*2.2. Body Composition and Metabolism*

Although aging is generally associated with a loss of lean muscle mass, exercise can modulate such losses. According to population-based studies, the prevalence of sarcopenia in healthy adults aged 60 years and older is about 10% for men and 10% for women, respectively [30]. Intriguingly, in a cross-sectional study including 156 female and male masters athletes aged between 40–79 years, no individual was categorized as sarcopenic, i.e., below normal levels of muscle mass and muscle strength or performance, according to the definition of the European Working Group for Sarcopenia in Older People [31,32]. Data from cross-sectional studies indicate that lean muscle mass and muscle strength did not decline with age in individuals aged 40–81 years who trained 4 to 5 times per week [33]. These data could indicate that declines in physical function may not be related to age alone but are rather confounded by muscle disuse and decreased levels of PA in the elderly general population [34]. On the other hand, two previous cross-sectional studies based on an anthropometric assessment of lower limb muscle volume [35] in male 115 track and field masters [36] and in 54 male master weight lifters [37] indicated a volume reduction of approximately 6% per age decade. This figure has been confirmed by a recent longitudinal study in 71 track-and-field master athletes with a mean follow-up of 4.2 years that found a reduction in calf muscle cross section, as assessed with computed tomography, by 0.6% per year in men, but no significant change in women [38]. Moreover, a recent cross-sectional study in 256 track and field masters aged 35–91 years in which whole body skeletal muscle mass was assessed via bio-electrical impedance indicates a reduction by 3.2% and 2.8% per decade in women in men, respectively [39], which was equalized by a commensurate increase in fat mass. On the other hand, older athletes of 68 years showed 17% lower body fat percentage and 12% greater leg lean mass, respectively [40]. In the general population, fat-free mass is expected to decrease the 6th decade of life onwards by approximately 2% per decade in men but not in women, while both men and women gain fat mass by 7.5% per decade [41]. Thus, whilst masters athletes likely experience muscle wasting and adipose tissue accumulation, their body composition may still be better preserved than in the non-athletic counterparts.

The question arises, which factors may trigger the age-related adipose tissue accumulation. Lifelong training increased the proportion of type I muscle fibres with a concomitant decrease of carbohydrate oxidation independent of intensity level in older athletes compared to younger man [42]. While fat oxidation capacity was similar in both groups, older athletes compensated with a higher exercise metabolic efficiency [42]. While sports and recreational activity decreased to a higher extent in men than in women over a course of ~10 years, activity levels as well as baseline age were inversely related to changes in fat mass in women only [41]. Low RMR may predispose to future weight gain. As PA contributes considerably to total energy expenditure, the question is whether regular exercise can curb age-related reductions in RMR. Although it is difficult to disentangle changes in metabolic rate from alterations of body composition with aging, evidence suggests that RMR is lower in older men and women, even after adjusting for differences in body composition, waist-to-hip ratio and smoking status [43,44]. A paper, based on the cohort of 256 track and field masters athletes mentioned above, has found that the effect of age on RMR is mostly attributable to changes in body composition [45]. While fat-free mass

is a main determinant of RMR, other factors that are unrelated to differences in body composition can also explain differences in RMR of young and older individuals. Although the decline of RMR also occurs in highly physically active individuals, it is associated with reductions of exercise volume and energy intake that occur with age. However, these age-related adaptations are blunted in individuals who maintain these two components at a similar level as young physically active men [46]. Also, a higher aerobic capacity is related to a higher RMR in older athletes [47]. In a sample of 65 healthy women ranging from 21–72 years, those individuals who were regularly performing endurance exercise were spared from an age-related decline in RMR assessed by indirect calorimetry compared to their sedentary counterparts [48]. This metabolic difference may in part explain lower body weight and fat mass in active, older individuals.

In summary, the decline in metabolic rate, along with simultaneous declines and inclines, respectively, in lean mass and fat mass in the elderly can only in part be ascribed to the aging process per se. Rather, these effects seem confounded by declining levels of PA and inadequate energy intake in this population. Although body composition of older athletes is considerably better than that of less physically active age-matched individuals, the age-related decline and alterations in body composition and metabolism also takes place in this group and can in part be compensated for by higher exercise metabolic efficiency.

#### *2.3. Effects of Aging on the Endocrine System and Metabolic Pathways*

The aging process is accompanied by several endocrine alterations, along with changes in nearly all biological systems, including body composition, functional performance and bone mass. These aging-induced effects are often confounded by other factors, such as chronic diseases or changes of dietary patterns, and malnutrition often occur concomitantly during the process of aging. This section is not intended to give a thorough overview of endocrine changes, but discusses the most important hormonal alterations occurring during aging pertaining regulation of appetite, glucose homeostasis and energy expenditure and the modulatory role of PA.
