Source: Based on references [7, 8].
The amount of non‐muscle tissue within the muscle was also assessed using five CT scans of the thigh in 11 older men and 13 young men [4]. Older men had 59.4% more non‐muscle tissue within the quadriceps and 127.3% within the hamstring muscle. In a similar study, the amount of non‐muscle tissue in older men was 81% higher in the plantar flexors as compared with young men [6]. Thus, apart from the smaller muscle size in old age, these studies suggested that the composition of the muscle also changed with aging, leading to less “lean” muscle tissue in old age.
With the greater availability of body composition methods such as bioelectrical impedance and dual‐energy x‐ray absorptiometry (DXA) over time, cross‐sectional data on muscle size in large study samples including a broad age range have been collected. Examples of these studies using lean mass from DXA (the non‐bone, non‐fat soft tissue mass) and fat‐free mass from bioelectrical impedance, presented by 10‐year age groups of men, are presented in Figure 2.1 [7, 8]. Older age groups had a lower total body fat‐free mass, lower total body lean mass, and lower arm and leg lean mass. Figure 2.2 presents the differences in muscle size between 10‐year age groups in men and women. With increasing age group, the data suggested a lower whole‐body lean mass and leg lean mass as assessed by DXA [9], a smaller arm muscle cross‐sectional area (from anthropometric measures [10]), and a smaller calf muscle cross‐sectional area (from peripheral qualitative CT [11]). These cross‐sectional data derived from samples from Italy, Australia, India, Japan, and the United States consistently suggested a decline in muscle size with aging. These data also suggested a steeper decline in muscle size with aging in men compared with women.
Figure 2.2 Differences in muscle cross‐sectional area and lean mass using different body composition methodologies between men and women of different age groups. DXA = dual‐energy x‐ray absorptiometry; CT = computed tomography; Anthrop. = anthropometry, using arm circumference and triceps skinfold.
Source: Based on references [9–11].
Cross‐sectional 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
|