Sarcopenia, the age-related reduction in skeletal muscle mass in the elderly, is a multifactorial problem. Contributing factors to the muscle loss include physical inactivity, increased cytokine production, decreased concentrations of hormones (growth hormone, testosterone, and IGF-1), motor-unit remodeling of type II skeletal muscle fibers, and decreased protein synthesis within the muscle.1,2 Sarcopenia is a well-accepted phenomenon in humans and is an area of extensive research because the associated loss of LBM has been determined to have important consequences. It is a major factor in the risk of falls in the elderly, has a profound effect on an elderly person's ability to perform simple tasks, and has been associated with weakness, functional limitations, immobility, and fractures.3,4 Because of the deleterious effects and morbidity in humans, increasing resources have been devoted to developing both preventative and management techniques for dealing with this age-related loss of muscle mass and strength, including development of resistance training and aerobic activity programs, hormone replacement, and nutritional evaluation and modifications.5
Sarcopenia can easily go unnoticed because it is often a gradual process and can be masked by a concurrent increase in body fat. A study6 in humans found considerable loss of LBM despite maintenance of body weight. Tests that have been used to investigate LBM in elderly humans include regional CT, DEXA, urinary creatinine excretion testing, and whole-body potassium and total body water determination.4,7,8 Cross-sectional area of the thigh muscle on CT scans is an important clinical and research measurement used extensively in the assessment of muscle loss in people because of its high precision and accuracy.6,9 Marked loss of LBM despite maintenance of midthigh diameter can easily be appreciated when comparing the CT images of young and old humans.10
Decreased LBM in aging humans may be due to a reduction in growth hormone secretion and parallel reductions in IGF-1 concentrations.11 Most of growth hormone's actions are mediated by IGF-1, and given that IGF-1 concentrations do not fluctuate throughout the day as do growth hormone concentrations, determination is used as a screening test for growth hormone deficiency.
Another proposed mechanism contributing to age-related loss of LBM is increased inflammation and subsequent muscle catabolism. A recent study9 measured inflammatory markers in a group of 2,177 humans and then followed changes in LBM for 5 years. That study revealed that higher concentrations of inflammatory markers, including CRP, interleukin-6, and TNF-α were associated with a greater 5-year decrease in LBM, compared with humans with decreased inflammation.
Inasmuch as the existence and clinical importance of sarcopenia in aging humans have been well accepted, limited information is available on its existence and clinical implications in old dogs. Those studies that have been performed provide an important framework for further research but have relied on methods of variable accuracy or are not clinically applicable. One study12 comparing the mean body fat content of dogs via proximate (carcass) analysis reported higher mean body fat in old dogs, compared with younger dogs, as well as age-related decrease in total body protein. These data support the existence of a similar age-related loss of LBM as is seen in humans, but lack of details on breed and health of the dogs limits the use of the data. More importantly, proximate analysis is not clinically applicable.
Two studies13,14 have used DEXA to assess changes in body composition during aging in dogs. One study13 following the body weight and composition of 48 healthy Labrador Retrievers from 8 weeks of age until death identified a considerable loss of LBM in dogs > 9 years of age. Another study14 of 40 adult Labrador Retrievers aged 2 to 13 years compared LBM and fat mass across age groups. That study identified a negative linear relationship between age and LBM as well as a decrease in lean muscle-to-fat ratio with age. Unfortunately, although DEXA is a feasible means of measuring body composition, it has a number of inherent limitations for measuring LBM and is not available in most clinical practices.15 These 3 studies investigating age-related changes in body composition in dogs are an important starting point, but further research is needed to identify the most accurate means of assessing LBM with a well-defined population of pet dogs. In addition, to advance the study of sarcopenia in companion animals, it is critical to identify clinically practical, noninvasive methods of assessing and quantifying loss of LBM. Therefore, the goals of the study reported here were to assess several methods of assessing LBM in dogs, to compare muscle mass in healthy young and old dogs, and to compare possible mediators of sarcopenia (IGF-1, CRP, and TNF-α) between young and old dogs. The hypothesis was that healthy old dogs would have significantly lower measurements of specific muscle groups, compared with measurements in healthy young dogs.
Materials and Methods
Subjects—For this study, 20 Labrador Retrievers were recruited from the staff and clients of the Foster Hospital for Small Animals at Tufts Cummings School of Veterinary Medicine. Dogs were recruited for both a young group (1 to 5 years old) and an old group (> 8 years old).16 To be eligible, dogs had to have a BCS of 5 to 6 on a 9-point scale17 and be in good health as determined by history, physical examination, CBC, serum biochemical profile, and urinalysis. Exclusion criteria included a history of orthopedic surgery or osteoarthritis and current use of prescription or over-the-counter medications or supplements. All owners signed an informed consent form prior to enrollment in the study. The study protocol was approved by the Tufts Clinical Studies Review Committee.
Body composition measurements—All dogs were weighed on the same scale to the nearest 0.1 kg. A BCS was assigned to all dogs on the basis of assessment by a single investigator (DH) on a scale of 1 to 9.17 A single investigator also assigned an MCS to each dog on a scale, where 0 is no muscle loss, 1 is mild muscle loss, 2 is moderate muscle loss, and 3 is severe muscle loss.a The MCS subjectively evaluates muscle mass and so differs from the BCS, which evaluates fat mass. An assessment of MCS includes visual examination and palpation of the muscle over the thoracic and lumbar vertebrae, pelvic bones, scapulae, and temporal bones.18,19,a
Radiography—A radiograph of the thoracolumbar region was obtained with dogs in left lateral recumbency, with the limbs at 90° angles from the body. Images were captured with a computed radiography system.b A 30-mm metallic calibration device was positioned in the radiograph to allow for magnification correction. When corrected for magnification, the height of the epaxial muscles was measured in a transverse plane between the junction of the lamina and spinous process of T13 ventrally and the ventral aspect of the subcutaneous fat layer dorsally. The radiographic images were measured by a single investigator (JSS) without knowledge of the dog's age with the line tool of DICOM (ie, Digital Imaging and Communications in Medicine) workstation softwarec and magnification correction performed with the following formula:
Epaxial muscle height also was normalized for variable bone size by dividing epaxial muscle height by the midbody height of T13 as determined by sagittal CT images.
CT scan—Dogs were lightly sedated with dexmedetomidine,d butorphenol,e and midazolamf (young dogs) or with a combination of butorphenol, acepromazine,g and glycopyrolateh followed by propofoli as needed to maintain sedation (old dogs) via an IV catheter for the short CT scan (total CT procedure time was < 15 minutes with a total active scan time < 20 seconds). Dogs were then positioned in sternal recumbency with sandbag immobilization of the limbs with identical positioning by a single investigator (DH) for each dog. Images were rapidly obtained with a 16-slice helical CT scanner.j Two scout views of the thoracolumbar junction were obtained initially, guiding the acquisition of a helical volume from mid-T12 to the midbody of L3. A second series of scout views of the skull guided the acquisition of a 10-cm temporal helical volume with a 1-mm slice thickness centered at the level of the bullae. All images were reconstructed in the transverse plane relative to the structures of interest via both soft tissue and bone algorithms with a 1-mm slice thickness and were maintained in a DICOM format.
Epaxial muscle cross-sectional area in cm2 was measured (mean of left and right measurements) by a single investigator (JSS) without knowledge of the dog's age at the level of the T13 vertebral body with a closed polygon tool in the DICOM viewer (Figure 1). The DICOM images contain dimensional calibration exported directly from the equipment from which the images were acquired. Epaxial muscle area also was normalized for variable skeletal size by dividing epaxial muscle area by midbody height of T13 as determined by sagittal CT images. Temporal muscle thickness was measured (mean of 3 measurements from both the left and right sides) in centimeters at the level of the external ear canal with the line measurement tool. The temporal muscle thickness was normalized by dividing the muscle thickness by the distance from the dorsalmost aspect of occipital crest to the zygomatic arch (obtained from zoometric measurements).
Transverse CT image of the epaxial musculature at the level of T13 from a young (4-year-old) spayed female Labrador Retriever. Notice the contrast between bone, muscle, and fat. The left epaxial musculature area (asterisk inside area indicated with a white line) has been measured with a closed polygon tool and determined to be 9.38 cm2.
Citation: American Journal of Veterinary Research 73, 11; 10.2460/ajvr.73.11.1794
Transverse CT image of the epaxial musculature at the level of T13 from a young (4-year-old) spayed female Labrador Retriever. Notice the contrast between bone, muscle, and fat. The left epaxial musculature area (asterisk inside area indicated with a white line) has been measured with a closed polygon tool and determined to be 9.38 cm2.
Citation: American Journal of Veterinary Research 73, 11; 10.2460/ajvr.73.11.1794
Transverse CT image of the epaxial musculature at the level of T13 from a young (4-year-old) spayed female Labrador Retriever. Notice the contrast between bone, muscle, and fat. The left epaxial musculature area (asterisk inside area indicated with a white line) has been measured with a closed polygon tool and determined to be 9.38 cm2.
Citation: American Journal of Veterinary Research 73, 11; 10.2460/ajvr.73.11.1794
Regional ultrasonographic measurements—Transverse static ultrasonographic images were obtained with a dorsal parasagittal transducer location at the 13th rib with an 8- to 5-MHz curvilinear transducer.k The unshaved skin was prepared with isopropyl alcohol and ultrasonography gel. Minimal ultrasonographic transducer pressure was used to minimize tissue distortion. The DICOM images were exported, reviewed, and analyzed with the DICOM viewer. Maximal transverse left and right epaxial muscle areas at the level of the 13th rib were obtained by drawing a freehand region of interest around the hypoechoic epaxial musculature. The epaxial muscle margins were defined medially by the spinous process of T13, ventrally by the 13th rib, and dorsally and laterally by subcutaneous fat (Figure 2). Each side was measured 3 times, and a mean value of the left and right sides was obtained. Epaxial muscle area also was normalized for variable skeletal size by dividing epaxial muscle area by the height of the body of T13. Three transverse images of the right and left quadriceps muscles were measured at the midway point between the greater trochanter and lateral condyle of the femur perpendicular to the underlying axis of the femur. Dogs were in left lateral and right lateral recumbency for measurement of the right and left quadriceps, respectively. The mean of the 2 limbs was used. Quadriceps muscle thickness was normalized for dogs with different skeletal size by dividing the muscle thickness by the circumference of the tarsus (mean of left and right tarsus). The temporal muscle thickness was measured at a midway point between the occipital crest and the right pinna on both the left and right sides; the mean of 3 measurements on both the left and right sides was used for analysis.
Transverse ultrasonographic image of the right epaxial musculature at the level of T13 from a young (4-year-old) spayed female Labrador Retriever. The muscle is highlighted by the ultrasonographic cursors with corresponding linear dimensions. The epaxial muscle is bordered medially by the spinous process of T13 (SP), ventrally by the right 13th rib (R), and dorsally by subcutaneous fat (SQF).
Citation: American Journal of Veterinary Research 73, 11; 10.2460/ajvr.73.11.1794
Transverse ultrasonographic image of the right epaxial musculature at the level of T13 from a young (4-year-old) spayed female Labrador Retriever. The muscle is highlighted by the ultrasonographic cursors with corresponding linear dimensions. The epaxial muscle is bordered medially by the spinous process of T13 (SP), ventrally by the right 13th rib (R), and dorsally by subcutaneous fat (SQF).
Citation: American Journal of Veterinary Research 73, 11; 10.2460/ajvr.73.11.1794
Transverse ultrasonographic image of the right epaxial musculature at the level of T13 from a young (4-year-old) spayed female Labrador Retriever. The muscle is highlighted by the ultrasonographic cursors with corresponding linear dimensions. The epaxial muscle is bordered medially by the spinous process of T13 (SP), ventrally by the right 13th rib (R), and dorsally by subcutaneous fat (SQF).
Citation: American Journal of Veterinary Research 73, 11; 10.2460/ajvr.73.11.1794
Zoometric measurements—All measurements were taken 3 times on the right side and 3 times on the left side of the body, and the mean of these 3 measurements was used as the result for each side. The mean result for the 2 sides was then calculated to get the final result for analysis. The measurements were all performed by the same investigator (DH) to reduce interobserver variation. Measurements were performed with dogs in standing position with a metric measuring tape.20,21 Measurements included height at the highest point of the shoulders (ie, withers), circumference of the tarsus, body length (tip of nose to base of tail), and distance from the dorsalmost aspect of occipital crest to the zygomatic arch.
Activity monitoring—All dogs underwent activity monitoring for 14 days immediately following completion of the other procedures. Owners were instructed to maintain their dogs' activity at typical levels. Activity was measured with a commercial activity monitorl worn on a collar around the dog's neck. Activity was analyzed with the monitor's analysis software to determine total 14-day counts, daily mean, and hourly mean.m
Laboratory analyses—Blood was collected by jugular venipuncture. Blood for analysis of a serum biochemical profile and CBC was submitted immediately. Blood for additional analyses was centrifuged, and serum was frozen in separate aliquots at −80°C until analysis for concentrations of CRP, IGF-1, and TNF-α. A voided urine sample also was collected for a urinalysis. Serum samples were analyzed in duplicate by ELISA in a single batch to determine concentrations of CRPn and TNF-α.o Concentration of IGF-1 was measured by high-performance liquid chromatography at a commercial laboratory.p
Data analysis—Data were evaluated graphically. Data are presented as mean ± SD (normally distributed data) or median and range (skewed data). Continuous data that were normally distributed were compared between young and old dogs via independent t tests. Data that were not normally distributed were logarithmically transformed prior to analysis, with the exception of MCS, which was compared between groups via a Mann-Whitney U test. Categorical variables were compared between the young and old groups via χ2 analysis. Correlations between continuous variables were examined via the Pearson correlation test. Multiple linear regression also was performed to assess the independent effects of variables found to be significant on univariate analysis on epaxial muscle area. Data were analyzed with standard statistical software,q and values of P < 0.05 were considered significant.
Results
Twenty dogs were enrolled in the study (young, n = 9; old, 11). One dog in the old group was later found to have been receiving an NSAID at the time of participation and so was excluded from all analyses. Therefore, the study included 9 young dogs (mean age, 2.4 ± 1.0 years) and 10 old dogs (mean age, 9.1 ± 2.1 years). Sex, body weight, and BCS were not significantly different between dogs in the young and old groups (Table 1). However, the MCS was significantly (P < 0.001) higher, indicating more muscle loss, in the old dogs. None of the zoometric measurements of skeletal size were significantly different between age groups (data not shown). The 2 groups were not significantly different in CRP or IGF-1 concentrations. Serum TNF-α concentration was undetectable in all dogs.
Comparison of signalment, physical examination, and laboratory measurements of young and old Labrador Retrievers.
| Variable | Young (n = 9) | Old (n = 10) | P value |
|---|---|---|---|
| Age | 2.4 ± 1.0 | 9.1 ± 1.2 | < 0.001 |
| Sex | 0.25 | ||
| Spayed female | 5 | 8 | |
| Castrated male | 4 | 2 | |
| Weight (kg) | 31.5 ± 5.4 | 30.1 ± 5.1 | 0.56 |
| BCS (1–9) | 5.4 ± 0.5 | 5.7 ± 0.4 | 0.36 |
| MCS | 0 (0–0) | 1.3 (0.5–2.0) | < 0.001 |
| Daily calorie intake (kcal/d) | 975 (674–1,855) | 960 (678–1,588) | 0.87 |
| Dietary protein intake (g/d) | 77 ± 26 | 78 ± 17 | 0.94 |
| CRP (μg/mL) | 0.15 (0.04–0.36) | 0.24 (0.05–0.57) | 0.39 |
| IGF-1 (nmol/L) | 39.11 ± 12.45 | 36.8 ± 10.8 | 0.67 |
Data are mean ± SD or median (range).
Muscle condition was scored as follows: 0 = no muscle loss, 1 = mild muscle loss, 2 = moderate muscle loss, and 3 = severe muscle loss.
The epaxial muscle area measured by ultrasonography was significantly (P = 0.03) smaller in old dogs, compared with that in young dogs (Table 2; Figure 1). Although epaxial muscle area measured by CT was not significantly (P = 0.32) different between groups, it was significantly smaller in old dogs when normalized for T13 body height (P = 0.04), as was the normalized ultrasonographic measurement (P = 0.007). Neither actual nor normalized radiographic epaxial muscle height was significantly different between groups (P = 0.54 and 0.11, respectively), although the old group had a smaller mean measurement for both.
Comparison of muscle measurements of young and old Labrador Retrievers.
| Variable | Young (n = 9) | Old (n = 10) | P value |
|---|---|---|---|
| Epaxial muscle | |||
| CT area (cm2) | 12.9 ± 2.0 | 12.0 ± 2.1 | 0.32 |
| CT area normalized* | 1.3 ± 0.2 | 1.1 ± 0.2 | 0.04 |
| US area (cm2) | 8.5 ± 1.6 | 6.9 ± 1.3 | 0.03 |
| US area normalized* | 0.8 ± 0.2 | 0.6 ± 0.1 | 0.007 |
| Radiography height (mm) | 23.2 ± 3.1 | 22.4 ± 2.8 | 0.54 |
| Radiography height normalized* | 2.3 ± 0.3 | 2.0 ± 0.3 | 0.11 |
| Quadriceps muscle | |||
| US thickness (mm) | 15.9 ± 2.0 | 14.8 ± 2.7 | 0.31 |
| US thickness normalized† | 0.7 ± 0.1 | 0.7 ± 0.1 | 0.46 |
| Temporal muscle | |||
| CT thickness (mm) | 19.8 ± 1.9 | 20.0 ± 2.6 | 0.83 |
| CT thickness normalized‡ | 12.9 ± 2.0 | 18.4 ± 1.6 | 0.32 |
| US thickness (mm) | 1.4 ± 0.2 | 1.4 ± 0.2 | 0.87 |
| US thickness normalized‡ | 1.4 ± 0.2 | 1.3 ± 0.1 | 0.33 |
| Measures of bone size | |||
| T13 vertebral height (mm) | 10.3 ± 0.7 | 11.2 ± 1.0 | 0.047 |
| Circumference of tarsus (mm) | 22.9 ± 2.1 | 22.5 ± 1.7 | 0.68 |
| Occipital crest to zygomatic arch (mm) | 14.1 ± 1.7 | 14.0 ± 1.2 | 0.89 |
Data are mean ± SD.
Actual measurement divided by the height of the body of T13.
Actual measurement divided by the circumference of the tarsus.
Actual measurement divided by the distance from dorsalmost aspect of occipital crest to zygomatic arch.
US = Ultrasonography.
Temporal and quadriceps muscle areas were not significantly different between age groups with any of the modalities, even when normalized for head size or tarsus circumference (Table 2). Although sex was not significantly different between the groups, body composition results were also compared on the basis of sex. Male dogs had significantly higher body weight (P = 0.02), epaxial muscle area measured by CT (P = 0.003) and ultrasonography (P = 0.02; Figure 1), and temporal muscle measured by CT (P = 0.03). However, age, BCS, MCS, and measures of skeletal size were not significantly different between male and female dogs. Via multiple regression analysis, after adjustment for age group, sex, and 14-day activity counts, age group and sex were independently associated with epaxial muscle area.
The activity monitor data for 1 of the 19 dogs had consistent motion artifact throughout the measurement period, so this dog was excluded from the activity monitoring analysis. Therefore, activity monitor data were analyzed for 18 of the 19 dogs. The total 14-day counts (P = 0.02), mean hourly counts (P = 0.04), and mean daily counts (P = 0.046) were significantly lower in the old group, compared with the young dog group. There was a significant (P = 0.007) negative correlation (r = −0.61) between age and activity level (total 14-day counts). Total 14-day count was not significantly correlated with muscle size measured by ultrasonography for epaxial muscle (r = 0.45; P = 0.06), quadriceps (r = 0.22; P = 0.38), or temporal muscle (r = 0.06; P = 0.81).
Discussion
In this study, mean epaxial muscle area was significantly lower in healthy old dogs, compared with healthy young dogs, when muscle area was measured by ultrasonography and more so when ultrasonography or CT was used after epaxial muscle area was normalized for vertebral body height. This study used dogs of only 1 breed, Labrador Retrievers, in an effort to maintain consistency in body shape and size for evaluation of these measurement techniques. Correction for vertebral height may enable comparison among dogs of different sizes, a factor of practical importance when making measurements in a species of such diverse body types. The data from this study suggest that ultrasonography may be an effective, inexpensive, and noninvasive method for assessing epaxial musculature. Radiographic measurement of epaxial muscle height in this study did not appear to provide sufficient precision to determine differences between the young and old groups, but additional studies are needed to more fully compare muscle mass in young and old dogs.
Ultrasonography has some potential limitations for routine clinical use. To obtain complete contact with the animal's skin, the curvilinear transducer inherently deforms the near field tissues. Although minimal transducer pressure and a standoff pad may minimize these changes, the potential for muscle shape distortion (especially in thin animals) needs to be considered. In addition, like all ultrasonography studies, operator experience and familiarity with the regional anatomy can influence the accuracy of the epaxial measurements.
Although BCS was not different between age groups because having a BCS of 5 to 6 of 9 was used as an inclusion criterion for the study, MCS was significantly higher (indicating more muscle loss) in the old versus the young group. This emphasizes the fact that both BCS and MCS should be assessed by veterinarians in every patient at every visit because there can be considerable muscle loss even in dogs that are overweight or obese and, conversely, dogs can be underweight (ie, have less than an optimal BCS) without muscle loss.18,19 However, in the present study, the single investigator who performed all the MCS evaluations was not unaware as to whether dogs were in the young or old groups, so there may have been some bias in assessment of MCS. Nonetheless, even mild muscle loss can be easily identified by a clinician who is attuned to assessing muscle mass. Although the MCS is subjective and assignment of this score was performed with knowledge of group assignments in this study, the results supported lower muscle mass in the old group. This finding was consistent with changes in the epaxial muscles seen via ultrasonography and CT. Further studies to validate the MCS and to assess its use in animals with different degrees of muscle loss from either aging or medical conditions are needed.
Other muscle regions were evaluated in this study, but the quadriceps and temporal muscle height were not significantly different between age groups. In humans, the CT cross-sectional area of the thigh is commonly used to measure muscle mass to identify sarcopenia. The quadriceps measurement was assessed in the present study, but because orthopedic disease is common in older dogs, it was anticipated that the epaxial and temporal muscles would be less affected by muscle loss from orthopedic disease. Measurements of quadriceps area were not significantly different in dogs evaluated for this study. Quadriceps muscles contain a mixture of type I and II fibers, whereas epaxial muscles are primarily composed of type II fibers.22 The type II (glycolytic) fibers are more susceptible to atrophy,23 so this difference in muscle fibers may also explain why the present study found that epaxial muscles were affected earlier and more severely than the quadriceps. The lack of differences between the temporal muscles might be related to this same issue of muscle fiber type.
In the present study, the authors made a concerted effort to rule out underlying disease; however, dogs in either group may have had subclinical disease that could not be identified. This is an important issue because muscle loss (cachexia) occurs in a variety of chronic conditions, such as congestive heart failure, chronic kidney disease, and cancer. Heart failure and considerable renal disease were not present in these dogs, but it is possible that they had cancer or other diseases that were not readily apparent from the history, physical examination, CBC, serum biochemical profile, and urinalysis. Inflammation is a possible mechanism contributing to sarcopenia, but CRP concentration was not significantly higher in the old group in this study, and TNF-α concentration was not detectable in any of the dogs. This may also have been due to lack of sensitivity or specificity of these variables for assessing inflammation in dogs or to lack of precision or accuracy of the assays. However, the findings do support the lack of any important subclinical medical conditions in this population of dogs. Anecdotally, serum CRP concentrations appear to be sensitive to even mild inflammation in dogs. The present study also assessed IGF-1 concentration as another factor that has been found to play a role in human sarcopenia,24 but no significant differences were detected between groups. As with the TNF-α and CRP concentrations, this may have been due to limitations in the sensitivity of the assay, to small sample size and resulting insufficient statistical power, or to a more complex mechanism involved in the relationship between aging and sarcopenia. Further assessment of underlying mechanisms of sarcopenia in dogs is needed, including evaluation of IGF-binding proteins and possibly other inflammatory cytokines, such as interleukin-1 or interleukin-6.
Another factor implicated as one of many elements responsible for the multifactorial phenomenon of sarcopenia in humans is the decrease in physical activity that occurs with age. In dogs, decreased activity is often blamed for the decrease in LBM.14 To identify the role of decreased physical activity in the loss of LBM in the old dog group in the present study, activity monitoring was performed. Although the old dog group had lower activity and there was a negative correlation between age and activity (total 14-day count), activity was not significantly correlated with epaxial, quadriceps, or temporal muscle area. Although additional research in a larger number of dogs of multiple breeds is needed, the lack of correlation between activity and muscle area suggests that the negative correlation between age and epaxial muscle area in the old dog group was not purely the result of lower activity.
Physical activity alone, as a means of building LBM, may also not be the most effective means of managing sarcopenia. Although studies25 in humans indicate that physical activity is useful in prevention of sarcopenia, research also suggests that once sarcopenia is present, elderly humans do not accrue muscle protein as efficiently in response to exercise as do younger individuals. Stimulation of muscle protein synthesis by resistance exercise is blunted in elderly humans, compared with younger humans.26 In cachexia, the muscle loss associated with disease, building or even maintaining muscle mass appears to require not only an anabolic stimulus (eg, exercise, IGF-1, and anabolic steroids), but also anticatabolic agents (eg, anti-inflammatory agents) and adequate substrates (eg, increased dietary protein and postexercise essential amino acids).25,27–29 Although exercise may be important to help prevent muscle loss during aging, it may not be sufficient by itself to build muscle once it is lost, and combination treatments may be required.
In addition to the differences in body composition between the young and old groups, male dogs weighed more (although they did not have a higher BCS) and had a larger epaxial muscle area. Therefore, both age and, as expected, sex are related to muscle area. Although all dogs in the present study were neutered, this sex difference in body composition emphasizes the importance of considering this factor when designing future studies of body composition in aging. The lower muscle mass in females also may have important clinical implications in sarcopenia because old female dogs have even less muscle mass to lose with age than males.
There were additional limitations to this study. The number of dogs was small, and confirmation of these findings in a larger homogeneous group of dogs would be useful before trying to evaluate muscle mass in old dogs of other sizes and breeds. Although all dogs in the old group were > 8 years of age, with a mean of 9.1 years, the oldest dog was 11 years old. This was a result of the study enrollment criteria, which eliminated any dogs with BCS > 6 or < 5, dogs with medical conditions, and those taking dietary supplements or medications. However, muscle loss may be more severe with even older dogs, so further evaluation of dogs > 10 years of age is warranted. Additionally, expanding this method to study dogs at lower and higher BCS will be important to address its utility, accuracy, and precision in dogs. All dogs in the present study were neutered, which may have resulted in different findings than if sexually intact dogs were enrolled. The potential effect of neutering on muscle mass requires further study. An additional limitation relates to the activity monitoring, which provides only a snapshot of the dogs' activity and may not be an accurate assessment of their long-term activity levels. Finally, no gold standard for assessing muscle mass was used in this study, so the true accuracy of the ultrasonographic and CT measurements remains to be tested.
There are numerous future research opportunities in the area of sarcopenia in dogs. Assessment of the use of ultrasonographic and CT measurement of epaxial muscle needs to be performed in dogs of other sizes, breeds, and body conditions to prove its usefulness outside of the homogeneous population of dogs evaluated in the present study. Furthermore, the dearth of information regarding the mechanisms underlying the loss of LBM in dogs, including the role of diet, decreased stimulation of muscle synthesis, and increases in other inflammatory mediators, warrants further investigation. Such data are needed to advance future research into the prevention and treatment of sarcopenia in dogs, which has the potential to improve the quality and quantity of life for dogs. Results from this study can aid in designing future, more definitive studies on sarcopenia in dogs.
ABBREVIATIONS
| BCS | Body condition score |
| CRP | C-reactive protein |
| DEXA | Dual-energy x-ray absorptiometry |
| IGF | Insulin-like growth factor |
| LBM | Lean body mass |
| MCS | Muscle condition score |
| TNF | Tumor necrosis factor |
Michel KE, Anderson W, Cupp C, et al. Correlation of a feline muscle mass score with body composition determined by DEXA (abstr), in Proceedings. WALTHAM Int Nutr Sci Symp 2010;47.
Kodak CR800, Carestream Health, Rochester, NY.
OsiriX Imaging Software, version 3.9.2, OsiriX Foundation, Geneva, Switzerland.
Dexdomitor, Pfizer Animal Health, New York, NY.
Torbugesic, Fort Dodge Animal Health, Fort Dodge, Iowa.
Midazolam, Hospira, Lake Forest, Ill.
AceproJect, Butler Animal Health Supply, Dublin, Ohio.
Glycopyrolate, American Regent, Shirley, NY.
Propofol, Hospira, Lake Forest, Ill.
Acquilion 16, Toshiba America Medical Systems, Tustin, Calif.
Philips iU 22 ultrasound machine, Philips Medical Systems, Bothel, Wash.
Actical Mini Mitter Monitor, Respironics, Bend, Ore.
Actical Mini Mitter Software, Respironics, Bend, Ore.
Tridelta Development Ltd, Maynooth, County Kildare, Ireland.
Quantikine canine TNF-α ELISA, R&D Systems, Minneapolis, Minn.
Endocrinology Section, Veterinary Diagnostic Center for Population and Animal Health, Michigan State University, Lansing, Mich.
Systat, version 12.0, SPSS Inc, Chicago, Ill.
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