- Previous research has identified cutoff values for the minimum physical fitness levels to gain health benefits with regard to future morbidity and mortality.
- Extremely unfit populations may not be able to reach those cutoff values, and it is unknown how improvements in fitness impacts health in these populations.
- Aging individuals with intellectual disabilities are extremely unfit and likely will not reach those cutoff values identified for the general population.
- Our data support that even small differences at the lower end of the physical fitness spectrum are associated with health benefits, which supports a stronger focus on improving fitness among individuals with intellectual disabilities.
Individuals with intellectual disability (ID) exhibit very low physical activity levels throughout their lives (1). This is related to population-specific factors, including lack of support to become active, accessibility of facilities, and motivational issues (2). Their lifelong low physical activity levels and associated low fitness levels transition into an extremely sedentary aging population with very poor fitness levels, expressed by cardiorespiratory fitness, strength, balance, and other health-related fitness components (3,4). For example, V̇O2max, the criterion standard measurement for cardiorespiratory fitness, ranges between 30 and 40 mL·kg−1·min−1 for young adults with ID (22–30 mL·kg−1·min−1 for young adults with Down syndrome) (5), whereas for those who are 20–29 yr old in the general population, the average is around 45–50 mL·kg−1·min−1 (6). Grip strength in 50-yr-old individuals with ID was almost half of that of the general population. Grip strength was 29.4 kg in 50- to 60-yr-old men with ID versus 50.6 kg in 50- to 60-yr-old men in the general population, and grip strength was 21.4 kg in 50- to 60-yr-old women with ID compared with 30.9 kg in 50- to 60-yr-old women in the general population (4). Gait speed, as a measure of dynamic balance, was also lower in 50-yr-old individuals with ID than in the general population (1.02 m·s−1 for 50- to 60-yr-old women vs 1.31 m·s−1 for 50- to 60-yr-old women in the general population) (4).
The poor fitness is compounded by other population-specific factors influencing health and daily functioning throughout their lives, such as cognitive limitations, physical and psychiatric comorbidities, as well as other specific comorbidities related to specific genetic syndromes (i.e., Down syndrome) (7). As a result, their aging process is neither optimal nor healthy, with high rates of morbidity including cardiovascular disease (CVD) and diminished skills for daily living (8,9). This further increases the need for support and care throughout their senior life (10). Because both the identification and treatment of several health conditions in individuals with ID have improved over time, the life expectancy of the majority of this population is currently close to the life expectancy of the general population (11). This further aggravates the impact of the increased care and support needs (12), both on an individual level and on a population level, with health care costs substantially increasing in the last decades of life. Because the disability-associated health care expenditures of individuals with disabilities currently comprise around 25% of the total US annual health care spending (13), any means of reducing these costs would be of major interest to both the individuals involved and the policy makers.
Remarkably, because individuals with ID seem to experience every disadvantage when it comes to a healthy aging process, fitness may still be a potent target to improve health. For the general population, research has defined what minimum amounts of physical activity or physical fitness level (i.e., cutoff values) are required to lower the risk of future negative health outcomes such as CVD and all-cause mortality. In populations with activity and fitness levels well below those cutoff values, one might not expect that variance within these lower regions would still result in better future health outcomes. However, strong associations do exist between fitness and health outcomes in this unfit population of individuals with ID. The baseline fitness levels, although very low, of a large group of older adults with ID were still predictive of daily functioning and mobility 3 yr after the baseline, and of the all-cause mortality, 5 yr after baseline (14–16), potentially even more so than obesity (Oppewal A and Hilgenkamp TIM, unpublished data, 2019). Our novel hypothesis is that among very unfit, older adults with ID, small changes in fitness translate to major changes in health (Fig. 1).
Even small changes within those lowest fitness categories, below any cutoff values, seem to be associated with better future health outcomes, making physical fitness a key target for healthy aging in individuals with ID. This could have major implications for other sedentary and unfit (patient) populations.
FITNESS PREDICTS MORBIDITY AND MORTALITY IN THE GENERAL POPULATION
Physical fitness, in particular, health-related physical fitness, predicts morbidity and mortality in the general population. Because CVD is the leading cause for mortality in the general population (17), and CVD risk factors are prevalent in individuals with ID (18), we focus on CVD morbidity and on all-cause mortality. Several physical fitness components strongly predict CVD morbidity and mortality in the general population, with the strongest evidence for cardiorespiratory fitness (19,20), gait speed (21,22), and grip strength (23).
Meta-analyses of studies in the general population demonstrate that CVD morbidity and all-cause mortality risk is related to baseline fitness levels. Cardiorespiratory fitness can be expressed in metabolic equivalent of task (MET), where 1 MET equals approximately an oxygen expenditure of 3.5ml per kilogram of body weight per minute. In healthy adults (mean age ranging from 37 to 57 yr), every 1 MET increase in cardiorespiratory fitness corresponded with a 13% [risk ratio (RR) = 0.87; 95% CI, 0.84–0.90] lower risk of all-cause mortality and a 15% (RR = 0.85; 95% CI, 0.82–0.88) lower risk of CVD morbidity (19). Improvements in cardiorespiratory fitness over time were associated with a reduction in mortality risk, independent of baseline cardiorespiratory fitness (24). In older adults (65 yr and older), a 0.1 m·s−1 faster baseline walking speed resulted in a 12% (hazard ratio (HR) = 0.88; 95% CI, 0.87–0.90) lower risk of all-cause mortality (22). For grip strength, a 1-kg higher baseline grip strength corresponded with a 4% (HR = 0.96; 95% CI, 0.93–0.98) lower risk of all-cause mortality in older adults (60 yr and older) (25). In another study in adults in the general population (35–70 yr), a 5-kg lower grip strength at baseline resulted in a 16% higher all-cause mortality risk (HR = 1.16; 95% CI, 1.13–1.20), 17% higher cardiovascular mortality risk (HR = 1.17; 95% CI, 1.11–1.24), 7% higher risk for myocardial infarction (HR = 1.07; 95% CI, 1.02–1.11), and 9% higher risk for stroke (HR = 1.09; 95% CI, 1.05–1.15) (26). The above-mentioned studies suggest a linear association per unit of increase/decrease in physical fitness; however, some studies suggest the greatest risk reductions happen when progressing from the least fit and the next least fit groups (20).
The above-mentioned studies might seem to be informative across the entire spectrum of physical fitness levels. However, two limitations on generalizing those results of unfit populations need to be addressed. Most of those studies include populations with a wide range of physical fitness levels, which makes it easier to detect an overall correlation, which does not necessarily apply to the entire range in the same magnitude (19,22,26). Some other studies focused on populations with better overall fitness than that of our target population (25,26), thus, lacking information on the impact of improvements on the lower end of the physical fitness spectrum.
NORMS AND REFERENCE VALUES IN THE GENERAL POPULATION
In addition to the relative data about the associations between fitness and health outcomes, the following paragraphs show that absolute cutoff values exist for physical fitness components that indicate the minimum amount that is required to lower the risk of future CVD morbidity and mortality.
Cardiorespiratory fitness is the most often studied physical fitness component with regard to CVD morbidity and all-cause mortality. The American College of Sports Medicine (ACSM) reports norms by sex and age for cardiorespiratory fitness (6). Low cardiorespiratory fitness, often defined as the lowest quartile or quintile for each sex/age category, is associated with a higher risk of CVD morbidity, CVD mortality, and all-cause mortality (6). In a meta-analysis in healthy adults, the minimum cardiorespiratory fitness level to be associated with lower CVD and mortality risk in men was at 9 METs at 40 yr, 8 METs at 50 yr, and 7 METs at 60 yr. In women, this was slightly lower with 7 METs at 40 yr, 6 METs at 50 yr, and 5 METs at 60 yr (19). In various patient populations including those with CVD, epidemiological studies found cardiorespiratory fitness levels above 8 to 10 METs to be associated with survival, and a cardiorespiratory fitness level under 5 METs to be associated high mortality risk (19,20).
The normal range for comfortable walking speed is 1.2–1.4 m·s−1, with an increase from childhood up to early adulthood where there is a period of maintenance, after which, walking speed decreases during or following mid adulthood (30–40 yr of age) (27). In older adults, comfortable walking speeds below 1.0 m·s−1 and below 0.8 m·s−1 often are used as cutoff values for an increased all-cause mortality risk (22,27–29). Cutoff values between 0.8 and 1.3 m·s−1 were identified for CVD morbidity risk in older adults (60 yr and older) (21). This range in cutoff values suggest that there may be an “area of risk” instead of a single cutoff value, with a walking speed of 0.8 m·s−1 seen as the lower end.
Grip strength increases up to early adulthood followed by a period of maintenance, and then declines after mid adulthood. The peak in grip strength for men is between 29 and 39 yr of age (51 kg) and between 26 and 42 yr of age for women (31 kg) (30). The ACSM reports norms by sex and age categorized from poor to excellent (6), and numerous studies have reported norm scores, mostly focused on older adults (30–32). Although no absolute cutoff values have been defined, low grip strength is an important predictor for CVD morbidity and mortality and all-cause mortality (33). Relative cutoff values are most often based on tertiles for men and women separately. The grip strength values of the consecutive tertiles for women were 17.2, 24.6, and 32.4 kg, and for men, 27.5, 38.7, and 49.9 kg, with a median age of 50 yr (interquartile range, 42–58) for the total study sample (26). For grip strength, quartiles for women were <14, 14.01–18.2, 18.21–22.5, and ≥22.51 kg, and for men, <22, 22.01–30, 30.01–35, and ≥35.01 kg, with a mean age of 72.8 yr for the total study sample (25,34).
As shown, these cutoffs seem to indicate that the risk of CVD morbidity and mortality decreases considerably only after you cross a certain threshold of physical fitness. Furthermore, this concept could imply that small improvements below those cutoff values would not really reduce the risk of CVD morbidity and mortality. With little to no supporting evidence for a focus on improving physical fitness in populations with extremely low fitness levels, this may result in a passive approach toward the unfit individuals that may need it most.
LOW FITNESS IN ADULTS WITH INTELLECTUAL DISABILITIES
Individuals with ID have consistently demonstrated low physical fitness levels (4,5,35). A recent review summarized the results of 13 studies on cardiorespiratory fitness in individuals with ID across different countries, different settings, and their lifespan (5). This review showed that cardiorespiratory fitness is lower in individuals with ID, and even lower in individuals with Down syndrome (5). The single largest dataset of V̇O2max results of individuals with ID with and without Down syndrome showed a lower V̇O2max for individuals with Down syndrome; however, no difference was found between individuals with ID and a control group (36). Upon closer examination, this was caused by a different ratio of men and women in each group (much higher number of men in the group with ID compared with a majority of women in the control group), After controlling for sex, a significant difference in V̇O2max between individuals with ID and the control group was confirmed in this dataset as well (37).
In the Healthy Aging and Intellectual Disabilities (HA-ID) study, the physical fitness of older adults with ID (defined as 50 yr and older) has been studied on a wide range of physical fitness components, among which are cardiorespiratory fitness, walking speed, and grip strength (38). Cardiorespiratory fitness was evaluated with the 10-m incremental shuttle walking test, and the test score was the distance covered by the participant during the test. This distance was used to calculate V̇O2max (39). Comfortable walking speed was evaluated by measuring the time it took to cover 5 m, and maximal grip strength was measured with a hand dynamometer. A more detailed description of the tests and the execution can be found elsewhere (4). In Figures 2–4, the cardiorespiratory fitness, walking speed, and grip strength levels of older adults with ID are plotted against the norm values of the general population (4). It can be seen that the largest part of the population scores below the lower limits of the average ranges in the general population. For speed, 43% of the men and 54% of the women with ID scored below the reference values (representing the average range, defined as 95% CI) in the general population, whereas this was (by design) only 2.5% for the general population (4). For grip strength, the difference was even larger, with 77% of the men and 67% of the women with ID scoring below the reference values (defined as 95% CI) compared with 2.5% in the general population (4). For cardiorespiratory fitness, 100% of the individuals with ID scored below what was considered the average reference range (based on the lowest quintile as the lower limit), whereas 20% of the general population was expected to score below that reference range (4).
When using the available cutoff values to determine CVD morbidity and mortality developed for the general population, older adults with ID clearly land in the bottom categories, or even below the lowest categories. For example, with regard to cardiorespiratory fitness, in Figure 2, it can be seen that older adults with ID score below 5 METs (V̇O2max of 17.5 mL·kg−1·min−1), with the minimum cardiorespiratory fitness level associated with risk reductions (4,19). For walking speed, older adults with ID scored around or below the 1.0 m·s−1 cutoff point, often used as a cutoff point for increased mortality risk (22,28). For grip strength, no clear cutoff values were available, but 67%–77% of older adults with ID scored below the lower limits of the norm values of the general population.
Based on these cutoff values, individuals with ID do not meet the minimum levels in fitness to expect any health benefits in terms of reducing risks of future morbidity or mortality. Moreover, they are categorized as being at a high/the highest risk of developing CVD morbidity and mortality, even the relatively more fit individuals within this population. This might suggest that differences in physical fitness within this lower end of the spectrum, and consequently improvements still below these cutoff values, are not worthwhile to focus on/are not relevant in terms of lowering the risk of CVD morbidity and mortality.
SMALL DIFFERENCES BETWEEN UNFIT INDIVIDUALS: DOES IT MATTER?
The HA-ID study was the first to provide data on the actual impact of these very poor physical fitness levels on mortality risk in this population with extremely low fitness levels (3 and 5 yr post-baseline).
Even in this extremely unfit population, better baseline cardiorespiratory fitness, comfortable speed, and grip strength were independently associated with a lower mortality risk (16). Each additional meter walked on the 10-m incremental shuttle walking test resulted in a 0.3% lower mortality risk (HR = 0.997; 95% CI, 0.995–0.999) (16). Higher speed, in increments of 1 km·h−1, resulted in a 35% lower mortality risk (HR = 0.65; 95% CI, 0.54–0.78) (16). Expressed as meter per second, a 1 m·s−1 increment resulted in a 78.7% lower mortality risk (HR = 0.213; 95% CI, 0.11–0.41). Comparing this with the 12% lower mortality risk seen in the general older population with an increment of 0.1 m·s−1, this is somewhat lower (78.7%/10 = 7.8%) but still a significant reduction of mortality risk (22). An increase in grip strength of 1 kg resulted in a 3% lower mortality risk (HR = 0.97; 95% CI, 0.94–0.99) (16). This is comparable with the 4% reduction seen in the general older population (25). These risk reductions in older adults with ID occurred at much lower fitness levels than those in the general population, and in a younger population (50 yr and older instead of 60/65 yr and older).
These data support for the first time the hypothesis that even at the low end of the physical fitness spectrum, small improvements do make a difference in mortality risk. This finding is extremely relevant for our understanding of the needs of the population of individuals with ID across their lifespan, but it may also translate to other unfit populations without ID. Although we recognize that individuals with ID may differ from other populations with regard to both their specific physical and physiological characteristics and their risk of developing CVD morbidity and mortality, our hypothesis is that small differences in physical fitness will still have a big impact on future outcomes in any very unfit population.
SMALL IMPROVEMENTS WITHIN AN UNFIT INDIVIDUAL: WILL IT MATTER?
Having established the importance of the small differences within the lower end of the physical fitness spectrum, the remaining question is whether this translates to individual risk reductions when someone is actively improving his or her physical fitness levels, without switching reference categories or surpassing cutoff values. This would support a causal association between fitness and future health outcomes, beyond the current evidence of a correlation, which could also be indicative of a healthy survivor bias.
Intervention studies have demonstrated the effects of physical fitness programs in individuals with ID. Heller et al. (2) reviewed 11 studies with a physical activity/exercise program in individuals with ID and reported improvements in balance, strength, and cardiorespiratory capacity. Another review by Bartlo and Klein (40) included 11 intervention studies with different physical activity/exercise programs, but with overall moderate to strong evidence for improvements in balance, muscle strength, and quality of life. Because this unfit population is able to improve fitness levels, future research needs to include health outcomes related to morbidity and mortality, preferably following participants over a longer period, to answer the question whether better fitness significantly improves future health outcomes.
Our hypothesis is also in line with the research on physical activity benefits, summarized in the new 2018 Physical Activity Guidelines for Americans (41), which report improvements in health with small increases in physical activity within the lower end of the physical activity spectrum. For the first time, the Key Guidelines for Adults state that “Some physical activity is better than none” (41). The guidelines even include a graph summarizing the dose-response relationship between physical activity and mortality, in which the risk of mortality decreases exponentially with more physical activity, with a steep decrease on the lower end of the physical activity spectrum (from inactive to a little bit of physical activity) (2018 Physical Activity Guidelines for Americans; Figure 6.2, “Relationships of moderate-to-vigorous physical activity to all-cause mortality”). A comparable exponential decrease in mortality risk may be seen for physical fitness as well, but this will require more dose-response research across the entire spectrum of physical fitness.
For individuals with ID, however, the physical activity guidelines define important gaps of knowledge that need to be addressed. One of the major issues is the generalizability of the physical activity research to the population of individuals with ID. Although the guidelines for the first time devote an entire chapter to individuals with ID, this chapter highlights the lack of evidence for the effects of physical activity on comorbidity, physical functioning, and health-related quality of life in this specific population (41). A recent review on the effectiveness of physical activity interventions for individuals with ID showed that programs not only are able to increase physical activity in this population, but also supported the need for methodologically strong research designs in future research (42).
Future research needs to address how to effectively improve fitness in those unfit populations with methodologically strong intervention studies that adapt their exercise programs to the lower starting level with expected incremental steps in workload. Furthermore, there is a need for longitudinal studies that include physical fitness as a baseline measure, but preferably monitoring physical fitness levels at the same time points as the morbidity and mortality outcome measures, to investigate how differences in physical fitness over time impact the risk of morbidity and mortality.
Our novel hypothesis is that small changes in fitness translate to major changes in health among unfit older adults with ID.
Whereas the lowest levels of fitness usually predict poor health outcomes, even small changes in those lower regions seem to be associated with big leaps in health, making physical fitness a key aspect for healthy aging in individuals with ID.
This work was partly funded by the Netherlands Organization for Health Research and Development, grant nos. 57000003 and 314030302, and the Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH K99R01 HD092606-01.
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