Total daily energy expenditure (DEE) ranges from 20 to 100 kcal/kg body weight (BW) in healthy adult research cats, reflecting resting energy expenditure (REE), physical activity, and postprandial thermogenesis.[1-4] After an overnight fast, REE usually represents 60–75% DEE. Although the effects of overconditioning, gonadectomy, and age on energy requirements have been investigated in research cats, few studies have examined energy expenditure in pet cats.[3, 5-8]
Different techniques for estimating DEE include quantitative food intake and weight balance studies, doubly labeled water metabolism and dispersal, and whole body calorimetry.[5-12] Regrettably, each method has clinical disadvantages. Practically, open-flow indirect calorimetry (IC) is the gold standard for short-term energy assessments and can estimate REE at a fixed time point. Although a validated open-flow IC-system can estimate REE in <1 hour in an acclimatized patient, few investigations of energy utilization in cats using this methodology have been reported.
Various formulas are used to calculate DEE in cats.[3, 5, 13] Yet, controversy surrounds application of each approach and the scaling factors designed to represent metabolically active tissue.[14-17] Differences in lean body mass (LBM), activity, and inherent metabolic adjustments (eg, disease, diet, feeding frequency) influence energy utilization, compromising accuracy of predictive equations. Studies in humans and experimental animals confirm that estimated DEE is most meaningful when normalized to LBM. Still, normalization against LBM can be criticized because fat mass (FM) is not metabolically inert, contributing up to 15–20% equivalent LBM energy utilization.[17, 18] Nevertheless, practical rapid estimation of REE and LBM may have utility in nonsedated cats.
Estimation of LBM in cats has been accomplished using body condition scores (BCS), dual-energy X-ray absorptiometry, stable isotope (deuterium [D2O] or doubly labeled water [D2O with H218O]) dilution, or multifrequency bioelectrical impedance analysis (MF-BIA).[19-22] Only BIA has potential for practical objective clinical application.[21, 22] Considering that LBM importantly influences energy utilization, its accurate estimation might be used to normalize energy data allowing relevant comparisons among patients and disease states. To date, illness energy quotients used to modify DEE estimates are poorly substantiated.[9, 23-26] Merging techniques of open-flow IC-REE and MF-BIA LBM in cats could allow normalization of energy expenditures among cats with various body conditions and disease states.
The present study of cats had 3 aims: (1) develop and validate an open-flow IC-system for REE estimation; (2) validate an MF-BIA instrument in nonsedated cats to estimate total body water (TBW) and extracellular water (ECW) compartments against criterion reference standards (D2O determined TBW [TBWD] and sodium bromide [NaBr] determined ECW [ECWBr]) to estimate LBM; and (3) normalize and compare IC-REE/LBM (kg) in various body conditions.
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- Materials and Methods
Our findings confirm that MF-BIA can reasonably estimate TBW and LBM in nonsedated cats of various body conditions. Results compare favorably with values previously established for this test system applied to sedated cats. LBMBIA in nonsedated cats had ±10% reproducibility error and ±0.19 kg error, comparing favorably to criterion reference standards and suggesting potential clinical utility. Although MF-BIA is a reasonable method for estimating LBM in nonsedated cats, adoption of this method requires validation against criterion reference standards, as described herein, and in a former pioneering study. Despite attempts to closely align study conditions to a previously described system, we were unable to directly adapt resistivity scalers for our equipment and procedure. Unique differences in equipment circuitry, methods of criterion reference determinations, and environmental conditions (eg, conductivity of table surfaces, building circuitry), and our choice to avoid animal sedation probably contributed to this dilemma. However, in nonsedated cats we demonstrated best MF-BIA performance using a different electrode array then previously reported, probably reflecting the ease in replicating sternal recumbent posture and NT array in nonsedated animals.[21, 22, 42]
We found that MF-BIA estimation of LBM surpassed subjective BCS assignment in identifying under-conditioned cats. Because individuals assigning BCS were amply experienced, poor correlation with LBM% quantified by MF-BIA and D2O may reflect inability of subjective physical assessment to discriminate the 80% LBM cutoff used to assign underconditioned status. However, it also may suggest that BCS of 2.5 is not consistent with an underconditioned animal, supported by another study in which BCS was compared to dual-energy x-ray absorptiometry. These findings suggest caution in using BCS assignment to validate MF-BIA equipment in lieu of criterion reference standards.22,
Using methodologies described herein, and in a prior single report, short-term open-flow IC can reasonably estimate REE in cats. However, measurement repeatability requires exacting control of equipment and variables and careful acclimatization to the procedure. Custom-built cages adapted to each subject thwarted formation of stagnant gas pockets and facilitated rapid flux of gases from the respiratory chamber in the present study. Care must be taken to acclimatize cats to IC to ensure that REE values reflect “least-observed-metabolism.” This study constitutes the largest data set of cats (n = 57) undergoing short-term IC and documents REE ranging between 120 and 215 kcal, 26 and 46 kcal/kg, and 38 and 68 kcal/LBM (kg). The high degree of REE repeatability allowed confident detection of ±15% change among serial REE assessments with 95% limit of agreement reconciling with ± 22 kcal/cat. Although short-term IC-REE and RQ reported herein were not compared with longer interval (ie, 12–24 hours) measurements, they do compare well with previously reported REE estimates (measured or estimated from DEE [ie, ≤ 60% DEE]) determined by a variety of methods.[8, 11, 12]
Diet-associated thermogenesis influences REE, with protein and fat digestion, and metabolism requiring energy and raising postabsorptive interval REE. Because all cats had food withheld for 12 hours before IC-REE, it is unlikely that meal-associated thermogenesis influenced findings. Although the 2 diets fed in this study had comparable protein, but different fat and carbohydrate contents, disparity in REE or RQ between groups was not realized. Measured REE herein also might reflect thermoregulatory metabolic activity considering that we maintained the ambient calorimetry temperature below the thermoneutral zone of cats (35–38°C) in an effort to avoid cage overheating during data collection. Calculation of REE using a modified Weir formula estimates protein contribution (nitrogen) to energy utilization. Yet, REE estimation from respiratory gases (in humans) reportedly imparts an approximate 1.0% error for every 12.3% increment in protein contribution to total oxidative metabolism. How much this error influences IC-estimated REE in cats remains unclear.
Isotope-estimated DEE (234–398 kcal/cat, 37–61 kcal/kg BW, 63–101 kcal/LBM [kg]) compared favorably with other reported estimates.[6, 7] Isotope distributional water space and elimination rate constants were consistent with previously published parameters in domestic cats. We did not identify differences in DEE among cats with different body conditions, possibly because of the small number of cats investigated. Regardless of body condition, IC-estimated REE approximated 60% of DEE.
Expression of REE or REE/kg in overconditioned and underconditioned cats can misrepresent REE relative to normal-conditioned animals. Transforming BW with allometric scaling (BW0.75) did not alleviate this conundrum. Evaluating REE with cats stratified by body condition (LBM%) confirmed higher REE in over-conditioned and lower REE in underconditioned cats compared with normal-conditioned animals. Conversely, REE/kg BW was higher in underconditioned and lower in overconditioned cats compared with normal-conditioned animals. However, when REE was normalized for LBM (kg), cats with diverse body conditions had similar energy utilization (~52–55 kcal/LBM [kg]). This observation has important clinical and investigational relevance, and emphasizes the value of accurate LBM assessments when energy measurements are evaluated or sequentially compared.
Although FQ for the 2 diets fed were 0.87 and 0.84, a lower mean RQ was estimated by IC reconciling with the fasting status of studied cats (in neutral weight balance). Measured RQ reflects energy utilization during the postabsorptive interval as well as protein catabolism, consistent with the high rate of amino acid turnover associated with the pure carnivore status of cats. The mean RQ determined herein (0.82 ± 0.04) is discordant with the mean RQ (0.86 ± 0.03) reported for a small group of moderately overconditioned experimental cats in stable BW. However, in that study, IC was done using large cage enclosures (~10-fold larger volume than in the present study) with data collected over 12 hours with food and water available ad libitum. Thus, estimated RQ probably reflected ingested fuel sources. Predictably, when those cats were energy restricted, mean RQ (0.81 ± 0.03) was similar to values reported herein.
In conclusion, this study validated an open-flow IC-system that is adaptable for short-term estimation of REE in domestic cats and an MF-BIA procedure that can be used for LBM estimation clinically in cats. These analytical tools may enable useful evaluations in cats with diverse health concerns. Indeed, serial MF-BIA hydration monitoring has been performed in animals undergoing hemodialysis (L.D. Cowgill, personal communication). The ability to rapidly determine quantitative LBM may critically modify dosing strategies for drugs and fluid treatment in over- and underconditioned cats, and might potentially assist in providing nutritional recommendations.