13CO2 Enrichment Research Articles

The recovery of labeled acetate in severely burned children 7 days after injury

Introduction A correction factor for acetate recovery must be measured for estimation of fatty acid oxidation in severely burned children. The purpose of the study was to determine the fractional recovery rate of labeled CO2 in the breath during the infusion of an acetate tracer in the basal state and during a hyperinsulinemic clamp. Methods Twelve children, aged 4 to 14 years with massive burns (total body surface area burned 61.2 ± 15.7 %), and admitted to the hospital within 4 days post-burn, participated in the study. All experiments were performed 7 days post-burn after an overnight fast. A primed (45 μmol/kg), constant (1.5 μmol·kg−1·min−1) infusion of [1,2-13C] acetate was given during a four hour basal period and during a four hour hyperinsulinemic-euglycemic clamp. A priming dose (150 μmol/kg) of NaH13CO3 was given at the beginning of the study. Breath samples were collected every 10 minutes during the last 40 minutes of each period. Indirect calorimetry was performed during the last 20 minutes of each period with a sensor medic metabolic cart. The isotopic enrichment of 13CO2 was determined by isotopic ratio-mass spectrometry. Results The fractional recovery of acetate label (i.e. the acetate correction factor) was 0.89 ± 0.05, and 0.91 ± 0.04, during the basal state and clamp, respectively. Conclusion The fractional recovery of labeled acetate in severely burned children is approximately three times the recovery of a non-burned adult, and close to the value of 1.0 in exercising adults. The high recovery rate reflects the rapid turnover of the TCA cycle in burned children relative to the rate of exchange reactions.

Short- and medium-term reproducibility of gastric emptying of a solid meal determined by a low dose of13C-octanoic acid and nondispersive isotope-selective infrared spectrometry

To evaluate the reproducibility of a modified 13C breath test-based measurement of solid phase gastric emptying (GE) within the frames of a simple-repeated measure study protocol. Twelve healthy subjects (6 females and 6 males, mean age 24.9+/-0.7 years) were recruited to undergo three identical GE examinations. In six subjects the first two examinations were performed 2 d apart, and the third session was carried out at a median interval of 19.5 d (range 18-20 d) from the second one. In another six subjects the first two measurements were taken 20 d apart (median, range: 17-23 d), whereas the third session took place 2 d after the second one. Probes of expiratory air collected before and during six hours after intake of a solid meal (378 kcal) labelled with 75 microL (68 mg) 13C-octanoic acid, were measured for 13CO2 enrichment with the nondispersive isotope-selective infrared spectrometry NDIRS apparatus. Taking coefficients of variation for paired examinations into account, the short-term reproducibility of the GE measurement was slightly but not significantly better than the medium-term one: 7.7% and 11.2% for the lag phase (T-Lag), 7.3% and 10.9% for the gastric half emptying time (T1/2). The least differences in GE parameters detectable at P=0.05 level in the 12 paired examinations were 9.6 and 15.6 min for T-Lag, 11.6 and 19.7 min for T1/2 by a two-day or two to three-week time gap, respectively. The low-cost modification of the breath test involving a lower dose of 13C-octanoic acid and NDIRS, renders good short- and medium-term reproducibility, as well as sensitivity of the measurement of gastric emptying of solids.

Postprandial glycaemic, lipaemic and haemostatic responses to ingestion of rapidly and slowly digested starches in healthy young women

The objective of the present study was to investigate the postprandial metabolism of two starches with contrasting rates of hydrolysis in vitro. Characterized using the Englyst method of in vitro starch classification, C*Set 06 598 contained predominantly rapidly digestible starch and C*Gel 04 201 contained predominantly slowly digestible starch. Each test starch, naturally enriched with 13C, was fed to ten healthy female volunteers as part of a moderate fat test meal (containing 75 g test starch and 21 g fat), in a double-blind randomized crossover design. The metabolic response to each starch was measured after an overnight fast, in an acute 6 h study, before and after 14 d of daily consumption of 75 g test starch. During each acute study, blood samples were taken at 15 min intervals for the first 2 h and at 30 min intervals for the remaining 4 h. Breath 13CO2 enrichment was measured at the same time points and indirect calorimetry was performed for 20 min every 40 min immediately before and throughout the study. Significantly more rapid, greater changes in postprandial plasma glucose, NEFA and serum insulin concentrations were observed after consumption of the rapidly digestible starch. Breath 13CO2 output over the first 3-4 h rose rapidly then began to decline following consumption of the rapidly digestible starch, but plateaued for the slowly digestible starch. The 14 d adaptation period did not affect any of the glycaemic or lipaemic variables but there was a reduction in postprandial plasminogen activator inhibitor-1 concentrations. These data confirm that starches characterized as predominantly rapidly digestible versus slowly digestible by the Englyst procedure provoke distinctly different patterns of metabolism postprandially.

Does pravastatin increase chylomicron remnant catabolism in postmenopausal women with type 2 diabetes mellitus?

We investigated the effects of pravastatin on chylomicron remnant catabolism measured with a 13C stable isotope breath test and plasma apolipoprotein (apo) B-48 and remnant-like particle (RLP)-cholesterol in postmenopausal women with type 2 diabetes mellitus. Nineteen postmenopausal women with type 2 diabetes were randomized to receive 40 mg/day pravastatin or no treatment for 6 weeks followed by a 2-week washout period, and crossed over for a further 6 weeks. Fractional catabolic rate (FCR) of a chylomicron remnant-like emulsion was determined from 13CO2 enrichment in the breath and plasma using isotope-ratio mass spectrometry and multicompartmental modelling. Plasma apo B-48 and RLP-cholesterol concentrations were also measured as static markers of chylomicron remnant metabolism. Pravastatin significantly reduced plasma concentrations of cholesterol (5.9 +/- 0.3 vs. 4.8 +/- 0.2 mmol/l; P < 0.001), low density lipoprotein (LDL)-cholesterol (3.5 +/- 0.2 vs. 2.6 +/- 0.2 mmol/l; P < 0.001), triglyceride (2.1 +/- 0.3 vs. 1.7 +/- 0.2 mmol/l; P = 0.017), non-high density lipoprotein (HDL)-cholesterol (4.4 +/- 0.3 vs. 3.3 +/- 0.2 mmol/l; P < 0.001), lathosterol/total cholesterol ratio (2.6 +/- 0.2 vs. 2.0 +/- 0.3, P = 0.035), apo B-100 (1.1 +/- 0.1 vs. 0.8 +/- 0.1 g/l; P = 0.001), apo B-48 (4.8 +/- 0.9 vs. 3.3 +/- 0.6 mg/l; P = 0.016), and RLP-cholesterol (31.4 +/- 8.2 vs. 18.6 +/- 4.6 mg/dl; P = 0.024). Pravastatin was also associated with an increase in sitosterol/total cholesterol ratio (2.8 +/- 0.3 vs. 3.1 +/- 0.3, P = 0.029). Chylomicron remnant-like emulsion catabolism was not, however, significantly altered by pravastatin estimated by either breath or plasma clearance measurements. In postmenopausal women, pravastatin decreases plasma concentrations of remnant lipoproteins by a mechanism that may relate chiefly to inhibition of remnant production, but this requires further evaluation.

Use of [13C]Bicarbonate for Metabolic Studies in Preterm Infants: Intragastric versus Intravenous Administration

The metabolic fate of substrates in humans can be examined by the use of stable isotopes, one of which, [13C]bicarbonate, may serve to estimate CO2 production rate. In view of minimizing the burden of metabolic studies for preterm infants, the authors determined whether intragastric and intravenous infusions of [13C]bicarbonate would achieve the same 13CO2 enrichment in expired air during steady state. A second aim of this study was to determine the minimum time required to reach steady state during intragastric infusion. Ten preterm infants received a primed continuous [13C]bicarbonate infusion intragastrically, followed by an intravenous infusion the next day. Breath samples were obtained every 30 min by the direct sampling method. 13CO2 isotopic enrichment, expressed as atom percent excess, was measured by isotopic ratio mass spectrometry. Two-tailed t tests were used to detect statistically significant differences between the infusion routes. The isotopic enrichment at plateau did not differ between intragastric and intravenous infusion. A steady state of 13CO2 enrichment was achieved after 60 min of intravenous infusion and after 120 min of intragastric infusion. In conclusion, intragastric infusion of [13C]bicarbonate may serve to estimate the whole-body CO2 production rate in preterm infants. To reach 13CO2 steady state, a minimum of 120 min of bicarbonate administration is required.

13C]Methionine breath test: a novel method to detect antiretroviral drug-related mitochondrial toxicity

A major side effect of antiretroviral drugs is nucleoside reverse transcriptase inhibitor (NRTI)-related mitochondrial toxicity, the in vivo diagnosis of which is difficult and not yet standardized. We used the [(13)C]methionine breath test to investigate hepatic mitochondrial oxidation in HIV-1-infected patients receiving antiretroviral therapy. The [(13)C]methionine breath test was performed in healthy subjects (n=10), HIV-infected patients on antiretroviral therapy with (n=6) and without (n=15) hyperlactataemia and naive HIV-infected patients (n=11). After oral administration of [(13)C]methionine (2 mg/kg body weight), hepatic methionine metabolism was measured by breath (13)CO(2) enrichment, expressed as delta over baseline (DOB) every 15 min for 120 min by mass spectrometry. The four study groups showed a significant difference in (13)CO(2) exhalation (P=0.001). HIV-infected patients on antiretroviral therapy with normal serum lactate had reduced exhalation of (13)CO(2) compared with healthy subjects (DOB mean peak: 8.82+/-0.62 versus 11+/-0.9, P<0.05). HIV patients with hyperlactataemia had even lower values when compared with patients with normal lactataemia (DOB mean peak: 4.98+/-0.68 versus 8.82+/-0.62, P<0.05). The [(13)C]methionine breath test possibly showed mitochondrial impairment in antiretroviral-treated HIV-positive patients, particularly with hyperlactataemia. This non-invasive test can be used to monitor drug-related mitochondrial toxicity in vivo and to discover early and asymptomatic damage of the respiratory chain.

Is there an advantage in normalising the results of the Helicobacter Pylori [13C]Urea breath test for CO2 production rate in children?

The urea breath test (UBT) is a non-invasive diagnostic test to detect the presence of Helicobacter pylori in the stomach, and is the simplest way to confirm eradication after treatment. The test is based on the capacity of H. pylori to secrete the enzyme urease, which hydrolyses urea to ammonia and carbon dioxide. The aim of this study was to determine whether there is an advantage in expressing the results of UBTs in terms of urea hydrolysis rate (UHR), rather than breath 13C enrichment alone. Retrospective analysis of data collected between 1995 and 2002 from 260 patients undergoing UBTs was performed. The cut-offs for positive tests using breath 30-minute enrichment (E30), UHR calculated using VCO2 estimated from height and weight (H/WT) and VCO2 estimated from weight only were determined using two-graph receiver operator characteristic (TG-ROC) analysis. The cut-off points were 3.5‰ or 38.7 ppm 13C excess, 7.04 µmol/h and 7.08 µmol/h, respectively. There was no advantage in expressing the results as UHR (θ0, Theta-zero, where sensitivity = specificity = 0.97 (UHR H/WT), 0.98 (UHR WT) and 1.00 (E30)) rather than breath 13CO2 enrichment alone. Differences in the extent of H. pylori colonisation and urease activity are more important than variation in VCO2 in determining breath 13CO2 enrichment in the UBT.

In vivo assessment of the mitochondrial response to caloric restriction in obese women by the 2-keto[1-13C]isocaproate breath test

The 2-keto[1-13C]isocaproate breath test has been proposed as a tool to detect mitochondrial dysfunction in alcoholic liver disease. The aim of this study was to evaluate if the 2-keto[1-13C]isocaproate breath test could detect in vivo dynamic changes on mitochondrial activity due to caloric restriction in obese women. Fifteen obese women (body mass index [BMI] > 30 kg/m2) participated in the study at baseline. Ten of these women agreed to participate on a diet program to induce body weight loss. Fifteen lean women (BMI < 25 kg/m2) were included as a control group. The breath test was performed by the oral administration of the tracer measuring 13CO2 enrichment in breath before and after ingestion using isotope ratio mass spectrometry. Body composition, resting energy expenditure, and plasma levels of insulin and leptin were measured. There were no relationships observed between the 2-keto[1-13C]isocaproate breath test and the plasma insulin (before diet: P =.863; after diet: P =.879), or leptin (before diet: P =.500; after diet: P =.637). In obese women before treatment, kilograms of fat free mass (P =.108), resting energy expenditure adjusted for body composition (P =.312), and the 2-keto[1-13C]isocaproate breath test (P =.205) were similar in comparison to lean women. However, 2-keto[1-13C]isocaproate oxidation tended to increase after dieting and was significantly higher than in controls (P =.015). These data suggest that the 2-keto[1-13C]isocaproate breath test reflected the adaptive modifications in mitochondrial oxidation in response to caloric restriction in obese women. Copyright 2003 Elsevier, Inc. All rights reserved.

Production of 13C Labelled Pea Flour for Use in Human Digestion and Fermentation Studies

Stable isotope breath tests offer a new approach to the study of digestion and fermentation of carbohydrates in man. In this study, 13C labelled peas were grown by pulsing 250ml 13CO2 into a sealed growth chamber. A second pulse was added to a portion of the peas to increase the 13C enrichment. This generated pea flour with an enrichment of 2.36 at.% excess (range 2.09–2.71 n = 3) and 8.64 atom % excess (range 7.37–9.78 n = 3) respectively. This represented incorporation of an absolute yield of 3.8% of the 13CO2 into peas in the ‘once-labelled’ treatment and 7.5% in the ‘twice-labelled’ treatment. Ingestion of a mixture of the labelled pea flour (300 mg) by two volunteers generated measurable 13CO2 excretion for breath test analysis. The profile of breath 13CO2 enrichment increased to a maximum within three hours after consuming the pea flour followed by a decrease almost back to baseline by 13 hours. Breath 13CO2 appeared to rise again after this apparent nadir at 13 hours until the end of the sampling period. Mathematical analysis of the data suggested that two peaks best described the profile of breath 13CO2 up to 13 hours. A third peak was necessary to describe the late rise in breath 13CO2 enrichment. This use of 13C enriched pea flour may provide a useful non invasive method for measurement of digestion and fermentation in vivo.

Magnesium chloride slows gastric emptying, but does not affect digestive functions.

An inverse relationship has been established between serum magnesium and serum lipid levels. By means of breath tests, we tested the hypothesis that magnesium inhibits intraluminal lipid digestion and subsequently causes changes in lipid metabolism. We also investigated the influence of the administration of magnesium chloride on protein digestion and gastric emptying. Five healthy volunteers performed simultaneous breath tests for gastric emptying and intraluminal lipid digestion, and six others for gastric emptying and protein digestion. Each test was performed in basal conditions and after the intake of 800 mg of magnesium chloride dissolved in water. Breath samples were taken at regular time intervals and analysed for 13CO2 and 14CO2 enrichment in order to calculate gastric emptying and lipid and protein digestion rates. The oral administration of a single dose of magnesium chloride resulted in a diminished rate of intraluminal lipid and protein digestion. The most pronounced effect of magnesium chloride, however, was a decreased gastric emptying rate of both test meals. After correction for gastric emptying, no differences were noted in intraluminal lipid or protein digestion. Therefore, the lower lipid levels noted after magnesium supplementation are unlikely to be the result of altered lipid assimilation. Magnesium chloride slows gastric emptying but does not influence lipid digestion.

Modeling 13C breath curves to determine site and extent of starch digestion and fermentation in infants.

The colon salvages energy from starch, especially when the capacity of the small intestine to digest it is limited. The aim of this study was to determine the site and relative extent of starch digestion and fermentation in infants. Thirteen infants (10 male and 3 female infants), median age 11.8 months (range, 7.6-22.7 months), were fed a starchy breakfast containing 13C-labeled wheat flour after an overnight fast. Duplicate breath samples were obtained before breakfast and every 30 minutes for 12 hours. Breath 13CO2 enrichment was measured using isotope ratio mass spectrometry, and results were expressed as percentage dose recovered (PDR) for each 30 minutes. The PDR data were analyzed and mathematically modeled assuming either a constant estimate of CO2 production rate or adjusted for physical activity. Mean +/- SD cumulative 13C PDR (cPDR) at 12 hours was 21.3% +/- 8.4% for unadjusted data and 26.5% +/- 11.6% for adjusted data. A composite model of two curves fit significantly better than a single curve. Modeling allowed estimation of cPDRs of small intestine (17.5% +/- 6.5% and 22.7% +/- 9.3% for unadjusted and adjusted data, respectively) and colon (4.6% +/- 2.9% and 6.3% +/- 5.4%). Modeling of 13CO2 enrichment curves after ingestion of 13C-enriched wheat flour is an attractive means to estimate the contribution of the upper and lower gut to starch digestion and fermentation.

Modeling 13C Breath Curves to Determine Site and Extent of Starch Digestion and Fermentation in Infants

ABSTRACTBackgroundThe colon salvages energy from starch, especially when the capacity of the small intestine to digest it is limited. The aim of this study was to determine the site and relative extent of starch digestion and fermentation in infants.MethodsThirteen infants (10 male and 3 female infants), median age 11.8 months (range, 7.6–22.7 months), were fed a starchy breakfast containing 13C‐labeled wheat flour after an overnight fast. Duplicate breath samples were obtained before breakfast and every 30 minutes for 12 hours. Breath 13CO2 enrichment was measured using isotope ratio mass spectrometry, and results were expressed as percentage dose recovered (PDR) for each 30 minutes. The PDR data were analyzed and mathematically modeled assuming either a constant estimate of CO2 production rate or adjusted for physical activity.ResultsMean ± SD cumulative 13C PDR (cPDR) at 12 hours was 21.3% ± 8.4% for unadjusted data and 26.5% ± 11.6% for adjusted data. A composite model of two curves fit significantly better than a single curve. Modeling allowed estimation of cPDRs of small intestine (17.5% ± 6.5% and 22.7% ± 9.3% for unadjusted and adjusted data, respectively) and colon (4.6% ± 2.9% and 6.3% ± 5.4%).ConclusionsModeling of 13CO2 enrichment curves after ingestion of 13C‐enriched wheat flour is an attractive means to estimate the contribution of the upper and lower gut to starch digestion and fermentation.

Markers of Triglyceride-rich Lipoprotein Remnant Metabolism in Visceral Obesity

Triglyceride-rich lipoprotein remnants are atherogenic, and this may be particularly important in visceral obesity. We investigated remnant metabolism in obese men by measuring remnant-like particle-cholesterol (RLP-C), apolipoprotein (apo) B-48, apoC-III, and the clearance of a labeled remnant-like emulsion. Fasting RLP-C, apoB-48, and apoC-III concentrations were measured in 48 viscerally obese men and 10 lean controls. RLP-C was determined by immunoseparation assay, apoB-48 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and enhanced chemiluminescence, and apoC-III by immunoturbidimetric assay. The catabolism of chylomicron remnants was measured by intravenous injection of a remnant-like emulsion containing cholesteryl [(13)C]oleate, with isotopic enrichment of (13)CO(2) in breath determined by isotope-ratio mass spectrometry and a multicompartmental model to estimate fractional catabolic rate (FCR) of the emulsion. Compared with controls, obese men had significantly increased plasma concentrations of RLP-C, apoB-48, and apoC-III (P <0.001 for all). Plasma total apoB-100, non-HDL-cholesterol, LDL-cholesterol, triglycerides, and insulin resistance (HOMA score) were also significantly higher in the obese group (P <0.001 for all). Obese men had a significantly lower FCR of the remnant-like emulsion compared with controls (P = 0.020). Viscerally obese individuals have insulin resistance and increased plasma concentrations of triglyceride-rich lipoprotein remnants, which may be attributable to decreased catabolism of these particles.

Nephrotic proteinuria has no net effect on total body protein synthesis: Measurements with 13C valine

In nephrotic syndrome, significant amounts of plasma proteins, mostly of hepatic origin, are lost in urine. Total hepatic protein synthesis increases, suggesting that other protein pools must be conserved to maintain steady state. This can be accomplished either by decreased amino acid oxidation or decreased protein synthesis in other organs to replace lost liver-derived proteins. To determine the effect of nephrotic syndrome on total-body protein metabolism, we compared whole-body valine use in seven nephrotic patients and five controls using a primed continuous infusion of [1-13C]-valine, with additional priming of NaH13CO3 . Plasma [13C]-valine, 13C α ketoisovaleric acid, and the expired 13CO2 enrichments were used to assess whole-body valine flux, valine oxidation, and nonoxidative valine disposal (NOVD). The valine flux into the blood compartment (97.7 ± 3.0 versus 95.3 ± 3.3 μmol/kg/h), oxidation of valine (19.4 ± 1.9 versus 21.2 ± 2.8 μmol/kg/h), and NOVD (78.3 ± 2.5 versus 74.2 ± 2.7 μmol/kg/h) were not statistically different in patients compared with controls. Valine oxidation correlated positively with urinary urea excretion (r = 0.70; P = 0.01) in all subjects. Compared with control subjects who have similar urinary urea excretion, nephrotic subjects do not compensate for urinary loss of protein by decreased amino acid oxidation or decreased nonoxidative valine disposal. Previous studies have shown that synthesis of several hepatic proteins increases when subjects are fed the same dietary regime, whereas the present study shows that total-body protein synthesis does not increase. This would imply reduced synthesis of nonhepatic protein pools.

Increase in fat oxidation on a high-fat diet is accompanied by an increase in triglyceride-derived fatty acid oxidation.

The aim of this study is to investigate the mechanism behind the slow increase in fat oxidation on a high-fat diet. Therefore, we determined 24-h substrate oxidation using respiration chambers and the rate of appearance and oxidation of plasma-derived fatty acids in seven healthy nonobese men (age 23 +/- 2 years, height 1.85 +/- 0.03 m, weight 70.4 +/- 2.3 kg, % body fat 13 +/- 1). Before testing, they consumed a low-fat diet (30% fat, 55% carbohydrate) at home for 3 days. Measurements were performed after 1 day consumption of either a low-fat diet (LF), a high-fat diet (HF1, 60% fat, 25% carbohydrate), or a high-fat diet preceded by a glycogen-lowering exercise test (HF1+EX), and after 7 days on a high-fat diet (HF7). After an overnight fast, an infusion of [U-13C]palmitate (0.00806 micromol x min(-1) x kg(-1)) was started and continued for 2 h at rest followed by 1 h of exercise at 50% of maximal power output (Wmax). Whole-body fat oxidation was measured using indirect calorimetry, and plasma-derived fatty acid oxidation was evaluated by measuring breath 13CO2 enrichment and corrected with the acetate recovery factor. Twenty-four-hour fat oxidation gradually increased on the high-fat diet. Both at rest and during exercise, there was no change in rate of appearance of fatty acids and plasma-derived fatty acid oxidation. Triglyceride-derived fatty acid oxidation tended to be higher after 7 days of high-fat diet at rest (P < 0.07). This difference was significant during exercise (P < 0.05). In conclusion, the results from this study suggest that triglyceride-derived fatty acid oxidation (VLDL or intramuscular triglycerides) plays a role in the increase in fat oxidation on a high-fat diet, but plasma-derived fatty acids remain the major source for fat oxidation.

Relative roles of LDLr and LRP in the metabolism of chylomicron remnants in genetically manipulated mice

Remnant-like emulsions labeled with cholesteryl [13C]-oleate were prepared with lipid compositions similar to remnants derived from triacylglycerol-rich lipoproteins. When injected into the bloodstream of conscious mice, the remnant-like emulsions were metabolized in the liver leading to the appearance of 13CO2 in the breath. Previously, using this technique, we found that remnant metabolism was significantly impaired but not completely inhibited in mice lacking low density lipoprotein receptors (LDLr). We have now found in mice with non-functional low density lipoprotein receptor-related protein (LRP) that breath enrichment of 13CO2 was significantly decreased, indicating that the LRP also plays an important role in the metabolism of chylomicron remnants (CR). The enrichment of 13CO2 in the expired breath was negligible in mice lacking both LDLr and receptor-associated protein (−/−), essential for normal function of LRP. In mice pre-injected with gluthatione S-transferase–receptor-associated protein to block LRP binding, there was a marked inhibition of the appearance of 13CO2 in the expired breath of homozygous LDLr-deficient mice, supporting the role of LRP in vivo. Whether or not LDLr were present, in mouse and human fibroblast cells human apoE3 or E4 but not apoE2 were essential for binding of remnant-like emulsions, while lactoferrin and suramin completely inhibited binding. We conclude that in normal mice LDLr are important for the physiological metabolism of CR. When LDLr are absent the evidence supports a role for the LRP in the uptake of CR in liver cells and in fibroblasts, with binding characteristics for CR-associated apoE similar to LDLr. —Martins, I. J., E. Hone, C. Chi, U. Seydel. R. N. Martins, and T. G. Redgrave. Relative roles of LDLr and LRP in the metabolism of chylomicron remnants in genetically manipulated mice.

Non-Invasive Assessment of Human Hepatic Mitochondrial Function through the 13C-Methionine Breath Test

Background: Mitochondrial metabolism is essential for liver function, and the availability of simple and non-invasive tests able to assess hepatic mitochondrial function could be extremely useful. Since the amino acid methionine is mostly oxidized by liver mitochondria, we investigated the feasibility of a breath test with 13C-methionine in healthy subjects to assess hepatic mitochondrial function before and after ethanol-induced oxidative stress. Methods: Twenty healthy male volunteers received 2 mg/kg body weight [methyl-13C]-methionine. Breath 13CO2 enrichment was measured at base line and every 15 min thereafter for 180 min. Forty-eight hours later the test was repeated 30 min after the ingestion of ethanol (0.3 g/kg body weight). Results: After oral administration of methionine, 13CO2

Performance of the [13C]-acetate gastric emptying breath test during physical exercise.

The gastric emptying rate of liquids can be determined non-invasively using the [13C]-acetate breath test at rest. The aims of our study were to validate this test during physical exercise against the double-sampling method and to evaluate the time needed for intestinal absorption and the delay between absorption and appearance of 13CO2 in breath, both at rest and during exercise. Fifteen well-trained male subjects were investigated. Gastric emptying was determined simultaneously measuring the 13CO2 breath enrichment after intragastric administration of 0.5 L of carbohydrate solution with 150 mg of [13C]-acetate added and by the double-sampling technique (n = 9). In separate tests, 150 mg of [13C]-acetate was also applied intraduodenally and intravenously (n = 6), both at rest and during exercise. Time-to-peak (TTP) 13CO2 enrichment was determined using a curve fit and was considered as the parameter for gastric emptying. TTP enrichment derived from the breath test significantly correlated with the gastric emptying half-time obtained from the gastric aspirates. During exercise, median TTP enrichment values after intragastric, intraduodenal (i.d.) and intravenous (i.v.) administration of [13C]-acetate were 22.3, 10.3 and 5.4 min respectively. During exercise, i.d. and i.v. values were reached significantly earlier than at rest. The [13C]-acetate breath test can be used as a non-invasive method to determine relative gastric emptying rates of liquids during exercise, but the results are influenced by the rate of absorption and the time needed for subsequent oxidation of [13C]-acetate and exhalation of 13CO2.

13C and H2 breath tests to study extent and site of starch digestion in children with cystic fibrosis.

Starch is an important source of energy for children with cystic fibrosis, but little is known about their capacity to digest it. A 13C breath test was used to measure starch digestion and oxidation in 16 children with cystic fibrosis (median [range] age, 7.9 [4-15] years; 7 girls, 9 boys) and 5 normal healthy control subjects (median age, 8.3 [7-13] years; 3 girls, 2 boys). A test meal of 13C flour and lactulose was consumed and breath samples were obtained half-hourly thereafter for 6 hours to measure 13C enrichment by isotope ratio mass spectrometry and H2 by electrochemistry. The test was repeated on 10 children with cystic fibrosis when they were taking pancreatic supplements. The median (range) cumulative percentage 13C dose recovery (cPDR), was 35% (18-52%) in control subjects, 18% (9-33%) in children with cystic fibrosis without enzymes, and 29% (22-51%) in those with pancreatic supplements. cPDR differed significantly between healthy control subjects and children with cystic fibrosis without enzymes (p = 0.01) and between children with cystic fibrosis with and without enzymes (p < 0.0001), but there was no difference between control subjects and children with cystic fibrosis taking enzymes (p = 0.5). Eight children with cystic fibrosis had a cPDR within control range, and in six there was a second peak in 13CO2 enrichment coincident with an increase in H2. Starch digestion and oxidation are diminished in children with cystic fibrosis, but pancreatic enzymes restored them to near normal levels. A second peak in 13CO2 enrichment, suggestive of colonic starch fermentation was absent in healthy children, but present in some children with cystic fibrosis and abolished by pancreatic enzymes.

Normalizing results of 13C-urea breath testing for CO2 production rates in children.

The 13C-urea breath test detects the presence of Helicobacter pylori from an enrichment of breath 13CO2, which, in turn, is critically dependent on the amount of dilution by endogenous CO2 production. The production of CO2 differs according to age (adults > children), sex (male > female) weight, and height. The cutoff value of 2.4 delta%(delta over baseline, DOB) for the 13C-urea breath test, defined in adults, does not take into account actual CO2 production. Therefore, this cutoff value (2.4 delta%) may or may not be appropriate for children. The purpose of this study was to determine a cutoff value that would provide accurate results in pediatric patients, independent of their differences in anthropometric parameters. Estimates of CO2 production were combined with DOB values to calculate the host-dependent urea hydrolysis rate. Calculated as urea hydrolysis rate, the cutoff range for adults was 10.4 to 10.9 microg/min. Individual ranges were concentric (men, 9.6-10.9 microg/min; women, 8.5-12.2 microg/min). Results in studies of 312 children show that a urea hydrolysis rate of more than 10 m microg/min may also be appropriate to predict H. pylori infection. Calculating 13C-urea breath test values as urea hydrolysis rate removes the effect of individual anthropometric differences on test outcome and provides a single cutoff value for pediatric patients of all ages.