The potential use of the labelled bicarbonate method for estimating energy expenditure in man

The oral 13C-bicarbonate technique for determination of energy expenditure in dogs: dietary and environmental factors affecting the respiratory quotient and 13C recovery factor

The oral ¹³C-bicarbonate technique (o¹³CBT) can be used for short-term measurements of CO₂ production (RCO₂) and energy expenditure (EEx). The method relies on appropriate estimates for the respiratory quotient (RQ) and recovery factor (RF) of ¹³C. Four Retriever dogs were included in four experiments to validate the o¹³CBT against indirect calorimetry (IC), and determine RQ and RF; Expt. 1: feeding different protein:fat:carbohydrate ratios [in % of metabolisable energy]: 25:33:42 in a maintenance (Mnt.) diet; 38:26:36 in a high-protein high-fibre (H_Fibre) diet and 27:56:17 in a high-fat (H_Fat) diet, simultaneously with start of measurements (T₀); Expt. 2: the Mnt. diet at T₀ or 4 h postprandial (T₄); Expt. 3: T₄ at different ambient temperatures, 22°C and 15°C; Expt. 4: T₄ after 1 h physical activity. The RCO₂ and EEx were determined from the respiration chamber measurements made simultaneously with IC and the o¹³CBT (o¹³CBT_online), and in Expts. 1 and 2, also on two consecutive days using o¹³CBT with collection of breath into breath bags (o¹³CBT_breathbags). The RQ values obtained at T₀ reflected dietary compositions, with the highest least square mean (LSM) of 0.954  for the Mnt. diet, 0.905 for the H_Fibre and 0.877 for the H_Fat diet (p < 0.05). An increased interval between meal and measurement period decreased RQ significantly (p < 0.05) in Expt. 2, LSM being 0.954 at T₀ and 0.909 at T₄. Ambient temperature (Expt. 3) and physical activity (Expt. 4) did not influence postprandial RQ. The RF values were not significantly affected by diet (Expt. 1). Measurements starting at T₀ (Expt. 2) resulted in higher (p < 0.05) RF values than at T₄ (LSM = 0.971 and 0.836, respectively). The ambient temperatures (Expt. 3) did not influence postprandial RF. However, when dogs were physically active prior to measurements (Expt. 4), RF values (LSM = 1.019) were higher (p < 0.05) than when resting only (LSM = 0.836). Calculations based on RQ and RF determined in each experiment resulted in RCO₂ and EEx values which were not different regardless of method used, except for Expt. 1 where EEx-values [kJ · kg BW^-0.75 · d^-1] were higher (p < 0.05) when measured with o¹³CBT_breathbags (460) than by IC (421) and o¹³CBT_online (420). Provided accurate RQ and RF values, the o¹³CBT_breathbags can be used as an independent and minimally invasive research tool to determine EEx in dogs under carefully standardised conditions.

A methodological and clinical approach to measured energy expenditure in the critically ill pediatric patient

The metabolic response to injury and stress is characterized initially by a decreased energy expenditure (Ebb phase) followed by an increased metabolic expenditure (Flow phase). Indirect calorimetry is a methodology utilized to measure energy expenditure and substrate utilization by measuring gas exchange in exhaled air and urinary nitrogen. The use of indirect calorimetry in critically ill patients requires precise equipment to obtain accurate measurements. The most recent guidelines suggested that measured energy expenditure by indirect calorimetry be used to determine energy requirements. This article reviews the methodological and clinical use of indirect calorimetry in critically ill pediatric patients.

Estimation of energy expenditure using the oral 13 C-bicarbonate technique in privately owned adult and senior dogs of three different body sizes

Knowledge about energy requirements (ER) of dogs is important in order to provide dogs with an appropriate energy supply. In this study, the oral ¹³ C-bicarbonate technique (o¹³ CBT) was used to estimate energy expenditure (EE) in 50 privately owned dogs of different body sizes (small: Danish-Swedish Farmdog (DSF, n = 16), medium: Beagle (n = 15), large: Labrador Retriever (n = 19)), of different ages (adult: 2-7 years (n = 33), senior: ≥8 years (n = 17)). The aim was to evaluate the feasibility of the method in the field and to get more information about ER of dogs. All dogs were measured twice, on two separate days under standardized, resting conditions. The dogs were fasted overnight prior to measurements, which were carried out in home environment. Only measurements of dogs resting calmly were included in the statistical analyses. No significant difference (p > 0.05) in EE was found between days of measurements. The EE measured in Labrador Retrievers (405 kJ (97 kcal)/kg BW^0.75 /day) was significantly lower (p < 0.001) than of Beagles (530 kJ (127 kcal)/kg BW^0.75 /day) and DSF (497 kJ (119 kcal)/kg BW^0.75 /day), the difference between Beagles and DSF being non-significant. Senior dogs had significantly lower (p < 0.001) EE values than adult dogs with least square means (LSM) of 441 kJ (105 kcal)/kg BW^0.75 /day and 513 kJ (123 kcal)/kg BW^0.75 /day respectively. Analysed within breeds, EE of senior Labrador Retrievers (370 kJ (88 kcal)/kg BW^0.75 /day)) was significantly lower (p < 0.01) than adults (439 kJ (105 kcal)/kg BW^0.75 /day), as well as in DSF (453 kJ (108 kcal)/kg BW^0.75 /day and 541 kJ (129 kcal)/kg BW^0.75 /day for senior and adult dogs respectively). However, the EE was not significantly different (p > 0.05) between adult (548 kJ (131 kcal)/kg BW^0.75 /day) and senior (499 kJ (119 kcal)/kg BW^0.75 /day) Beagles. This study suggests that when measured under standardized resting conditions, the o¹³ CBT can provide reliable results of EE and be a helpful tool to get more knowledge about ER of dogs of different sizes, breeds and ages.

(13)C-Breath testing in animals: theory, applications, and future directions

The carbon isotope values in the exhaled breath of an animal mirror the carbon isotope values of the metabolic fuels being oxidized. The measurement of stable carbon isotopes in carbon dioxide is called (13)C-breath testing and offers a minimally invasive method to study substrate oxidation in vivo. (13)C-breath testing has been broadly used to study human exercise, nutrition, and pathologies since the 1970s. Owing to reduced use of radioactive isotopes and the increased convenience and affordability of (13)C-analyzers, the past decade has witnessed a sharp increase in the use of breath testing throughout comparative physiology--especially to answer questions about how and when animals oxidize particular nutrients. Here, we review the practical aspects of (13)C-breath testing and identify the strengths and weaknesses of different methodological approaches including the use of natural abundance versus artificially-enriched (13)C tracers. We critically compare the information that can be obtained using different experimental protocols such as diet-switching versus fuel-switching. We also discuss several factors that should be considered when designing breath testing experiments including extrinsic versus intrinsic (13)C-labelling and different approaches to model nutrient oxidation. We use case studies to highlight the myriad applications of (13)C-breath testing in basic and clinical human studies as well as comparative studies of fuel use, energetics, and carbon turnover in multiple vertebrate and invertebrate groups. Lastly, we call for increased and rigorous use of (13)C-breath testing to explore a variety of new research areas and potentially answer long standing questions related to thermobiology, locomotion, and nutrition.

The ¹³C bicarbonate method: an inverse end product method for measuring CO₂ production and energy expenditure

We reconsider the principle of the (13)C bicarbonate (NaH(13)CO3) method ((13)C-BM) for the determination of the CO2 production to obtain an estimate of energy expenditure (EE). Its mathematical concept based on a three-compartmental model is related to the [(15)N]glycine end product method. The CO2 production calculated by the (13)C-BM, RaCO2((13)C) is compared to the result from the indirect calorimetry, RCO2(IC). In an interspecies comparison (dog, goat, horse, cattle, children, adult human; body mass ranging from 15 to 350 kg, resting and fasting conditions) we found an excellent correlation between the results of (13)C-BM and IC with RCO2(IC) = 0.703 × RaCO2((13)C), (R(2) = 0.99). The slope of this correlation corresponds to the fractional (13)C recovery (RF((13)C)) of (13)C in breath CO2 after administration of NaH(13)CO3. Significant increase in RF((13)C) was found in physically active dogs (0.95 ± 0.14; n = 5) vs. resting dogs (0.71 ± 0.10, n = 17; p = .015). The (13)C recovery in young bulls was greater in blood CO2 (0.81 ± 0.05) vs. breath CO2 (0.73 ± 0.05, n = 12, p < .001) and in ponies with oral (0.76 ± 0.03, n = 8) vs. intravenous administration of NaH(13)CO3 (0.69 ± 0.07; n = 8; p = .026). We suggest considering the (13)C-BM as a 'stand-alone' method to provide information on the total CO2 production as an index of EE.

Evaluation of the oral ¹³C-bicarbonate tracer technique for the estimation of CO₂ production and energy expenditure in dogs during rest and physical activity

For feeding of working dogs during their daily life, illness, routine jobs or sporting activities, an accurate determination of their nutritional requirements is essential to ensure their optimal health and performance. To predict the appropriate guidelines about how to feed dogs, it appears essential to determine the energy expenditure (EE) in a reliable and feasible way. In the present experiment, the non-invasive oral ¹³C-bicarbonate tracer technique (o¹³CT), i.e. collection of breath samples after oral administration of NaH¹³CO₃, was used for the estimation of CO₂ production and EE in dogs. Measurements were conducted during two days of rest, and during three days with 3 h of exercise per day. Average EE was 483 and 876 kJ kg⁻⁰·⁷⁵ d⁻¹ during rest and exercise, respectively. The o¹³CT seems appropriate to use as a minimal restrictive and non-invasive method to obtain reliable estimates of EE in dogs at different activity levels under near natural conditions.

Energy expenditure of cattle grazing on pastures of low and high availability

The energy expenditure of freely grazing cattle was investigated in the National Institute of Agricultural Technology and Agricultural Science College of Balcarce, Argentina (37° 45'S, 58° 18'W), by the CO2 entry rate technique. Two experiments were carried out in the autumn in March 1994 (experiment 1) and in April 1995 (experiment 2) with animals prepared with catheters in the parotid gland (collection of saliva) and into the peritonea for infusion of a solution of 14C. Six Angus steers (259 (s.e. 11) kg) were used in experiment 1 and seven (298 (s.e. 36) kg) in experiment 2. In experiment 1 animals grazed ryegrass pastures for 5·5 h in two periods of 1·5 h in the morning and of 4 h in the afternoon and in experiment 2 animals grazed oat pasture in one period of 1 h in the morning. Twenty hours before and during the experiments a solution ofNaH14CO3 was infused at a rate of 9·4 (experiment 1) and 8·1 (experiment 2) μiCi/h for 48 h with portable peristaltic pumps carried by each animal. Saliva samples were collected at least after the first 20 h of infusion. The first sample was collected in the corral just before grazing and two samples were collected in each grazing period. Also, in experiment 1 three samples were taken during resting (noon, 1 h after grazing and the next morning). In addition, bite frequency, pasture availability, plant height, in vitro digestibility and crude protein were measured. Carbon dioxide production was calculated as the ratio between the rate of infusion of 14C (μCi/h) and the specific activity of CO2 (μCi/l CO2) in saliva samples. Bite frequency was 59 and 28 bites per min on the respective pasture of ryegrass (148 g dry matter (DM) per m2 and 10·5 cm height) and oat (228 g DM per m2 and 27 cm height). Energy expenditure (EE, kJ/h per kg M0·75) in corrals was 14·9 (experiment 1) and 14·3 (experiment 2), increasing to 22·6 (proportionately 0·52) when grazing at 59 bites per min and to 16·6 (0·16) when grazing was at 28 bites per min. One hour after grazing at 59 bites per min (experiment 1) the EE was as high as during grazing, and in the next morning (after 5·5 h) of grazing remained at 19·7 kJ/h per kg M0·75 (0·32). No differences in energy expenditure were found between periods of grazing in experiment 1. It was concluded that the increase in energy expenditure of cattle due to the activity of grazing depends on the rate of biting. Grazing for 10 h at a moderate rate may boost EE proportionately by only 0·06, however grazing at the highest rates could easily add proportionately 0·20.

Energy expenditure of cattle walking on a flat terrain

A study was carried out to evaluate the effect of horizontal walking upon CO2 production rate by the carbon dioxide dilution rate technique. This was used as an indicator of animal energy expenditure. Two groups of three 18-month-old Aberdeen-Angus steers were assigned to two experiments. Average weights were 290 (s.e. 7·6) kg and 285 (s.e. 1·0) kg for experiments 1 and 2 respectively. Animals were allocated to individual pens and given 5·0 and 4·5 kg dry matter of a mixed diet for experiments 1 and 2 respectively. After a 45-day training period they were assigned to three walking treatments: 0 (T0), 3 (T3) and 6 (T2) km at 3 km/hfor 3 days in a Latin square design (3 × 3). 14C labelled sodium bicarbonate (5·4 μCi/h), diluted in carbonate-bicarbonate buffer sterile solution 0·1 mol/l, was infused for 92 h intraperitoneally with portable peristaltic pumps carried by the animals. The CO2 production rate was calculated as the ratio between the rate of infusion (μCi/h) and the specific activity of CO2 (μCi/ml CO2) in saliva samples, which were taken, in experiment 1, as an average of the day (09.00 to 16.00 h) and the night (16.00 to 09.00 h of the following day). In experiment 2 the day was divided as follows: prior to activity (09.00 to 13.00 h), activity (14.00 and 15.00 h) and post activity (16.00 h). CO2 production rate (ml CO2 per h per kg M0·75) at resting was 817 (412 kj/kg M0·75), increasing during walking to 1·46 of the resting level (T1 and T2, experiment 2) with no differences between the 1st and 2nd h of activity. One hour post activity, the CO2 production rate returned in T2 to the level of T0 but in T2 remained at 1·28 times that of T0. The average CO2 production rate during a complete day or night (experiment 1) was not affected significantly by the activity. Assuming that CO2 production rate during walking is 1·46 of resting (experiment 2) and remains at that level even at lower speeds, it can be estimated that a daily 6 km walk would increase resting energy expenditure from 1·04 when walking takes 2 h, as in this experiment (3 km/h), to 1·11 when the animal spends 6h(1 km/h).

Measuring energy expenditure in birds using bolus injections of 13C-labelled Na-bicarbonate

The (13)C-labelled Na-bicarbonate technique uses stable isotopes to measure energy expenditure in birds. After administration, the isotopes reach equilibrium within the body's bicarbonate pools at a fast rate due to the small size of the bicarbonate pool in relation to CO(2) flux. This technique is therefore ideal for measuring energy expenditure over short-term activities. The major advantage of this technique is that it can be applied without the animal having to wear a respirometry mask or being enclosed in a respirometry chamber. Despite the technique's suitability for use in birds and other animals, there have been few studies that have used it to date and so its potential is not fully understood. Here we discuss the methodology and review previous applications.

Whole-body CO2 production as an index of the metabolic response to sepsis

Whole-body carbon dioxide (CO2) production (RaCO2) is an index of substrate oxidation and energy expenditure; therefore, it may provide information about the metabolic response to sepsis. Using stable isotope techniques, we determined RaCO2 and its relationship to protein and glucose metabolism in medical patients with sepsis and septic shock. Whole-body CO2 production, an index of basal metabolic rate, was measured in 13 patients with sepsis or septic shock and 7 healthy controls using an i.v. infusion of 13C-sodium bicarbonate. Endogenous leucine flux, leucine oxidation, and nonoxidative disposal, indices of whole-body protein breakdown, catabolism, and synthesis, were measured with an infusion of 1-13C-leucine, and glucose production and clearance were measured with an infusion of 2H2-glucose. There was no difference in mean RaCO2 between the patients and controls, but the patients had a wider range of values. The four patients with the lowest RaCO2 died. Protein breakdown and synthesis and glucose production were significantly faster in patients than in controls (P < 0.05). Whole-body CO2 production was positively correlated with protein breakdown (P = 0.001), protein synthesis (P < 0.01), and glucose clearance (P = 0.01). Patients with low metabolic rates (mean-2 SDs of controls) had slower protein breakdown and decreased glucose clearance compared with patients with high metabolic rates (mean + 2 SDs of controls). Septic patients were both hypometabolic and hypermetabolic. The correlation between RaCO2 and protein breakdown and synthesis as well as glucose clearance suggests that RaCO2 can provide information about substrate metabolism in septic patients. Because hypometabolism was associated with mortality and changes in protein and glucose metabolism in septic patients, it may be a useful clinical indicator of an inadequate metabolic response.

Energy expenditure due to forage intake and walking of grazing cattle

The relative increment in daily maintenance requirement by physical activity was estimated in free-grazing cattle. The effects of forage harvesting and walking were measured and expressed as an index in relation to the value of energy expenditure of animals in corrals. All indices were obtained from experiments conducted in Balcarce (Argentina) from 1993 to 1995. Energy expenditure of Aberdeen Angus steers was estimated by the 14C-entry rate technique, on animals standing still in a corral, grazing at two different biting rates or walking at four different speeds. Information on time spent on grazing and distance traveled was obtained from the literature. These estimations indicate that the maintenance cost of cattle in corrals could be increased on pastures by a factor of 1.08 to 1.30, depending upon the grazing conditions. It was observed that grazing at high biting rate was the variable of highest effect on maintenance energy cost, and that walking or grazing at moderate biting rate was of lower incidence.

Measurement of carbon dioxide production in very low birth weight babies

BACKGROUND

CO2 production is most commonly measured by using indirect calorimetry to quantify elimination of CO(2) in breath (VCO2). An alternative is to measure the rate at which CO2 appears in the body pool (RaCO2) by infusing a (13)C labelled bicarbonate tracer. VCO2 and RaCO2 generally differ but are related by c, a factor that adjusts for the incomplete recovery of infused tracer in the breath. The literature relating to human studies cites a wide range of values for c but the only neonatal study to determine c empirically estimated a mean value of 0.77.

AIM

To estimate fractional recovery rate, c, in very low birthweight babies, and assess the feasibility of using the isotopic technique to measure CO2 production during mechanical ventilation.

METHOD

Eleven spontaneously breathing, continuously fed, very low birthweight infants (median birth weight 1060 g, median gestational age 29 weeks) were studied.

RESULTS

Mean (SD) VCO2 was 9.0 (2.0) ml/min (standard temperature and pressure dry, STPD) and mean (SD) RaCO2 was 9.6 (2.1) ml/min (STPD). The mean (SD) value of c was estimated as 0.95 (0.13). The 95% confidence intervals of the mean were 0.87-1.03.

CONCLUSIONS

The results emphasise the importance of measuring c for a given study population rather than assuming a value based on adult studies. The close approximation of RaCO2 and VCO2 in this group of babies implies that the labelled bicarbonate infusion technique could be used to measure simply CO2 production during mechanical ventilation.

Validation of a [13C]bicarbonate tracer technique to measure neonatal energy expenditure

The use of a stable isotope-labeled [13C]bicarbonate infusion to measure energy expenditure is advantageous, as a complete collection of expired air is not required. This technique allows for facile measurements of energy expenditure in intubated neonates. The aim of the present study was to determine the accuracy of energy expenditure estimates in postsurgical neonates by using the [13C]bicarbonate method compared with the current standard, indirect calorimetry. Eight neonates who were receiving total parenteral nutrition [98 +/- 21 (SD) kcal x kg(-1) x d(-1); 3.1 +/- 0.7 (SD) protein g x kg(-1) x d(-1)] were studied on postoperative d 15.5 +/- 11.9. A primed continuous 3-h intravenous infusion of NaH13CO3 and indirect calorimetry were performed simultaneously. Energy expenditure was calculated separately from the Weir equation and from the dilution of 13CO2 in the breath in combination with the individual energy equivalents of CO2 from the diet. The rate of CO2 appearance and energy expenditure calculated from the bicarbonate method (0.725 +/- 0.021 mol x kg(-1) x d(-1); 89.5 +/- 2.5 kcal x kg(-1) x d(-1)) highly correlated (r = 0.94 and 0.98, respectively) with the CO2 excretion and energy expenditure determined by indirect calorimetry (0.489 +/- 0.016 mol x kg(-1) x d(-1); 60.2 +/- 2.0 kcal x kg(-1) x d(-1)) when analyzed nonproportionately to weight. Bland-Altman analysis demonstrated the 95% confidence interval to be +/- 8.2 kcal x kg(-1) x d(-1). Linear regression analysis revealed a highly statistically significant equation relating the two energy expenditures: Indircal (kcal/d) = -9.341 + [0.705 x Bicarb (dcal/d)]; p < 0.001, r2 = 96.4%. We conclude that energy expenditure in neonates can be accurately determined using the [13C]bicarbonate method and a regression equation. Therefore, the bicarbonate method may be useful for determining energy expenditure in neonates not readily accessible to indirect calorimetry, such as those being mechanically ventilated or on extracorporeal life support.

Tissue sequestration of C-labelled bicarbonate [HCO3-] in fed and fasted young sheep

Carbon dioxide entry rates (CER) based on isotopic activities in either expired air or blood following a 24-h intravenous infusion of [13C]- and [14C] sodium bicarbonate were compared with CO2 production quantified by respiration hood in young sheep (28-30 kg) either fed (three animals) or fasted (three animals). CO2 production increased with intake (5.2 vs 10.3 mol/day; P < 0.001) as did CER values based on expired air (9.9 vs 18.6 mol/day; P < 0.001) or blood (7.5 vs 16.5 mol/day; P < 0.001). The differences between air and blood CER values were significant (P < 0.001). There were no differences, however, when data were compared between [13C] and [14C] measurements. How much of these differences could be attributed to sequestration of label in body tissues was examined at the end of the infusion. The highest specific radioactivities (dpm/g dry matter) in acid-fast tissue material were observed for the more metabolically active tissues, liver, jejunum and kidney, with the lowest values for fat and muscle. When tissue mass was taken into account, however, the largest proportions of the dose sequestered were in bone muscle, skin and fat with significantly more retained for the former three (P < 0.01) during fasting. Separately, losses as urinary urea were also quantified. Total measured sequestration of label only accounted for approximately 24-44% of the difference between CER and CO2 production.

Measuring energy costs of leisure activity in adolescents using a CO2 breath test

To determine whether a 13C-bicarbonate, isotope dilution technique could be used to estimate relative changes in energy expenditure of leisure activities of short duration, we studied eight adolescents who performed the following activities: watching television (120 min); playing a stringed instrument (60 min plus 60 min of sitting); and walking plus rest during two approximately isocaloric sessions (slow walk at 40% of peak VO2 for 43 min plus 77 min of sitting; fast walk at 73% of peak VO2 for 22 min plus 98 min of sitting). The rate of appearance of CO2 (RaCO2) was determined from the ratio of the oral dose of 13C-bicarbonate and the isotopic enrichment of breath CO2. The net rates of excretion of CO2 (VCO2) and oxygen consumption were measured. VCO2 and RaCO2, were correlated (r = 0.93; P < 0.05). To adjust for the systematic difference in CO2 production between methods, determinations were expressed as a fraction of that during television viewing. For RaCO2, the ratios for instrument playing, walking at 40% peak VCO2, and walking at 73% peak VO2 were respectively 133 +/- 20%, 186 +/- 38%, and 206 +/- 34%; for VCO2, the respective ratios were 129 +/- 19, 210 +/- 50, and 232 +/- 39 (P > 0.05 for methods and interaction, two-way ANOVA). RaCO2 may be a useful method for detecting relative differences in energy expenditure associated with leisure activities of brief duration.

Estimation of CO2 production in enterally fed preterm infants using an isotope dilution stable tracer technique

BACKGROUND

Estimates of the rate of CO2 production may be useful in preterm infants, but assessment of the rate of respiratory excretion of CO2 (VCO2) may not always be practical in infants requiring constant care. We hypothesized that the rate of dilution of 13CO2 (RaCO2) would be a valid index of CO2 production in preterm infants.

METHODS

Twelve studies of RaCO2 and VCO2 were performed in six enterally fed preterm infants. RaCO2 was measured using a 2-hour, primed, constant, orogastric infusion of NaH13CO3 with formula and an assessment of the plateau 13C enrichment of expired CO2. VCO2 was measured over two 10-minute intervals during the infusion using a flow-through system. Energy expenditure was estimated from these data and the food quotient.

RESULTS

Mean (+/- SD) rate of CO2 production using RaCO2 (348 +/- 32 mumol/kg/min) was 114% of that estimated using VCO2 (304 +/- 51 mumol/kg/min). The ratio of VCO2/RaCO2 is equal to the fractional recovery of tracer CO2 in the expired air during the course of the tracer infusion. In studies of short duration, this ratio is generally less than 100% because of isotope exchange. For five pairs of studies performed on consecutive days, each individual value of RaCO2 on day 2 was multiplied by the mean of the individual ratios of VCO2/RaCO2 on day 1 (0.78); corrected RaCO2 was 306 +/- 19 mumol/kg/min compared with 307 +/- 59 mumol/kg/min for VCO2.

CONCLUSIONS

Thus, RaCO2, particularly when corrected for isotope recovery, may be a useful index of group mean CO2 production and energy expenditure in preterm infants.

Exercise and the oxidation and storage of glucose, maize-syrup solids and sucrose determined from breath 13CO2

In order to determine which of maize syrup solids, glucose and sucrose were more readily oxidised during exercise and least readily oxidised afterwards, the rates of oxidation of three almost identical isoenergetic solutions of carbohydrates (330 ml of 18.5% w/v solutions of glucose, maize syrup solids and sucrose, 989-1050 kJ total energy) naturally enriched with 13C were examined at rest and during and after 1 h uphill walking at 75% maximum oxygen uptake (VO2max) in nine subjects [mean (SEM) VO2max, 45.4 (0.9) ml.kg-1.min-1]. Rates of production of expired 13CO2 were used to estimate rates of oxidation of each exogenous substrate. Energy expenditure and the contributions from total carbohydrate and fat oxidation were calculated from whole-body gas exchange. At rest, maize syrup solids were oxidised less than sucrose during the 1st h [glucose 2.7 (0.2) g.h-1, maize syrup solids 1.9 (0.3) g.h-1, sucrose 3.7 (0.2) g.h-1; maize syrup solids vs sucrose P < 0.01], but this difference disappeared after a further 3 h at rest [glucose 8.3 (0.5) g.h-1, maize syrup solids 7.7 (0.5) g.h-1, sucrose 8.1 (0.4) g.h-1]. During exercise, all the carbohydrates were oxidised to the same extent [glucose 23.0 (2.8) g.h-1, maize syrup solids 23.9 (3.4) g.h-1, sucrose 27.5 (2.6) g.h-1) but during 4 h of recovery after exercise, maize syrup solids were oxidised least [glucose 4.6 (0.1) g.h-1, maize syrup solids 3.7 (0.1) g.h-1, sucrose 6.4 (0.1) g.h-1; P < 0.05] suggesting that it may be stored to a greater extent. The results suggest that 18.5% glucose, maize syrup solids and sucrose solutions were equally well oxidised during exercise. During recovery from exercise maize syrup solids were oxidised less than glucose, which in turn was oxidised less than sucrose.

Kinetics of 13CO2 elimination after ingestion of 13C bicarbonate: the effects of exercise and acid base balance

In order to investigate the effects of muscular work and preceding exercise on the retention of exogenous labelled bicarbonate, we studied the effects of oral administration of [13C]bicarbonate (0.1 mg kg-1) in five subjects at rest before exercise and during and after 1 h of treadmill walking at 73% VO2max on three separate occasions. Elimination of CO2 from labelled bicarbonate was 62.6 +/- 8.1% at rest, 103.6 +/- 11.3% during exercise (P < 0.01) and 43.0 +/- 4.7% during recovery from exercise (P = 0.01). During exercise mean residence time (MRT) was shorter than at rest (35 +/- 7 min vs. 54 +/- 9 min, P < 0.02) and CO2 pool size was larger (998 +/- 160 ml CO2 kg-1, vs. 194 +/- 28 ml CO2 kg-1, P < 0.001). Compared to values obtained at rest, during recovery from exercise, MRT and CO2 pool size were reduced (34 +/- 5 min, P < 0.05; 116 +/- 19 ml CO2 kg-1, P < 0.02, respectively). In an additional five subjects acidosis and alkalosis were induced prior to administration of oral [13C]bicarbonate at rest. Elimination of bicarbonate was lower during acidosis (46.1 +/- 5.6%, P < 0.01) but was unaltered (50.9 +/- 5.6%, NS) during alkalosis, compared to the values obtained at resting pH. During acidosis MRT and CO2 pool size decreased (37 +/- 3 min, P < 0.01 and 123 +/- 10 ml CO2 kg-1, P < 0.01, respectively) whereas in alkalosis MRT was unchanged (65 +/- 8 min NS) but CO2 pool size was increased (230 +/- 23 ml CO2 kg-1, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Feeding-induced changes in energy expenditure in children with cystic fibrosis

Seven children with cystic fibrosis (aged 7 to 12 years) were studied in the fasted and fed states. Using a primed, constant, intravenous infusion of NaH13CO3, the rate of appearance of CO2 (RaCO2) was estimated. Net CO2 excretion (VCO2) was also measured. Energy expenditure was calculated using the food quotient. RaCO2 (mean +/- SD) (mumol.kg-1.min-1) in the fasted and fed states (297 +/- 59 and 359 +/- 67) was 117% and 105% of VCO2 (259 +/- 48 and 352 +/- 72). Feeding induced a 23% and a 37% increase in RaCO2 and VCO2, respectively, and respective 19% and 33% increases in energy expenditure (p < .05). Measurement of CO2 production by isotopic dilution is a useful index of group changes in energy expenditure, including those induced by feeding.

Non-invasive techniques for assessing carbohydrate flux: II. Measurement of deposition using 13C-glucose

A non-invasive method for studying the dynamics of post-exercise carbohydrate storage by means of whole-body calorimetry and 13CO2 breath tests is described. Seven untrained glycogen-depleted subjects were offered naturally 13C-labelled high carbohydrate meals (97% by energy) at 30 min intervals for 5 h and asked to consume as much as possible. Mean intake averaged 757 +/- 211 (SD) g. Exogenous carbohydrate oxidation over 16 h calculated from gas exchange and isotope ratio measurements averaged 161 +/- 45 g, and endogenous carbohydrate oxidation averaged 31 +/- 25 g. Net carbohydrate storage, calculated as the difference between amount ingested and oxidized, was 563 g which was more than twice the measured hepatic and muscle carbohydrate oxidized during the depletion phase. After correction for body size the major determinant of glycogen storage was the amount of carbohydrate consumed (r = 0.97, P < 0.001) which in turn was determined by each subject's dietary tolerance. Post-repletion exercises (12 h after last meal) were used to remobilize freshly stored glycogen. 13CO2 enrichments indicated that a substantial part of the new glycogen was derived from the exogenous carbohydrate provided by the repletion meals.

Measurement of bicarbonate turnover in humans: applicability to estimation of energy expenditure

Bicarbonate turnover and energy expenditure were assessed in six healthy male volunteers, by the use of a constant infusion of radiolabeled bicarbonate (NaH14CO3) administered over 36 h, while the volunteers were confined to a whole body indirect calorimeter. Recovery and dilution of isotope were assessed from measurements made on continuous collections of CO2, entering and leaving the calorimeter, urine, and intermittent spot breath and saliva samples. Mean recovery of infused label in gaseous CO2 was 95.6 +/- 1.1% (SD) between 12 and 36 h. Applying a 95% mean recovery of label to each subject individually enabled the use of integrated mean specific activity of CO2 in spot breath and urine samples to predict measured net CO2 production and energy expenditure to within about +/- 6%. Estimates based on urinary measurements were compromised slightly by the exchange of label through the bladder wall (this was dependent on pH and volume of urine). It is concluded that this constant-infusion labeled bicarbonate method offers a potentially useful means of assessing net CO2 production and total energy expenditure over the short term (e.g., 1-3 days).

Use of [3-3H]glucose and [6-14C]glucose to measure glucose turnover and glucose metabolism in humans

[3-3H]glucose is frequently used to measure glucose turnover in humans. If fructose 6-phosphate-fructose 1,6-diphosphate cycling (Fpc) is negligible in both liver and muscle, then [3-3H]- and [6-14C]glucose (corrected for Cori cycle activity) should provide equivalent measures of glucose turnover. In addition, if glycogenolysis is fully suppressed, then [14C]lactate specific activity should equal that of [6-14C]glucose from which it was derived, and oxidation of [6-14C]glucose, as measured by rate of generation of 14CO2, should equal total glucose oxidation (i.e., that derived from intra- and extracellular pools) as measured by indirect calorimetry. To address these questions, glucose turnover was measured simultaneously with [3-3H]- and [6-14C]glucose in the basal state and in presence of low (approximately 200 pM) and high (approximately 750 pM) insulin concentrations. Glucose turnover rates measured with [3-3H]- and [6-14C]glucose were equivalent at all insulin concentrations, indicating that Fpc had no detectable effect on measurement of glucose appearance. [14C]lactate specific activity was lower (P less than 0.01) than that of [6-14C]glucose in the basal state but not during either low- or high-dose insulin infusion, implying that all lactate was derived from extracellular glucose. On the other hand, glucose oxidation as measured by rate of generation of 14CO2 was lower (P less than 0.05) than glucose oxidation as measured by indirect calorimetry during both insulin infusions, implying either that suppression of glycogenolysis was not complete in all tissues or that one or both of these techniques do not accurately measure glucose oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

Energy equivalents of CO2 and their importance in assessing energy expenditure when using tracer techniques

Carbon dioxide production in free living animals and humans can be measured using tracer techniques, but the prediction of energy expenditure also requires an estimate of the energy equivalents of CO2 (energy expended/CO2 produced; EeqCO2). This work is concerned with assessing the variation in EeqCO2 with the use of dietary information, indirect calorimetry, and theoretical concepts. The EeqCO2 for diets (EeqCO2 diet) ingested by 63 individuals living in a Cambridgeshire village, UK, was found to vary by less than 10%. The EeqCO2 diet for different populations varied by greater than 10% and for artificial enteral feeds by approximately 20%. Alcohol increases this variability because it has a particularly high EeqCO2. Variation in the nitrogenous end products of metabolism may also have a substantial effect on the EeqCO2 for a subject (EeqCO2 body), especially when a large proportion of energy expenditure is derived from protein oxidation, as in strict carnivores. Nutrient/energy imbalances such as those associated with growth, hypercaloric feeding, or starvation may also have major effects on EeqCO2 body. It is concluded that the calculation of energy expenditure from CO2 production should not employ a universal value for EeqCO2 body. The value should take into account the physiological and clinical state under investigation. Practical recommendations are suggested.

Does mitochondrial compartmentation of CO2 exist in man?

A comparison was made between the specific radioactivity of urea, and that of CO2 in breath, in urine and in arterialized blood, during a 36-h continuous infusion of 0.5 mCi and 100 mmol of sodium bicarbonate (NaH14CO3) into six normal male volunteers. After a period of equilibration, the mean specific radioactivity of urea was found to be only 16% below that of end expiratory CO2 and a similar amount below that of CO2 both in arterialized blood and in urine. This difference may be explained by isotopic dilution of 14CO2 by metabolic CO2 produced in the splanchnic tissues. It is concluded that, in these normal subjects, there is little or no compartmentation between cytosolic CO2 and the mitochondrial CO2 used for urea synthesis.