Issues concerning sarcopenia in ageing adults.

Sir, Cruz-Jentoff et al. present a valuable review of the difficult field of clinical measurements of sarcopenia [1]. However, several problems remain that limit full acceptance of their calculations. First, all or most of the current methods employed for measurement of muscle mass or fat-free mass (FFM) have associated assumptions and technical issues, and questionable accuracy in assessing small changes [2]. Further, although extensively discussed by the authors, the major current anatomical definitions of sarcopenia are clearly error-prone. Given the very wide variations of muscle mass (about half of FFM) within the normal population, the use of the ‘sex-specific lowest 20% of study group’ or of ‘>2 standard deviations below mean of young adults’ seem poor definition criteria. For example, using direct measurement of body fat by inert gas absorption to accurately calculate FFM by difference (±2%), the range of FFM for healthy Americans of similar sex, age and height is very wide (over 40%) [3]. Many (most?) bigger normal persons could lose over 20% of FFM (or muscle mass) without being appreciated as sarcopenic while smaller normals might be labelled sarcopenic with only minor loss [3]. Although the concept of ‘primary age-related’ muscle loss has been advanced for over 70 years, most supporting animal studies have been largely misleading due to problems associated with cross-sectional observations, to unappreciated disease within animal colonies or to the selectively longer lifespan of smaller mammals [4]. Some years ago, our laboratory carried out longitudinal observations of several colonies of male Sprague-Dawley rats, using serial in vivo measurements for body fat of individuals from early maturity to old age, as well as parallel histochemical studies. No loss of muscle mass or FFM was apparent in healthy rats observed to 900 days (when colonies were terminated) [4, 5]. Nine hundred days is thought approximately equivalent to human age 90. Similarly, when direct in vivo measurement of body fat was used to establish individual fat-free masses (±2%) for carefully studied small groups of young men, young women, older men and older women (ages 60–75), no significant reductions of FFM with age were demonstrated [3]. Were the older human subjects still too young to exhibit sarcopenia? Early, often occult, illnesses are highly prevalent at the advanced years when sarcopenia becomes evident, so the isolation and documentation of ‘primary age-related sarcopenia’ prove increasingly complicated. In view of the inherent broad ranges of muscle mass and muscle function within the normal population, it would appear that establishing valid changes (and defining the causes) of individual muscle mass and function will require detailed longitudinal observations over periods of time.