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D. R. Seals: Department of Integrative Physiology, University of Colorado, 354 UCB, Boulder, CO 80309, USA. Email: firstname.lastname@example.org
The demographics of ageing are changing dramatically such that there will be many more older adults in the near future. This setting is projected to produce a new ‘boomer-driven’ epidemic of physiological dysfunction, disability and risk of chronic degenerative disorders, including cardiovascular diseases. Standing out against this dreary biomedical forecast are Masters athletes, a group of middle-aged and older adults who engage in regular vigorous physical training and competitive sport. Compared with their sedentary/less active (untrained) peers, Masters athletes who perform endurance training-based activities demonstrate a more favourable arterial function–structure phenotype, including lower large elastic artery stiffness, enhanced vascular endothelial function and less arterial wall hypertrophy. As such, they may represent an exemplary model of healthy or ‘successful’ vascular ageing. In contrast, Masters athletes engaged primarily/exclusively in intensive resistance training exhibit less favourable arterial function–structure than their endurance-trained peers and, in some instances, untrained adults. These different arterial properties are probably explained in large part by the different intravascular mechanical forces generated during endurance versus resistance exercise-related training activities. The more favourable arterial function–structure profile of Masters endurance athletes may contribute to their low risk of clinical cardiovascular diseases.
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The world is ageing; the number of older adults is on the rise. This phenomenon comes with serious physiological and health implications, including increases in cardiovascular dysfunction and disease (CVD). Indeed, it has been projected that without effective intervention 40% of all adults in the USA will have at least one form of CVD by 2030, with a tripling of attendant medical costs, due largely to the ageing of the population (Heidenreich et al. 2011).
In the midst of this impending epidemic of age-associated dysfunction and disease stands a physiologically exceptional group of middle-aged and older adults referred to as ‘Masters athletes’. These individuals exercise vigorously on most, if not all, days of the week, often engaging in athletic competitions and demonstrating enhanced age-normalized physical function and remarkable sports performance (Tanaka & Seals, 2008). Importantly, at least for those performing aerobic exercise-related training and competitions, Masters athletes have greater cardiovascular capacity (e.g. maximal cardiac output and oxygen consumption; Tanaka & Seals, 2008) and a lower risk of CVD (Laure & Binsinger, 2009) compared with their more sedentary peers.
Many physiological and/or pathophysiological changes are likely to contribute to declines in cardiovascular function and increases in CVD risk with ageing. Among the most important are changes to the arterial system, including stiffening of the large elastic arteries (aorta and carotid arteries), development of endothelial dysfunction and wall thickening (Lakatta & Levy, 2003). Here we summarize and update recent discussions (Seals et al. 2008, 2009) of evidence suggesting that these adverse vascular changes may be less manifest (or even absent) in certain subgroups of Masters athletes and, therefore, might help explain their more favourable cardiovascular capacity and health.
Large elastic artery stiffness
Large elastic artery stiffness, most commonly assessed by aortic pulse wave velocity (aPWV) or the local compliance of the carotid artery (via ultrasound and tonometry), has emerged as a major independent risk factor for CVD in older adults and is linked to a greater risk of systolic hypertension, left ventricular hypertrophy and other disorders of ageing, such as cognitive impairment (Lakatta & Levy, 2003; Mitchell et al. 2010). As reflected by increased aPWV and decreased carotid compliance, large elastic artery stiffness increases with age even in non-hypertensive adults free of clinical CVD (Tanaka et al. 1998; Lakatta & Levy, 2003).
Middle-aged and older male and female Masters endurance athletes (triathletes, cyclists, runners and swimmers) demonstrate lower aPWV (Vaitkevicius et al. 1993; Tanaka et al. 1998) and greater carotid artery compliance (Tanaka et al. 2000; Monahan et al. 2001; Moreau et al. 2003, 2006a; Nualnim et al. 2011) compared with their non-exercise-trained or sedentary (herein referred to as ‘untrained’) peers (Fig. 1). The aPWV in these Masters athletes is similar to that in trained and/or untrained young adults (Vaitkevicius et al. 1993; Tanaka et al. 1998), whereas carotid compliance is lower than that observed in young adult control subjects (Tanaka et al. 2000; Moreau et al. 2003). The lower large elastic artery stiffness in Masters endurance athletes compared with middle-aged/older untrained adults is associated with other cardiovascular benefits, including lower 24 h systolic and pulse pressures (Seals et al. 1999) and enhanced baroreflex sensitivity (Monahan et al. 2001; Nualnim et al. 2011). Little is known about the mechanisms by which these Masters athletes maintain lower large elastic artery stiffness with age, but less oxidative stress-related suppression of arterial compliance may play an important role (Moreau et al. 2006a). A lower ‘subclinical’ CVD risk factor burden in the Masters endurance athletes also could contribute, although subjects with major risk factors were excluded in the aforementioned studies.
In contrast to their peers performing endurance training/competitions, Masters athletes engaged in sports requiring intensive resistance training have greater large elastic artery stiffness than untrained adults, as indicated by lower carotid artery compliance (Miyachi et al. 2003). Interestingly, Masters rowers, a group of athletes who perform both intensive resistance and endurance training, demonstrate enhanced carotid artery compliance compared with untrained control subjects (Cook et al. 2006), suggesting that even some element of endurance training can offset the apparent negative consequences of intensive resistance training. No differences in peripheral large (femoral) artery compliance have been observed among groups of Masters athletes and untrained healthy adults (Cook et al. 2006; Nualnim et al. 2011), suggesting age- and training-specific influences on large elastic arteries.
Reduced vascular oxidative stress is a key mechanism by which EDD is preserved with age in male Masters athletes (Taddei et al. 2000; Eskurza et al. 2004; Franzoni et al. 2005). Indeed, there is now direct evidence of reduced oxidant stress in the vascular endothelial cells of these athletes compared with untrained control subjects, and this is associated with reduced endothelial cell expression of the oxidant enzyme NADPH oxidase and redox-sensitive transcription factor nuclear factor κB, as well as increases in the expression of the antioxidant enzyme manganese (mitochondrial) superoxide dismutase (SOD) and activity of endothelium-bound SOD (Pierce et al. 2011a). Reduced endothelial oxidative stress in these Masters athletes causes less destruction/greater bioavailability of the endothelium-dependent dilating molecule, nitric oxide (NO), resulting in a greater NO-mediated EDD (Taddei et al. 2000). Greater bioavailability of the critical cofactor for NO production, tetrahydrobiopterin (BH4), also plays an important role in the maintenance of EDD in these athletes (Eskurza et al. 2005). This could be due to less oxidation of BH4, increased endogenous BH4 synthesis, or both. Basal NO production also is preserved in male Masters endurance athletes (Seals et al. 2008), perhaps also a result of reduced oxidative stress and enhanced BH4 bioavailability.
The mechanisms for this endothelium-protective phenotype of male Masters athletes remain to be established, though it is not clearly or consistently related to differences in clinical characteristics (Seals et al. 2008, 2009, 2011). Rather, training-induced increases in intravascular laminar shear (via increases in systemic and active limb blood flow), differences in one or more presently unidentified (protective) circulating humoral factors and/or greater resistance to a given level of potentially endothelium-damaging factors (e.g. plasma low-density lipoprotein cholesterol or glucose) all have been proposed (Seals et al. 2008, 2009, 2011).
In comparison to men, far fewer data are available on vascular endothelial function in female Masters endurance athletes, and all of these data are based on brachial artery FMD. Initial reports on small groups of women suggested greater EDD in female Masters endurance athletes compared with untrained age-matched control subjects (Hagmar et al. 2006; Black et al. 2009). A recent study of a much larger sample found no differences in brachial FMD in endurance-trained and untrained postmenopausal women, while confirming past observations in men (Pierce et al. 2011b). Extensive analysis revealed no obvious physical or clinical characteristics that could explain the sex-specific differences. However, all of the women were estrogen deficient, and it is possible that a certain critical level of estrogen bioavailability is necessary for exercise-generated physiological signals to modulate vascular endothelial function in this group.
Finally, among Masters endurance athletes, it is possible that vascular endothelial function is influenced by the type of activity performed. A recent investigation found that brachial artery FMD was greater in middle-aged and older Masters runners compared with age- and sex-balanced groups of Masters endurance swimmers and healthy untrained control subjects (Nualnim et al. 2011). To our knowledge, no cross-sectional studies are available on vascular endothelial function in primarily/exclusively resistance-trained Masters athletes.
Arterial wall thickness
Carotid and femoral artery intima–media thickness (IMT) are independent predictors of CVD and increase two- to threefold with adult ageing in the absence of major risk factors or clinical diseases (Lakatta & Levy, 2003; Seals et al. 2008). This large artery wall thickening with age is mediated by hypertrophy of both the intimal and the medial layers and is likely to represent one aspect of a vascular remodelling process in response to changes in intravascular mechanical forces with ageing (Seals et al. 2008). Age-associated increases in IMT also may reflect the development of subclinical or clinical-grade atherosclerotic plaques, although the latter is less likely in healthy adults.
The carotid IMT of male and female Masters endurance athletes does not differ from untrained age- and sex-equivalent untrained adults, nor are the age-related differences in carotid IMT different in endurance athletes compared with untrained adults (Moreau et al. 2002, 2003; Tanaka et al. 2002). This also is the case in resistance exercise-trained Masters athletes (Miyachi et al. 2003). The absence of an effect is probably due to the fact that ‘central’ (e.g. carotid artery) blood pressure, a key determinant of IMT among healthy adults, does not differ in Masters athletes and health untrained control subjects.
In contrast, femoral artery IMT is smaller in male and female Masters endurance athletes compared with age- and sex-matched untrained control subjects, and the age-associated difference is smaller in endurance-trained athletes compared with untrained adults (Fig. 3; Dinenno et al. 2001; Moreau et al. 2002, 2006b). The smaller femoral IMT and accompanying increase in lumen diameter in Masters endurance athletes are features of ‘expansive arterial remodelling’, a process presumably aimed at normalizing wall stress in response to exercise-evoked increases in femoral blood flow required to meet the demands of the active muscles in the legs (Dinenno et al. 2001). Rather than being smaller, femoral IMT is greater in resistance-trained male Masters athletes compared with untrained age-matched control subjects (Miyachi et al. 2005). This may be the result of the different intravascular mechanical forces generated in the systemic circulation during resistance compared with endurance training, particularly the marked increases in arterial pressure during weight-lifting manoeuvres.
Summary and conclusions
Large elastic artery stiffness, vascular endothelial function and large artery wall thickness are major indicators of arterial health and risk of age-associated CVD (Lakatta & Levy, 2003). Overall, Masters endurance athletes demonstrate a more favourable arterial phenotype compared with untrained middle-aged and older adults, which may explain, at least in part, their greater cardiovascular functional capacity and lower risk of CVD. As such, the Masters endurance athlete may be viewed as a model of ‘exceptional vascular ageing’. In contrast, Masters athletes for whom training and competitive sport require primarily or exclusively intensive resistance muscle activities exhibit a less favourable arterial function–structure profile than their endurance-trained peers and, in some cases, compared with untrained adults. The differences in arterial properties between Masters athletes engaging in sports requiring endurance versus resistance training are probably explained by differences in the intravascular mechanical forces generated during these activities.
Our thanks go to all of the students, postdoctoral fellows and staff who contributed to the work in our laboratory. This work was supported by NIH R37 AG013038, T32 AG000279 and UL1 RR025780.