What changes in lean body mass occur with aging?
There is an inevitable loss of lean body mass, mostly skeletal muscle, with aging. Cross-sectional studies demonstrate that starting at age 30 years, 1% to 2% of muscle mass is lost per year, such that a 20% to 30% loss of lean mass is observed by age 80. The mechanisms underlying the loss of muscle mass with aging include declines in sex hormones, myocyte apoptosis and mitochondrial dysfunction. Secondary causes of sarcopenia include disuse resulting from immobility and a sedentary lifestyle, inadequate nutrition, and disease-related muscle loss (e.g., endocrine and inflammatory diseases). The age-related loss of muscle mass has been blamed for much, but not all, of the age-related decline in muscle strength and power. Longitudinal studies have shown that the decline in strength outpaces the loss of lean mass with up to a 60% loss in strength from age 30 to 80 years. Furthermore, the loss of strength and power is not as linear as the loss of muscle mass and seems to accelerate at older ages. A 25% decline in strength has been detected between ages 70 and 75 years. Power (work per unit time) may decline at double the rate of strength. These changes in lean mass, muscle mass, strength and power are complex but have important functional consequences for older people. For example, testosterone supplementation is consistently associated with increases in lean mass but is less consistently associated with improvements in strength or function.
The term sarcopenic obesity refers to the age-associated loss of muscle mass and/or muscle function (i.e., strength) coupled with increased adiposity. Consensus on diagnostic tools for or the definition of sarcopenic obesity is lacking, and therefore, prevalence, clinical relevance, and optimal treatment have not been fully established. Prospective studies show that older men with sarcopenia and obesity have the highest risk of all-cause mortality even after adjustment for lifestyle factors (e.g., smoking, alcohol intake, physical activity, and occupation), cardiovascular disease (CVD), inflammation, and weight loss. These data support the importance of implementing weight-loss programs that maintain lean mass in older adults. One contributing factor to the development of sarcopenic obesity could be weight cycling with loss of both lean and fat mass followed by regain of fat mass only. This may have a greater effect in older patients who are less anabolic and generally less active. The loss of lean mass with aging can have a profound effect on resting metabolic rate and thus predispose to further accretion of fat mass if caloric intake is not reduced.