Validation of an equation for energy expenditure that does not require the respiratory quotient

Abstract

BackgroundEnergy expenditure (EE) calculated from respirometric indirect calorimetry is most accurate when based on oxygen consumption (VO2), carbon dioxide production (VCO2) and estimated protein metabolism (PM). EE has a substantial dependence of ~7% on the respiratory quotient (RQ, VCO2/VO2) and a lesser dependence on PM, yet many studies have instead estimated EE from VO2 only while PM has often been ignored, thus reducing accuracy. In 1949 Weir proposed a method to accurately calculate EE without using RQ, which also adjusts for estimated PM based on dietary composition. This RQ- method utilizes the calorimeter airflow rate (FR), the change in fractional O2 concentration (ΔFO2) and the dietary protein fraction. The RQ- method has not previously been empirically validated against the standard RQ+ method using both VO2 and RQ. Our aim was to do that.MethodsVO2 and VCO2 were measured repeatedly in 8 mice fed a high protein diet (HPD) during exposure to different temperatures (n = 168 measurements of 24h gas exchange). The HPD-adjusted RQ+ equation was: EE [kcal/time] = VO2 [L/time]×(3.853+1.081RQ) while the corresponding RQ- equation was: EE = 4.934×FR×ΔFO2. Agreement was analyzed using the ratios of the RQ- to RQ+ methods along with regression and Bland-Altman agreement analyses. We also evaluated the standard equation using the dietary food quotient (FQ) of 0.91 as a proxy for RQ (FQ+ method).ResultsRatio analysis revealed that the mean error of the RQ- method was only 0.11 ± 0.042% while the maximum error was only 0.21%. Error using the FQ+ method was 4 -and 10-fold greater, respectively. Bland-Altman analysis demonstrated that the RQ- method very slightly overestimates EE as RQ decreases. Theoretically, this error can be eliminated completely by imposing an incurrent fractional oxygen concentration at a value only slightly greater than the atmospheric level.ConclusionsThe Weir ‘RQ-free’ method for calculating EE is a highly valid alternative to the ‘gold standard’ method that requires RQ. The RQ- approach permits reduced cost and complexity in studies focused on EE and provides a way to rescue EE measurement in studies compromised by faulty CO2 measurements. Practitioners of respirometry should consider adjusting EE calculations for estimated protein metabolism based on dietary composition.

CalR: A Web-Based Analysis Tool for Indirect Calorimetry Experiments

“…However, this nuance is lost in ANOVA/ANCOVA/GLM analyses. Regardless of the high-resolution time data, the consensus approach of ANCOVA-like analysis depends upon a single mean value per metabolic variable per mouse (Kaiyala; Speakman et al, 2013; Tschöp et al, 2012). Future advances may be possible by implementing time series and body composition into a statistical framework for indirect calorimetry.…”

Gas7 knockout affects PINK1 expression and mitochondrial dynamics in mouse cortical neurons

“…Monitoring was performed for 5 days after mice had acclimatized to cages for 2 days. The respiration quotient (RQ) was calculated as the ratio of VCO 2 /VO 2 . The system acquired data on fractional O 2 and CO 2 concentrations, running wheel speed, water vapor pressure (WVP), barometric pressure (BP), ambient temperature and light levels, flow rates, food and water dispenser masses (to 1 mg), body masses (to 1 mg via a weighed enrichment habitat), running wheel revolutions, and X, Y and Z locations together with beam breaks.…”

“…The respiration quotient (RQ) was calculated as the ratio of VCO 2 /VO 2 . 20 The system acquired data on fractional O 2 and CO 2 concentrations, running wheel speed, water vapor pressure (WVP), barometric pressure (BP), ambient temperature and light levels, flow rates, food and water dispenser masses (to 1 mg), body masses (to 1 mg via a weighed enrichment habitat), running wheel revolutions, and X, Y and Z locations together with beam breaks. Measurements were acquired at a sample rate of 1 sample/sec for all sensors and cages simultaneously via an error-correcting control area network (CAN).…”

Do Calorimetric Results in Mice Depend on the System Being Used?

“…Compared with the OxyMax system, Promethion activity counts were lower; food intake during HC feeding was lower; and the respiratory exchange ratio during HC feeding was both lower and below the diet’s food quotient (FQ) (RER is carbon dioxide production [VCO 2 ] ÷ oxygen uptake [VO 2 ]; FQ is the RER predicted by diet composition when mice are in energy balance during measurements ). The RER result is puzzling because neither VO 2 or VCO 2 nor estimated heat production during HC feeding differed between calorimeter systems; because both groups remained nearly weight stable with the overall group difference in weight change being trivial (< 0.5 g); and because RER during HC feeding agreed with FQ in other Promethion studies .…”

“…Notable strengths of Soto et al include a randomized crossover design, control of ambient temperature at thermoneutrality to preclude confounding by cold stress , and the inclusion of group difference testing between male and female C57BL/6J mice. However, problematic issues include a large number of statistical comparisons with no protection for multiple comparisons.…”