data from a sample of 72 women aged 18–69 years suggested a strong correlation between age and the amount of low‐density lean tissue as assessed by a CT scan of the mid‐thigh. The density of muscle tissue as assessed by CT is indicative of the amount of fat infiltration into the muscle [12]. Higher age was associated with greater amounts of low‐density lean tissue (correlation coefficient = 0.52 [13]). This result again suggested a greater fat infiltration into the muscle with the increasing age.
These cross‐sectional data, however, should be interpreted carefully as cohort and period effects, and not aging per se, may have caused the observed differences in muscle size and muscle composition between the age groups. For example, well‐known cohort differences in body height, a strong determinant of muscle size, may partly explain the lower muscle mass in older persons compared with younger persons. In addition, period differences in lifestyle (e.g. sports participation, diet, and obesity status) and job demands may have differentially affected muscle size and muscle composition between age groups. Therefore, prospective data are needed within the same individuals to investigate the true change in muscle mass with aging.
CHANGE IN MUSCLE MASS WITH AGING
Forbes was among the first researchers to report prospective data on the age‐related decrease in lean body mass in a small group of adults using potassium40 counting data [14]. The reported decline was −0.41% per year as observed in 13 men and women aged 22–48 years.
Many prospective studies followed using body composition techniques such as bioelectrical impedance, isotope dilution, skinfolds, and underwater weighing to study change in fat‐free body mass and total body water with aging [15–21]. However, due to the body composition methodologies used in these studies, no precise measurement of skeletal muscle mass could be obtained because fat‐free mass and total body water also include lean, non‐muscle tissue such as the visceral organs and bone. Therefore, these studies only provide a crude estimate of the sarcopenia process with aging.
More recent prospective studies have measured the decline in appendicular lean mass using DXA [22–25] and the decline in muscle cross‐sectional area by CT in relatively large samples of older men and women [26, 27]. The characteristics of these studies are presented in Table 2.1. From these studies a precise and accurate estimation of the sarcopenia process can be obtained. The relative annual decline in skeletal muscle mass was estimated to be between −0.65 and −1.39% per year for older men and between −0.61 and −0.80% per year for older women (Figure 2.3). Even in weight‐stable older persons, a decline in appendicular lean mass was observed [24, 25]. In older persons the absolute as well as the relative decline of skeletal muscle mass with aging was generally larger in men compared with women. Moreover, prospective studies show that the relative annual decline in skeletal muscle mass increases with higher age group between the ages 40 and 90 years [23, 28]. For example, the relative 6‐year change in leg lean mass increased from −0.33% in women in their 40s to −0.65% in women in their 70s. For men, these percentages increased from −0.07 to −0.65% [23].
Table 2.1 Characteristics of prospective studies investigating the age‐related change in skeletal muscle mass in older men and women as assessed by dual‐energy x‐ray absorptiometry (DXA) or computed tomography (CT).
| Reference | N and sex | Country | Age (mean [SD]) or range (y) | Mean follow‐up time (y) | Body composition method | Muscle measurement |
| 22 | 1129 men 1178 women | United States | 70–90 | 7 | DXA | Leg lean mass |
| 23 | 114 man 95 women | Japan | 70–79 | 6 | DXA | Leg lean mass |
| 24* | 24 men 54 women | United States | 60–90 | 4.7 | DXA | Appendicular lean mass |
| 25* | 60 men 101 women | Italy | 68–78 | 2 | DXA | Appendicular lean mass |
| 26 | 869 men 934 women | United States | 70–79 | 5 | CT | Mid‐thigh muscle cross‐sectional area |
| 27 | 188 men 166 women | United Kingdom | 68.9 (2.5) 69.2 (2.6) | 7 | Peripheral quantitative CT | Arm muscle area |
*Weight‐stable sample.
SD = standard deviation.
Figure 2.3 Annual decline (%) in skeletal muscle mass in older men and women from prospective studies with follow‐up times from 2 to 7 years.
Limited data are available on the prospective change in muscle fat with aging. Data from the Health, Aging and Body Composition Study showed an increase in intermuscular fat at the mid‐thigh of 3.1 cm2 in older men and 1.7 cm2 in older women during the 5‐year follow‐up [29]. This is translated to an annual increase of 9.7% in men and 5.8% in women. This increase was paralleled by a decline in subcutaneous fat at the mid‐thigh and shows specifically the increasing fat infiltration into the muscle tissue with the increasing age. Moreover, data from 99 adult male twins with a mean age of 47.3 years at baseline, showed a decline in the ratio of muscle cross‐sectional area/functional cross‐sectional area over 15 years as assessed by magnetic resonance imaging (MRI) at the L3–L4 level, indicative of greater fat infiltration into the paraspinal muscles with aging [30].
From these body composition studies it can be concluded that the amount of skeletal muscle mass declines substantially with aging. At the same time, the composition of the muscle changes and a greater fat infiltration into the muscle occurs. It is important to understand the potential impact of these changes on healthy aging.
REFERENCES
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