Characterizing Differences in Mortality at the Low End of the Fitness Spectrum

Sandra Mandic; Jonathan N. Myers; Ricardo B. Oliveira; Joshua P. Abella; Victor F. Froelicher

Disclosures

Med Sci Sports Exerc. 2009;41(8):1573-1579. 

In This Article

Discussion

The overall findings of the present study are consistent with previous reports demonstrating a nonlinear gradient between fitness and both all-cause and cardiovascular mortalities.[2,8,18,21,24] A unique finding was the suggestion that the striking mortality difference between the least-fit and the next-least-fit quintiles of fitness in healthy individuals are difficult to discern but may be related to a more sedentary lifestyle in the least-fit group rather than to differences in health status at the time of evaluation.

Several factors determine an individual's fitness level, including age, health status, physical activity patterns, behavioral and environmental factors, and genetics.[4] We observed that older age, higher prevalence of diabetes, lower prevalence of dyslipidemia, and more extensive use of ACE inhibitors, calcium channel blockers, and other antihypertensive agents were observed in Q1 versus Q2. No other differences in risk factors or cardiovascular medications were found between the groups. Although statistically significant, the differences in these clinical characteristics were small and are unlikely to explain the nearly twofold increase in mortality risk between Q1 and Q2. Therefore, other factors that are difficult to measure such as genetics[3] or lifestyle behaviors related to higher cardiovascular risk likely play a significant role.

Low levels of self-reported physical activity have been associated with an increased risk of mortality in healthy individuals[11,13,23] with the greatest survival benefits occurring between the least-active and the next-least-active category.[13] Although physical fitness is a stronger predictor of mortality compared with activity pattern,[17,28] physical activity is an important determinant of fitness and is an appropriate therapy for unfit individuals. In the present study, the least-fit quintile had a significantly lower level of recent recreational physical activity compared with the next-least-fit quintile after adjustment for age. Energy expenditure for lifetime (adulthood) recreational physical activity was not significantly different between the groups. Taken together, these findings provide further support for the current guidelines on physical activity and health, which emphasize the importance of maintaining modest regular physical activity throughout life.

Consistent with recent studies,[2,9,10,17,18,21] we observed that exercise capacity was the strongest independent predictor of all-cause and cardiovascular mortalities after adjusting for age, medications, and cardiovascular risk factors. In addition, every 1-MET increase in exercise capacity in the lowest end of the fitness spectrum (Q1 and Q2 combined) was associated with a 21% reduction in risk of all-cause and a 24% reduction in risk of cardiovascular mortality. These risk reductions were almost two times greater than the respective values in the total population, suggesting that increasing exercise capacity is associated with the greatest survival benefit among individuals at the low end of the fitness spectrum. In addition, mortality risk for the second, third and fourth quintiles was approximately two times greater compared with that for the fifth (the most fit) quintile (Figures 1 and 4). Thus, these findings provide further support for the importance of encouraging individuals who are poorly fit to adopt health-promoting behaviors to improve fitness and potentially reduce their mortality risk.

According to the current public health guidelines on physical activity, all adults should attempt to accumulate ≥30 min of moderate-intensity physical activity on ≥5 d·wk-1.[7,20,25] This amount of physical activity is roughly equivalent to an energy expenditure of 1000 kcal·wk-1. In the present study, 64% of the individuals in Q1 did not meet these minimal physical activity recommendations (Figure 4). Therefore, considerable health benefits, prolonged lifespan, and, ultimately, reduced health care costs[26] could be achieved by initiating regular physical activity including as little as 30 min of brisk walking on most days of the week in these comparatively unfit individuals. Although evidence suggests that mortality benefits could be achieved by engaging in less than recommended activity levels,[12] further health outcome benefits occur at exercise levels that exceed current minimum guidelines.[27]

In apparently healthy individuals, it is possible that poor fitness and a sedentary lifestyle may be due to the presence of subclinical illness. Subclinical disease could cause poor performance on an exercise test and also lead to increased mortality rates in individuals presumed to be healthy at baseline.[2] In the present study, individuals with known chronic diseases, including cardiovascular and pulmonary disease, cancer, chronic renal insufficiency, and endocrine and neurologic disorders, were excluded. In addition, the gradient between fitness and mortality was not altered across quintiles after excluding individuals who died within 3 yr after baseline testing. Although an influence of subclinical disease cannot be excluded, these results suggest that the relationship between fitness and mortality was not likely due to subclinical disease.

Although the present findings were not based on the results of a prospective randomized controlled trial, a notable feature was the lack of overt differences in baseline clinical characteristics and a significant difference in the extent of recent recreational physical activity between Q1 and Q2. This observation strengthens the conclusion that the differences in energy expenditure from physical activity in the last year contribute to differences in fitness and possibly the difference in mortality risk between the least-fit and the next-least-fit quintile.

Clinical Implications

These results extend the public health message that emphasizes the importance of increasing regular recreational physical activity, particularly in poorly fit individuals. Achieving public health recommendations for minimal physical activity can have an important impact on improving health outcomes.[7,20,25] Nearly two thirds of the least-fit individuals were performing less than the minimal amount of recommended physical activity, yet this group achieves the greatest health benefits from a small increment in fitness. Thus, it is a paramount that health professionals develop strategies to encourage physical activity among patients and the public. Health professionals should consider a sedentary lifestyle and poor fitness as treatable and major risk factors.

Limitations

We used a single measurement of exercise capacity and physical activity patterns at baseline. It is unknown whether changes in fitness due to changes in activity or other health behaviors influenced the results during the follow-up. Second, physical activity data were collected in more recent years and were available only in a subset of patients. Therefore, the physical activity cohort had a significantly shorter follow-up period and lower mortality rates compared with the remainder of the cohort. Despite small differences in medication use reflecting a difference in medical treatment of cardiovascular disease prevention in the last decade, the physical activity cohort was not healthier or more fit than the rest of the cohort. Moreover, information about education level and socioeconomic status between the groups was not available. Third, there are inherent limitations to self-reporting of physical activity, including potential measurement errors, recall bias, variation in the level of activity during adulthood, and how attentive subjects may have been in their responses. Fourth, aside from likely being more physically active, individuals at the high end of the fitness spectrum (Q4 and Q5) may tend to adopt other kinds of health-promoting behavior (such as healthy diet)[27] that may reduce their risk of cardiovascular disease, other illnesses, and mortality. Fifth, environmental factors and genetics affect an individual's fitness level and physical activity pattern, but their effects are difficult to quantify. Finally, our results are based on US Veterans and may not necessarily apply to the general population.

Summary

The present study examined the association between exercise capacity and mortality in the lowest end of the fitness spectrum in healthy individuals. Our results suggest that reduced recent physical activity rather than differences in clinical characteristics contribute to the striking difference in mortality rates between the least-fit and the next-least-fit quintile of fitness. These findings make a unique contribution to the existing literature by providing information related to factors that may explain the nonlinear gradient between fitness and mortality. Given the considerable survival benefit associated with improving fitness in the least-fit category, increasing fitness by increasing regular physical activity should be a priority in those who are unfit. Health professionals should particularly encourage unfit individuals to engage in regular physical activity.


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