Stable isotope methods for nutritional diagnosis and research

Nutrition from the Inside Out

Nearly 50 years ago, I set out to investigate the clinical problem of hypoglycemia in children with illnesses that limited their food intake. My goal was to gather accurate and precise measurable data. At the time, I wasn't interested in nutrition as a discipline defined in its more general or popular sense. To address the specific problem that interested me required development of entirely new methods based on stable, nonradioactive tracers that satisfied the conditions of accuracy and precision. At the time, I had no inclination of the various theoretical and practical problems that would have to be solved to achieve this goal. Some are briefly described here. Nor did I have the slightest idea that developing the field would result in a fundamental change in how human clinical investigation was conducted, with the eventual replacement of radiotracers with stable isotopically labeled ones, even for adult clinical investigation. Additionally, I had no inclination that the original questions would open avenues to much broader questions of practical nutritional relevance. Moreover, only much later as the editor of The American Journal of Clinical Nutrition did I appreciate the policy implications of how nutritional data are presented in the scientific literature. At least in part, less accurate and precise measurements and less than full transparency in reporting nutritional data have resulted in widespread debate about the public policy recommendations and guidelines that are the intended result of collecting the data in the first place. This article provides a personal recollection (with all the known faults of self-reporting and retrospective memory) of the journey that starts with measurement certainty and ends with policy uncertainty.

Effects of nutrient solutions on concentration analysis of non-absorbable dilution markers--implications for studies of gastric emptying

Gastrointestinal luminal contents may interfere with concentration analysis of non-absorbable dyes. However, non-absorbable markers are broadly used for studies of gastric emptying rates of nutrient solutions. This prompted us to evaluate the properties of non-absorbable markers to mark such nutrient solutions. In vitro concentrations of polyethylene glycol, phenol red, dextran blue, two anthroquinone dyes and inulin were determined spectrophotometrically in the presence or absence of a formula diet, single compounds of the diet or an oligo-peptide diet, and the reproducibility and validity of the analyses were evaluated. The presence of the formula diet or the oligopeptide diet seriously impaired the analyses of marker concentrations, whereas single nutrient compounds did not uniformly interfere. The analysis of polyethylene glycol and phenol red concentrations was impaired by proteins, while the analysis of inulin concentration was impaired by carbohydrates. Dextran blue and the anthroquinones were completely eliminated by protein-precipitation procedures. In conclusion, phenol red and polyethylene glycol should only be used as marker substances for protein-free meals or nutrient solutions, while inulin should not be used with meals or nutrient solutions containing carbohydrates. Marker dilution techniques cannot be recommended for measurements of gastric emptying rates of complete meals.

Chemical synthesis of deuterated folate monoglutamate and in vivo assessment of urinary excretion of deuterated folates in man

the synthesis and in vivo application of stable-isotopically labeled folic acid was investigated to devise methods suitable for studies of folate metabolism in human subjects. Glutamate-labeled tetradeutero-pteroylglutamic acid (d4-folic acid) was prepared by mixed anhydride coupling of N10-trifluoroacetylpteroic acid and dimethyl L-[3,3,4,4-2H4]glutamic acid, saponification in sodium deuteroxide, and chromatographic purification. Retention of the isotopic label was verified by proton NMR and mass spectrometry of the para-aminobenzoylglutamic acid product of C9-N10 bond cleavage. A method was devised for determination of of isotopic enrichment of urinary d4-folates derived from orally administered d4-folic acid using affinity chromatographic purification, chemical cleavage of the C9-N10 bond, HPLC isolation of the p-[2H4]aminobenzoylglutamate product, followed by negative-ion chemical-ionization gas chromatography/mass spectrometry. Data concerning the urinary excretion of d4-folates derived from an oral dose of d4-folic acid in an adult human are presented.

Comparison of 15N analysis by optical emission spectrometry and mass spectrometry for clinical studies during total parenteral nutrition

During total and stable parenteral nutrition, a branched chain amino acid enriched solution containing [15N]leucine was infused into a patient to determine the fate of the nitrogen administered through this formulation. Measurements of 15N isotopic enrichments were performed on the same biological samples (urinary urea, total plasma proteins and albumin) by optical emission spectrometry (OES) and mass spectrometry (MS) to determine if OES with its specific advantages (cost, handling maintenance) constituted even with low enrichments a useful alternative technique to MS considered as the reference method. The average 15N% enrichments are 0.5% for urinary urea, 0.21% for total plasma proteins and 0.14% for albumin. The coefficients of correlation between both series of 15N% enrichment measurements realized by MS and OES vary from 0.999 to 0.998 and 0.988, respectively, for urinary urea, total plasma proteins and albumin. These results show that OES constituted a very useful analytical technique to obtain reliable information in clinical metabolic studies when low 15N enrichments must be determined.