Fitness and death

Physical fitness refers to a physiologic state of well-being that allows one to meet the demands of daily living or that provides the basis for sport performance, or both. Health-related physical fitness involves the components of physical fitness related to health status, including cardiovascular fitness, musculoskeletal fitness, body composition and metabolism. In large epidemiologic investigations, physical activity and physical fitness are often used interchangeably, with fitness commonly being treated as a more accurate (albeit indirect) measure of physical activity than self-report.100

Physical fitness appears to be similar to physical activity in its relation to morbidity and mortality2,34 but is more strongly predictive of health outcomes than physical activity.6,29,100 Most analyses have shown a reduction of at least 50% in mortality among highly fit people compared with low-fit people (see Appendix 2, available at www.cmaj.ca/cgi/content/full/174/6/801/DC1).29

Nonetheless, both physical activity and fitness are strong predictors of risk of death.29 To obtain accurate estimates of physical activity, many fitness consultants rely on primary (criterion and “gold”) standards for the measurement of energy expenditure, such as direct observation of movement or, in the laboratory, the doubly labelled water technique or indirect calorimetry.101 On a practical basis, however, measures of physical activity and energy expenditure are obtained by using heart rate monitors and motion sensors (pedometers and accelerometers). These devices will be briefly reviewed in the companion article in the Mar. 28 issue.

The assessment of physical fitness is often not feasible or practical in large population-based investigations. Fortunately such studies have consistently shown an inverse gradient of health risk across self-reported physical activity groups. From a public health perspective, Blair and colleagues6 have argued that it is preferable to encourage people to become more physically active rather than to become physically fit, since, as they stated, sedentary people will likely achieve the latter if they do the former.

How does physical activity and fitness lead to improved health outcomes?

Several biological mechanisms may be responsible for the reduction in the risk of chronic disease and premature death associated with routine physical activity. For instance, routine physical activity has been shown to improve body composition (e.g., through reduced abdominal adiposity and improved weight control),9,10,113–116 enhance lipid lipoprotein profiles (e.g., through reduced triglyceride levels, increased high-density lipoprotein [HDL] cholesterol levels and decreased low-density lipoprotein [LDL]-to-HDL ratios),9,10,117–123 improve glucose homeostasis and insulin sensitivity,3,9,10,124–126 reduce blood pressure,127–130 improve autonomic tone,131,132 reduce systemic inflammation;133 decrease blood coagulation,134,135 improve coronary blood flow,136 augment cardiac function137,138 and enhance endothelial function.139–142 Chronic inflammation, as indicated by elevated circulating levels of inflammatory mediators such as C-reactive protein, has been shown to be strongly associated with most of the chronic diseases whose prevention has benefited from exercise. Recent RCTs have shown that exercise training may cause marked reductions in C-reactive protein levels.143 Each of these factors may explain directly or indirectly the reduced incidence of chronic disease and premature death among people who engage in routine physical activity.

Routine physical activity is also associated with improved psychological well-being (e.g., through reduced stress, anxiety and depression9,10,144). Psychological well-being is particularly important for the prevention and management of cardiovascular disease, but it also has important implications for the prevention and management of other chronic diseases such as diabetes, osteoporosis, hypertension, obesity, cancer and depression.

Changes in endothelial function may be a particularly important adaptation to routine physical activity. Endothelial dysfunction has been observed with aging, smoking and multiple chronic disease states, including coronary artery disease, congestive heart failure, stroke, type 2 diabetes, hypertension, hypercholesterolemia and obesity.116 Regular aerobic activity has been found to improve vascular function in adults independent of changes in other risk factors142,145 and has been said to result in a shear-stress–mediated improvement in endothelial function,116 which confers a health benefit to a number of disease states.146

Although most research into the mechanisms of how physical activity and fitness improve health outcomes has dealt with the relation between cardiovascular disease and physical activity, researchers have also evaluated the primary mechanisms responsible for decreases in the risk and severity of individual disease states. In fact, despite the adaptations that are of global benefit for multiple disease states, physical activity also results in specific adaptations that affect individual disease states. For instance, in type 2 diabetes, adaptations that affect glucose homeostasis are of great importance. As reviewed by Ivy,147 a series of changes (independent of changes in body mass64) occur as a result of regular physical activity, including increased glycogen synthase148 and hexokinase activity,149 increased GLUT-4 protein and mRNA expression,148,150 and improved muscle capillary density (resulting in improved glucose delivery to the muscle).149 A series of mechanisms may explain the 46% reduction in cancer rates observed with regular physical activity,66 including reductions in fat stores,66 increased energy expenditure offsetting a high-fat diet,66 activity-related changes in sex hormone levels, immune function, insulin and insulin-like growth factors, free-radical generation,151 and direct effects on the tumour.151

The majority of proposed mechanisms have been discussed in the context of chronic adaptations brought about by routine physical activity. However, researchers have recently examined the importance of acute changes in risk factors for chronic disease.152 An excellent review of the topic by Thompson and colleagues revealed that acute, dynamic exercise may result in transient changes in the form of reductions in triglyceride levels, increases in HDL cholesterol level, decreases in blood pressure (for 12–16 hours), reductions in insulin resistance and improvements in glucose control.152 These acute changes indicate the important role individual exercise sessions have on health status.

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Summary

There is incontrovertible evidence that regular physical activity contributes to the primary and secondary prevention of several chronic diseases and is associated with a reduced risk of premature death. There appears to be a graded linear relation between the volume of physical activity and health status, such that the most physically active people are at the lowest risk. However, the greatest improvements in health status are seen when people who are least fit become physically active. The current activity guidelines promoted by Health Canada appear to be sufficient to reduce health risk. People who engage in exercise at levels above those recommended in the guidelines are likely to gain further health benefits. Health promotion programs should target people of all ages, since the risk of chronic disease starts in childhood and increases with age.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1402378/

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