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

Quantifying energy expenditure in Göttingen Minipigs with the 13C-bicarbonate method under basal and drug-treated conditions

Effective treatments of obesity focusing on energy expenditure (EE) are needed. To evaluate future EE-modulating drug candidates, appropriate animal models and methods to assess EE are needed. This study aimed to evaluate the stable isotope 13C-bicarbonate method (13C-BM) for estimating EE in Göttingen minipigs under basal and drug-treated conditions. Four experiments (Expt.1-4) were conducted to assess: 1) the 13C-BM reproducibility using breath sample collection (n = 8), on two consecutive days, 2) the effect of two dose levels (5 and 10 mg/kg body weight (BW)) of the mitochondrial uncoupler dinitrophenol (DNP) in a crossover design (n = 8), 3) sampling method agreement; blood vs. exhaled air (n = 6) and 4) 13C-BM using constant isotope infusion compared with indirect calorimetry (IC) (n = 3). Results correlated significantly (p < 0.001) between days (Expt.1), with an average coefficient of variance of 5.4 ± 2.3%. Administration of 10 mg DNP/kg BW increased (p < 0.01) EE by 33.2 ± 6.4% (Expt.2). Results based on different sampling methods correlated significantly (p < 0.001) and EE increased after 10 mg DNP/kg BW (p < 0.05) in Expt.3. However, results based on blood sampling were significantly higher (p < 0.01) than those of exhaled air. No effect of DNP and significantly different EE results (p < 0.05) was observed in a limited number of animals, when constant isotope infusion and blood sampling was compared with IC (Expt.4). In conclusion, the 13C-BM is useful for investigating treatment effects on EE in minipigs. However, further validation under standardized conditions is needed to provide accurate estimates of the 13C recovery factor and respiratory quotient, both of decisive importance when using the 13C-BM.

The 13C-bicarbonate technique as a tool for measurement of energy expenditure in overweight dogs undergoing body weight reduction and the effect of different dietary composition

Changes in body size and composition, i.e., body weight (BW) gain or loss, affect the daily energy expenditure (EE). To ensure an appropriate BW reduction and to find an efficient strategy to reduce and maintain a target BW, regular evaluations and adjustments of energy allowance are important. This study aimed to provide a detailed knowledge about the possible changes in resting EE using the oral 13C-bicarbonate technique (o13CBT) as a research tool in 16 overweight pet dogs undergoing BW reduction. Dietary composition (i.e., in % of dry matter [DM] being a high protein [33.3], low fat [9.6], and high crude fiber [18.0] diet [LFHFibre], and a high protein [37.9], high fat [52.0], carbohydrate-free diet [HFat]) during 16 wk of energy restriction were evaluated regarding effects on resting EE, rate of BW reduction, body composition, and plasma concentrations of metabolic hormones involved in energy metabolism and appetite regulation. The mean BW loss was higher (P < 0.05) for the dogs fed the LFHFibre diet (1.1%/wk) than that for dogs fed the HFat diet (0.8%/wk), but the total BW reduction of 14.6% and 12.0% of initial BW did not differ significantly (P > 0.05). Resting EE was lower (P < 0.02) after the BW reduction; 414 kJ (99 kcal)/kg BW0.75/d at the start (week 0) and 326 kJ (78 kcal)/kg BW0.75/d at the end (week 16) of the study. The BW reduction in both groups (P > 0.05) consisted of both fat mass (FM) and fat-free mass (FFM). Energy expenditure, calculated in relation to amount of FFM, was not significantly (P > 0.05) affected by BW reduction. Dietary composition did not significantly affect (P > 0.05) plasma concentrations of insulin, leptin, and ghrelin, and no effect (P > 0.05) of BW reduction was observed on hormone concentrations. In conclusion, the o13CBT proved to be a useful research method for studying short-term EE in overweight dogs. Even though all dogs lost BW, most dogs were still overweight at the end of the study. Due to a high individual variation among dogs, a longer experimental period with a larger sample size would be desirable.

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 novel doubly labelled 13C, 15N amino acid method for measuring energy and protein metabolism in man

A novel doubly [1-¹³C, α-¹⁵NH₂]-labelled amino acid method (DLAAM) is presented for the determination of the CO₂ production (RCO₂) and energy expenditure in humans. This method is based on the simultaneous administration of [1-¹³C]glycine and [¹⁵N]glycine followed by the measurement of excretion kinetics of breath ¹³CO₂ and urinary ¹⁵N. The basic idea of the DLAAM is that the unknown ¹³C recovery RF(¹³C) of the 1-¹³C amino acid, essential for the calculation of the net CO₂ production, can be approximated by the easily measureable ¹⁵N recovery RF(¹⁵N) of the α-¹⁵NH₂ labelled amino acid. In four healthy adult men (76-97 kg) the DLAAM was tested parallel to the IC and in one man (74 kg) parallel to the DLWM. Using the approximation RF(¹³C) ≈ RF(¹⁵N) the RCO₂ (in l CO₂ d^-1) was calculated to 387.0 ± 30.3 (DLAAM) vs. 382.8 ± 22.6. (IC). The Bland-Altman plot shows that the difference between the DLAAM and IC of individual RCO₂ is within the 95 % confidence interval (mean ± 2 SD): +4.3 ± 37.5 l CO₂ d^-1. We conclude that the DLAAM and IC may be used interchangeably. The physical activity level (PAL) was calculated based on the DLAAM vs. DLWM to about 1.5.