Sarcopenia.

Sarcopenia is a term utilized to define the loss of muscle mass and strength that occurs with aging. Sarcopenia is believed to play a major role in the pathogenesis of frailty and functional impairment that occurs with old age. Progressive muscle wasting occurs with aging. The prevalence of clinically significant sarcopenia is estimated to range from 8.8% in young old women to 17.5% in old old men. Persons who are obese and sarcopenic (the "fat frail") have worse outcomes than those who are sarcopenic and non-obese. There is a disproportionate atrophy of type IIa muscle fibers with aging. There is also evidence of an age-related decrease in the synthesis rate of myosin heavy chain proteins, the major anabolic protein. Motor units innervating muscle decline with aging, and there is increased irregularity of muscle unit firing. There are indications that cytokines-especially interleukin-1beta, tumor necrosis factor-alpha, and interleukin-6-play a role in the pathogenesis of sarcopenia. Similarly, the decline in anabolic hormones-namely, testosterone, dehydroepiandrosterone growth hormone, and insulin-like growth factor-I-is also implicated in the sarcopenic process. The role of the physiologic anorexia of aging remains to be determined. Decreased physical activity with aging appears to be the key factor involved in producing sarcopenia. An increased research emphasis on the factors involved in the pathogenesis of sarcopenia is needed.