Sarcopenia

Sarcopenia (from the Greek meaning "poverty of flesh") is the degenerative loss of skeletal muscle mass and strength associated with senescence. Sarcopenia (sometimes confused with sarcoidosis, an unrelated skin disease) is not a disease itself, but a medical term describing marked or visible loss of musculoskeletal mass, strength, and integrity. As exercise and activity levels decline over a lifetime -- or access to such exercise and activity is reduced -- muscle mass and strength also decline. Marked by an infiltration of fat and excess connective tissue into the muscle mass, sarcopenia unfortunately becomes a minor descriptor for many middle-aged people and a major characteristic of the frail elderly population.

Simple circumference measures do not provide enough data to determine whether or not an individual is suffering from severe sarcopenia. Sarcopenia is also marked by a decrease in type 2 fiber circumference, but no change in type 1 fiber circumference, and a pathological morphing of muscle structure. Furthermore, sarcopenia is associated with a distinct loss of quiescent satellite cells available for recruitment to repair muscle injury; hence, the ability to repair damaged muscles or respond to nutritional signals is impaired. Extreme muscle loss is often a result of both diminishing anabolic signals, such as growth hormone and testosterone, and promotion of catabolic signals, such as pro-inflammatory cytokines.

Due to the lessened physical activity and increased longevity of industrialized populations, sarcopenia is emerging as a major health concern. Sarcopenia may progress to the extent that an older person may lose his or her ability to live independently. Furthermore, sarcopenia is an important independent predictor of disability in population-based studies, linked to poor balance, gait speed, falls, and fractures. Sarcopenia can be thought of as a muscular analog of osteoporosis, which is loss of bone, also caused by inactivity and counteracted by exercise. The combination of osteoporosis and sarcopenia results in the significant frailty often seen in the elderly population.

Natural history

Strength losses with ageing for men and women are relatively similar. They are greater for lower than upper extremity muscles. Maximum attainable strength peaks in mid-twenties and declines thereafter. The decline is precipitous after 65 years of age, though few longitudinal studies exist on this topic. A direct assessment of the effects of sarcopenia, even in extremely physically fit individuals, can be seen in the age-related decline in Masters athletics (track and field) world records of muscle-intensive sports, such as weight lifting. No substance-free proven Olympic weight lifting record has been set by any athlete of any sex or weight class above the age of 31. However, in the sport of powerlifting, many world records in several weight divisions have been accomplished by athletes well into their forties. It has been postulated that while Olympic weightlifting requires great speed to accomplish the snatch and clean and jerk movements, the squat, bench press and deadlift require no such attribute and only great limit strength is required. These world records are verified by the International Powerlifting Federation, and are certified by that organization to have been accomplished drug-free.

Diagnosis

Making the clinical diagnosis of sarcopenia is difficult for the following reasons. There is no absolute level of lean mass, body cell mass, or muscle mass for comparison. There is no generally accepted clinical test to diagnose sarcopenia. Finally, there is no accepted threshold of functional decline at which sarcopenia is implied. However, the use of whole-body dual-energy x-ray absorptiometry (DEXA) or CT scans of the abdomen to assess muscle mass is being assessed in research settings. Baumgartner et al. published a working definition of sarcopenia based on 2 standard deviations below the mean for healthy young adults which has been used in research settings. [cite journal |author=Baumgartner RN, Koehler KM, Gallagher D, "et al" |title=Epidemiology of sarcopenia among the elderly in New Mexico |journal=Am. J. Epidemiol. |volume=147 |issue=8 |pages=755–63 |year=1998 |month=Apr |pmid=9554417 |doi= |url=http://aje.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=9554417]

Mechanism

Lack of exercise, over a long period of time is currently thought to be the primary, or preponderant, cause of sarcopenia. [cite journal |author=Abate M, Di Iorio A, Di Renzo D, Paganelli R, Saggini R, Abate G |title=Frailty in the elderly: the physical dimension |journal=Eura Medicophys |volume=43 |issue=3 |pages=407–15 |year=2007 |month=Sep |pmid=17117147 |doi= |url=http://www.minervamedica.it/index2.t?show=R33Y2007N03A0407] Just as fitness is cumulative, so sarcopenia is also cumulative, reflecting years of chronic inactivity or lack of access to exercise. Not only muscle but the entire musculoskeletal system of muscle, neuromuscular responsiveness, endocrine function, vasocapillary access, tendon, joint, ligament, and bone, depends on regular and lifelong exercise to maintain integrity. The slow attenuation, atrophy, or loss of muscle tissue that medical professionals sometimes describe as sarcopenia (literally, "flesh loss') is currently thought to be the result of cumulative loss of musculoskeletal strength and mass associated with chronic absence of exercise of sufficient intensity or volume.

In the past decade, it has become clear that without exercise, there is an increased catabolic signal to aged muscle, which is associated with increased catabolic cytokine production by the immune system (See below). Breakdown of neuromuscular junction as well as activity-dependent loss of neurons in the spinal cord are thought to be two of the proximate causes. Possibly, the decreased ability of satellite cells to propagate themselves may be another contributing factor. Satellite cells are required to fuse into skeletal muscle fibers, and help in settings where repair and regeneration are required. Therefore unexercised muscle loses its ability to respond to anabolic stimuli, such as insulin, growth hormone, and amino acids. Catabolic stimuli may also play a role: the inflammatory IL-6, IL1-Ra, and TNF-alpha are elevated in people with significant sarcopenia. As exercise and activity levels often decline with age, many anabolic stimuli are withdrawn in the elderly population. Depleted muscles atrophy and are replaced by connective tissue, though the mechanism in sarcopenia may be different than that seen in other settings of "muscle atrophy", since in younger individuals there is not an obvious problem with the satellite cells. Type II muscle fibers atrophy more so than type I.

Management

Primary management of sarcopenia is through the application of a graded exercise program, across both cardiovascular and strength domains, dosed in such a way as to provoke beneficial adapatation without overloading the weakened body. [cite journal |author=Taaffe DR |title=Sarcopenia--exercise as a treatment strategy |journal=Aust Fam Physician |volume=35 |issue=3 |pages=130–4 |year=2006 |month=Mar |pmid=16525526 |doi= |url=http://www.racgp.org.au/afp/200603/3608] Possible therapeutic strategies include resistance training and aerobic activity programs, as evidenced by recent studies. Nutritional evaluation may also be indicated if malnutrition is suspected, or current nutritional intake is insufficient to maintain adequate total body mass, although increased exercise also increases appetite. Physical activity incorporating resistance training is probably the most effective measure to prevent and treat sarcopenia.

References

*cite journal |author=Roubenoff R |title=Physical activity, inflammation, and muscle loss |journal=Nutr. Rev. |volume=65 |issue=12 Pt 2 |pages=S208–12 |year=2007 |month=Dec |pmid=18240550 |doi= |url=http://openurl.ingenta.com/content/nlm?genre=article&issn=0029-6643&volume=65&issue=12%20Pt%202&spage=S208&aulast=Roubenoff

*cite journal |author=Lynch GS |title=Tackling Australia's future health problems: developing strategies to combat sarcopenia—age-related muscle wasting and weakness |journal=Intern Med J |volume=34 |issue=5 |pages=294–6 |year=2004 |month=May |pmid=15151679 |doi=10.1111/j.1444-0903.2004.00568.x |url=

*cite journal |author=Edström E, Ulfhake B |title=Sarcopenia is not due to lack of regenerative drive in senescent skeletal muscle |journal=Aging Cell |volume=4 |issue=2 |pages=65–77 |year=2005 |month=Apr |pmid=15771610 |doi=10.1111/j.1474-9728.2005.00145.x |url=

*cite journal |author=Fujita S, Volpi E |title=Amino acids and muscle loss with aging |journal=J. Nutr. |volume=136 |issue=1 Suppl |pages=277S–80S |year=2006 |month=Jan |pmid=16365098 |doi= |url=http://jn.nutrition.org/cgi/pmidlookup?view=long&pmid=16365098

*cite journal |last=Visser |first=Marjolein |authorlink= |coauthors=Deeg D, Lips P |year=2003 |month= |title=Low vitamin D and high parathyroid hormone levels as determinants of loss of muscle strength and muscle mass (sarcopenia) |journal=J. clin. endocrinol. metab. |volume=88 |issue=12 |pages=5766–5772 |id= |url=http://cat.inist.fr/?aModele=afficheN&cpsidt=15356420 |accessdate= 2007-11-06 |quote=|doi=10.1210/jc.2003-030604|pmid=14671166