54 +/- 0.6 atom % excess) indicating tracer dilution by transamination and by proteolysis; however, leucyl-tRNA labeling by either isotope (13C, 10.26 +/- 0.50; 15N, 4.72 +/- 0.72 atom % excess) was significantly above mixed tissue free leucine (P less than 0.05). Labeling of leucyl-tRNA in human erector spinae muscle (obtained after preoperative L-[1-13C]leucine infusion) was, at 4.98 +/- 0.43 atom % excess, lower (27%) than venous plasma leucine (P less than 0.05) and intermediate between muscle free leucine (9% lower; P less than 0.01) and venous alpha-ketoisocaproate (11% higher; P less than 0.02). Human placental leucyl-tRNA labeling (after predelivery tracer infusion) was 37% lower (P less than 0.05) than maternal uterine vein labeling but not significantly different from placental free leucine or umbilical arterial leucine.

Use of a new gas chromatograph isotope ratio mass spectrometer to trace exogenous 13C labelled glucose at a very low level of enrichment in man

The use of 13C labelled glucose in human metabolic studies has been limited by the high cost of the tracer and the problems of measuring low 13C isotopic abundance in plasma glucose. In the present work we describe a new gas chromatograph-isotope ratio mass spectrometer allowing the measurement of a 0.001 atom % increase in 13C abundance over baseline, on a nanomole glucose sample. Studies were performed in rats and in human subjects. The rate of glucose appearance in 24 h fasted rats using D-[1-13C] glucose as tracer and analysed by this new method was found to be 10.4 +/- 0.7 mg.kg-1.min-1, a value 21% lower than that found using D-[6,6-2H2] glucose as tracer (13.1 +/- 1.1 mg.kg-1.min-1) analysed by classic gas chromatography-mass spectrometry. The new method was also used to trace, in combination with D-[6,6 2H2] glucose, the metabolic fate in human subjects of two oral glucose loads (0.5 g.kg.-1,1 g.kg.-1) labelled with 0.1% D-[U-13C] glucose. During the six hours following the glucose load, it was found that total glucose appearance was 0.97 +/- 0.04 g.kg.-1 and 1.2 +/- 0.04 g.kg.-1, exogenous glucose appearance was 0.51 +/- 0.02 g.kg.-1 and 0.84 +/- 0.04 g.kg.-1, endogenous glucose production was 0.44 +/- 0.04 g.kg.-1 and 0.35 +/- 0.06 g.kg.-1 after the 0.5 and 1 g.kg.-1 load respectively. These values are similar to those reported using glucose labelled with radioactive isotopes.(ABSTRACT TRUNCATED AT 250 WORDS)

The bioavailability of vitamin B6. Recent findings

Further clarification of the bioavailability of dietary vitamin B6 requires better analytical data concerning the forms of vitamin B6 in foods as well as the overall composition of foods. The application of radioisotopic and stable-isotopic studies such as those described will provide useful information concerning the inherent bioavailability of the various vitamin B6 compounds. Additional studies should then address the influence of other dietary components on the utilization of the vitamin B6 compounds, using intrinsic and extrinsic enrichment techniques. Care must be taken in interpretation of the results of animal bioassays in the determination of the bioavailability of vitamin B6 in complex diets.

Formation of androstanediol from 13C-labeled testosterone in humans

The determination of urinary 5 alpha-androstane-3 alpha,17 beta-diol (3a-Diol) by gas chromatography/mass spectometry during and after the infusion of stable-labeled testosterone (T) represents an alternative to the use of radioactive label for turnover studies in vivo. Using this methodology to assess the urinary excretion rates of T and 3a-Diol in healthy men (n = 6) and women (n = 5) during and after the intravenous infusion (t = 4 hours) of 20 mg (men) or 5 mg (women) [13C]testosterone, the cumulative renal excretion of 13C-labeled T was found to be 15.6 +/- 9.6 micrograms/24 hours (men) and 1.1 +/- 1.6 micrograms/24 hours (women), equivalent to 0.08% +/- 0.05% and 0.02% +/- 0.03% of the infused amount of 13C-T, respectively. The cumulative excretion of 13C-3a-Diol was 67.7 +/- 19.9 micrograms/24 hours (men) and 10.0 +/- 6.0 micrograms/24 hours (women), equivalent to 0.3 +/- 0.1% and 0.2 +/- 0.1% of the infused dose of 13C-labeled testosterone, respectively.