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.
Body condition score
Dual-energy x-ray absorptiometry
Insulin-like growth factor
Lean body mass
Muscle condition score
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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