Body composition analysis is essential for meaningful medical follow-up of individual patients,1 but measurement of body weight alone is not a reliable method of evaluation. Therefore, simple and effective techniques for evaluation of body composition are highly desirable.
Traditionally, the body is divided into 2 compartments (the water compartment and tissue compartment). The water compartment, or TBW, is divided into extracellular and intracellular compartments. The extracellular compartment consists of plasma and interstitial fluids in which cells are found. The tissue compartment consists of the FM and FFM. The FM is regarded as an energy reserve, whereas the FFM represents the metabolically active fraction of a living body. Thus, FFM is a key component in a variety of physiological states (eg, growth, aging, and physical activity) as well as in various disease conditions (eg, metabolic diseases, cancer, and obesity). Because the FM does not contain any water, all the body water within the tissue compartment is contained within the FFM. Therefore, the FFM can be estimated by dividing the TBW mass by the mean hydration coefficient of the body. Subtraction of the TBW mass from the body weight yields the mass of the tissue compartment, and the tissue compartment mass minus the FFM yields the FM. Consequently, evaluation of TBW enables determination of the body composition2 because the mean hydration rate is constant and is approximately 73.2% in humans3 and 74.4% in dogs.4
Isotopic dilutions are generally used as reference techniques for evaluation of body fluid content. Deuterium dilution is frequently used to estimate TBW content because deuterium is a stable nontoxic compound that can be accurately analyzed in biologic fluids and its distribution in the body is the same as that of water. In humans and dogs, deuterium concentrations are commonly measured by mass spectrometry5,6 or infrared spectrometry.7,8 Deuterium concentrations in dogs have also been measured by Fourier-transform infrared spectroscopy.9 Results of multiple studies10–12 indicate that the deuterium dilution technique for estimation of TBW content in dogs is accurate to within 1% to 2% of the actual TBW content. However, the deuterium dilution technique is a lengthy and costly process that requires animals to be anesthetized, the collection of several blood samples, and expensive equipment that is rarely found in general veterinary practices, which makes it impractical in most situations. Isotopic dilution is most useful for validation of alternative techniques for evaluation of body composition, such as DEXA13,14 or quantitative MRI.14
A 9-point scale has been proposed for assessment of the body condition of dogs that involves observation and palpation of the animal to evaluate the percentage of body fat.15 This semiquantitative technique provides an estimate of FM. Each point in the scale differs by approximately 5% from the contiguous points. A score of 5 on this scale corresponds to an ideal FM of 17.5 ± 10% for male dogs and 19.9 ± 10% for female dogs.15 An algorithm-based system called S.H.A.P.E. that also uses observation and palpation has likewise been validated for estimation of the body composition of dogs.16 Both techniques are easy, repeatable, and dependable when used by trained staff, and the results in terms of percentage of body weight can be correlated with those obtained by DEXA. However, only superficial muscles and fat volume are evaluated by those 2 methods, and the margin of error associated with the incomplete evaluation of body composition makes those techniques unsuitable for precise monitoring of changes in body composition over time.
Bioelectrical impedance is an alternative to dilution techniques and DEXA for evaluation of body composition. This technique has been validated and is routinely used to estimate the body composition of healthy and ill humans.17–19 It is quick and noninvasive and uses a low-intensity electrical current that is undetectable by the subject being evaluated. The equipment required for bioelectrical impedance is inexpensive. Bioelectrical impedance has been successfully used in unanesthetized dogsa and appears to be a viable method for the measurement of body composition in that species.
The principles of bioelectrical impedance have been described in detail.17,20,21 Briefly, impedance is a measure of the opposition to an alternating electrical current and is composed of tissue resistance and tissue reactance. It is directly proportional to tissue length and inversely proportional to tissue diameter.20A tissue has a certain degree of resistance to the passage of a current depending on its electrolyte and water content. Fat mass has a high degree of resistance to current transmission (ie, high impedance), and cellular membranes behave like electrical conductors and generate tissue reactance. Currents with low frequencies (< 5 kHz) do not penetrate through cell membranes and remain in the extracellular space, whereas currents with high frequencies (≥ 50 kHz) pass through both the extracellular and intracellular spaces.22
Single-frequency BIA with a current frequency of 50 kHz has been used and validated for evaluation of TBW content in healthy men with a mean error range of −3.3% to 1.1%.23–27 In the earliest study,28 2 electrodes were used to perform single-frequency BIA, but the technique evolved and is now commonly performed with 4 electrodes.29 The use of single-frequency BIA for evaluation of TBW content has been validated with various types of equipment for several species including dogs,30,31,b which resulted in the derivation of linear regression equations for estimation of TBW content. Unfortunately, those equations can no longer be used because the equipment used to derive them is no longer available or the results have been found to be inconsistent and unreliable,32 most likely because some measurements were obtained from anesthetized animals and others were obtained from unanesthetized animals. Because the regression equations traditionally used to estimate TBW content from single-frequency BIA were developed on the basis of results obtained from protocols and equipment that are now outdated, new equations associated with currently available protocols and equipment need to be developed.
Single-frequency BIA is an easy method for assessment of body composition that could be implemented in daily veterinary practice. The objectives of the study reported here were to develop equations for prediction of TBW content in unsedated healthy adult dogs by the use of single-frequency BIA with currently available equipment and to compare the results of those equations with the TBW content determined by deuterium dilution (TBWd).
This manuscript represents a portion of a dissertation submitted by Dr. Yaguiyan-Colliard to the Université Pierre-et-Marie-Curie L'école doctorale physiologie, physiopathologie et thérapeutique (ED 394) as partial fulfillment of the requirements for a Doctor of Philosophy degree.
Plates for biochemical analyses were provided by IDEXX France.
The authors thank Lauréline Colliard for the creation of Figure 1 and Marc Chodkiewicz for help with manuscript preparation.
The authors declare that they have no competing interests.
Body condition score
Coefficient of variation
Dual-energy x-ray absorptiometry
Total body water
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