Clinical Evaluation of the Obese Patient

5. Clinical Evaluation of the Obese Patient

Quantifying Obesity in Cats

Obesity is defined as an excess accumulation of body fat and all measures of adiposity involve defining body composition. The main conceptual division of importance is between:

• Fat mass (FM): the adipose tissue
• Fat-free mass (FFM) (Pace & Rathbun, 1945). The major constituents of the FFM are presumed to be present in fixed ratios and include the intracellular (ICW) and extracellular water (ECW), minerals, and protein. The FFM contains the body cell mass (BCM) which is the metabolically active part of the body responsible for determining most of the resting energy expenditure. BCM encompasses those lean tissues most likely to be affected by nutrition or disease over relatively short periods. Further, since FFM is an index of protein nutrition, changes in FFM likely represent alterations in protein balance.

The body weight recorded at the end of the first year can usually be a good reference of the optimum body weight of the cat during the rest of its life. (© Royal Canin).

Thus, assessment of FM and FFM provides valuable information about the physical and metabolic status of the individual; the FM represents an energy storage depot, whilst FFM represents the actual health of the animal (Table 4). 

Various techniques are available to measure body composition (Table 5), and these differ in applicability to research, referral veterinary practice and firstopinion practice. Broadly speaking, a number of techniques are available to assess the degree of adiposity, including:

• Clinical assessments (e.g., morphometric measurements, body condition scoring, sequential body weight measurement, sequential photography)
• Experimental procedures (e.g., chemical analysis, dilution techniques (e.g., deuterium determination of total body water), total body potassium, densitometry, total body electrical conductivity, and neutron activation analysis).
• Techniques that have potential for application in clinical work (e.g., dual energy xray absorptiometry, bioelectrical impedance analysis, computed tomography, magnetic resonance imaging).

Only those of greatest relevance to clinical practice will be discussed in detail.

Established Clinical Measures of Body Composition

Body Weight Measurement

It is the simplest technique available and should be included in the examination of every patient, especially in very young cats, at the end of the growth period. However, work by one of the authors has suggested that this remains an infrequent part of the routine examination of companion animals (unpublished observations). It provides a rough measure of total body energy stores and changes in weight parallel energy and protein balance. In the healthy animal, body weight varies little from day to day.

There can be wide variation between scales though, so it is important to use the same scale for an individual animal each time to avoid inter-scale variation. Body weight can be falsely altered by dehydration or fluid accumulation. Edema and ascites may mask losses in body fat or muscle mass. Likewise, massive tumor growth or organomegaly can mask loss in fat or lean tissues such as skeletal muscle. Further, breed influences can also lead to variability in body weight for cats in similar condition (Figure 8). Body weight correlates only moderately with body fat mass (Burkholder, 2001).

Figure 8. Indicative weight for several feline breeds. (Source: Royal Canin Encyclopedia of the Cat).

As a result, sporadic measurements at single time points are of only limited use (if not coupled with concurrent assessment of body condition - see below). Nevertheless, sequential body weight measurements (e.g., throughout life in an individual cat, instigated at the time of young adulthood) can provide a sensitive indicator of subtle changes in body composition and could provide a vital tool for the prevention of obesity.

Body Condition Scoring

Body condition scoring is a subjective, semi-quantitative method of evaluating body composition that is quick and simple to perform. All systems assess visual and palpable characteristics to assess subcutaneous fat, abdominal fat and superficial musculature (e.g., ribcage, dorsal spinous processes and waist). The technique of body condition scoring does depend on operator interpretation and does not provide any precise quantitative information concerning alterations in fat free or lean body mass relative to fat mass.

Different scoring systems have been described but the most common scoring systems used are the 5-point system (where a BCS of 3 is considered ideal, see Figure 9) or the 9-point system (where a BCS of 5 is considered ideal) (Laflamme, 1997; McGreevy et al., 2005). Given that half points are often employed in the 5-point system (giving a total of 9 categories), these two systems are virtually equivalent. A 7-point algorithm-based approach (Figure 10) is specifically designed to be used by owners to assess their own pets. A study has demonstrated a good correlation between the system and body fat measurements made by Dual Energy X-Ray Absorptiometry (DEXA) with excellent agreement between experienced operators (German et al., 2006). Most importantly, good agreement was found between measurements by operators and owners, suggesting that the method may be reliable when used without prior training. However, such data are preliminary and further validation would be required before it is used by owners.

Figure 10. Waltham S.H.A.P.E. tm guide for cats.

Limitations of the BCS include the subjectivity inherent in the scoring system and inter-observer variation. Finally, like body weight, BCS gives an overall assessment of body condition; it cannot differentiate between body compartments and does not provide any precise quantitative information concerning alteration in fat free or lean body mass relative to fat mass.

Morphometric Measurements

Morphometry (more appropriately "zoometry" for veterinary species) is defined as the measurement of "form" and, in relation to body composition analysis, refers to a variety of measured parameters that are used to estimate body composition. The three main approaches are:

• Dimensional evaluations (where various measures of stature are combined with weight)
• Measurement of skin fold thickness
• Ultrasound technique.

Dimensional Evaluations

Dimensional evaluations are usually performed by tape measure, and a number have been reported in cats. Measurements of "length" (e.g., head, thorax and limb) are correlated with lean body components (Hawthorne & Butterwick, 2000), whilst circumferential measurements have been shown to correlate both with lean body mass (LBM) (Hawthorne & Butterwick, 2000), and body fat (Burkholder, 1994). Segmental limb measures and (likely) truncal length are thought to be better measures of stature and thus correlate best to LBM. By combining more than one measure (usually one that correlates with FM, and one correlating with LBM), equations can be generated to predict different body components.

The best example of such a measure is the feline body mass index (FBMI)tm (Hawthorne & Butterwick, 2000). The FBMItm is determined by measuring the rib cage circumference at the level of the 9th cranial rib and the leg index measurement (LIM), which is the distance from the patella to the calcaneus (Figure 11a and Figure 11b).

Figure 11a. Measurement of the length of the lower limb (LIM) from the middle of the patella to the calcaneus. (©Waltham Centre for Pet Nutrition).

Figure 11b. Measurement of the rib cage circumference. (©Waltham Centre for Pet Nutrition).

The percent body fat can be calculated as:

- % fat = (1.54 x ribcage circumference) – (1.58 x leg index measurement) – 8.67 (rib cage circumference and LIM in cm)
- or, more simply: 1.5 (ribcage – LIM) / 9
- or determined by consulting a reference chart (Table 6).

The FBMItm is a very simple, yet objective tool to determine the body fat content of the cat. In addition, it is particularly valuable for convincing clients that their cat is indeed overweight and in need of weight loss.

Measurement of Skin Fold Thickness

This technique has been used extensively in people to determine the percent body fat using equations derived for various populations. Unfortunately, these measurements cannot be used in cats because feline skin is easily detached from underlying fat tissue which makes skin-fold measurement impractical and unreliable.

Ultrasound

Another method of measuring the subcutaneous fat layer is by ultrasound. This technique has been used in Beagles and equations have been derived to predict percent body fat from the subcutaneous fat thickness (Wilkinson & McEwan, 1991). These regression equations do not work in other dog breeds but future research may allow investigators to develop new, more accurate equations for this simple technique.

Bioelectrical Impedance Analysis (BIA)

Bioelectrical impedance analysis (BIA) is a safe, noninvasive, rapid, portable, and fairly reproducible method of assessing body composition in companion animals. This method has the potential of quantifying total body water (TBW), ECW, ICW, BCM, FFM and FM.

Electrical conductance is used to calculate the composition of the body by measuring the nature of the conductance of an applied electrical current in the patient. Body fluids and electrolytes are responsible for conductance whilst cell membranes produce capacitance. Since adipose tissue is less hydrated than lean body tissues, more adipose tissue results in a smaller conducting volume or path for current and larger impedance to current passage. The FFM contains virtually all the water in the body and thus if bioelectrical impedance is measured a value for FFM can be determined.

Bioelectrical impedance analysis (BIA) is a safe, noninvasive, rapid, portable, and reproducible method of assessing body composition in healthy cats. (© Larry Cowgill).

Two types of BIA systems are currently available; single frequency which applies a 50 kHz current, and multi-frequency which utilizes frequencies from 5 kHz to 1000 KHz. A BIA test is performed by placing four small electrodes on the body. The electrical current is introduced into the patient from the distal electrodes. As the current travels through the body it experiences a slight delay due to cells, and the current is then detected by proximal electrodes.

The proportion of the current in the ICW and ECW is frequency dependent:
- Low frequencies (e.g., 5kHz) pass primarily through the ECW because of high cell membrane capacitance
- In contrast, at higher frequencies the effects of cell membrane capacitance is diminished so the current flows through both the ICW and ECW environments (or TBW).

BIA allows estimation of body composition in healthy dogs, cats, and humans (Scheltinga et al., 1991; Stanton et al., 1992; Patel et al., 1994). However, BIA may be affected by hydration status, consumption of food and water, skin and air temperature, recent physical activity, conductance of the examination table, patient age, size, shape and posture in addition to electrode positioning. Reliable BIA requires standardization and control of these variables. BIA requires further evaluation and validation in disease states, especially those associated with major disturbances in water distribution and states such as sepsis which may alter cell membrane capacitance.

Calculation of ECW-ICW takes approximately 1 minute, hence BIA provides instantaneous on line information of body composition that has never before been available.

Deuterium (D2O) Dilution Technique

The water content of the FFM is among the best used techniques for determining body composition due to the relative stability of the FFM hydration between species. Briefly, TBW can be measured by several stable labelled isotopes dilution methods including D2O and the following relationship has been validated:

Fat mass = body mass – TBW/0.73
The first study on cats was published in 1950 by Spray and Widdowson.

In practice, after a 24h fasting period, a sub-cutaneous injection of D2O in an isotonic saline solution is administered (500 mg D2O/kg). The mass of the syringe (and needle) before and after injection should be accurately weighed to determine the exact quantity of labelled-isotope that will dilute in body water. The first blood sample is taken before injection, the second about 3 - 4 hours after D2O injection. Until recently, this technique was limited due to technological problems but today a new method of analysis has been developed which makes this technique less expensive and more widely available.

Dual Energy X-ray Absorptiometry (DEXA)

This technique originally developed for precise measurement of bone mineral content (BMC). However, it is now also used to measure both body fat and non-bone lean tissue. DEXA uses photons of two different energy levels (70 and 140 kVp) to distinguish the type and amount of tissue scanned. The X-ray source is positioned underneath the table supporting the patient, with the detector housed in an arm above the patient.

This technique originally developed for precise measurement of bone mineral content (BMC). However, it is now also used to measure both body fat and non-bone lean tissue. DEXA uses photons of two different energy levels (70 and 140 kVp) to distinguish the type and amount of tissue scanned. The X-ray source is positioned underneath the table supporting the patient, with the detector housed in an arm above the patient.

DEXA’s low coefficient of variation for measuring BMC (~1%) makes it a very precise technique but a few constraints have to be noted:

• Equipment is still expensive
• Short sedation is required
• Standardization of the technique is very important (Raffan et al., 2006).

DEXA is safe and quick; with the more modern fan-beam DEXA scanner, it takes under five minutes for a whole body scan in a cat (Figure 12). Similar to other body composition techniques, DEXA relies on the assumption that lean body mass is uniformly hydrated at 0.73 mL water/g.

Figure 12. DEXA examination in an obese cat. DEXA prior to weight loss shows a body fat content of 54.4% (reference range 18 to 25%).

Determination of Basal Metabolic Rate (BMR)

Precise knowledge of energy expenditure is important in obese animals to determine the exact amount of energy needed to loose weight. Energy expenditure is the result of internal and external work and of heat yields. Energy originates from nutrients that are converted to various energy forms that can be used by the body. Most chemical reactions in the body need oxygen and produce water and carbon dioxide. So relationships have been established between respiratory and energy expenditure.

Among methods available, indirect calorimetry allows the determination of the basal metabolic rate by measuring only oxygen consumption and carbon dioxide production. In practice, the cat is placed in a specific cage for about 4-h and gas exchanges are measured. The formula used to calculate BMR was validated by Weir (1949). An abbreviated Weir formula has also been developed:

BMR (kcal/day) = [3.9 (kcal/L) x V(O2( L)) + 1.1 (kcal/L) x V(CO2 (L))]