Substantial evidence shows that physical activity can protect against the development of chronic disease and increase longevity.1^–10 Therefore, promoting physical activity may be a good strategy for enhancing health for obese individuals. Considering the combined and independent effects of physical activity and obesity on various health outcomes is important to help develop health recommendations for obese individuals. However, the interrelationship of physical activity and obesity on health outcomes is complicated and sometimes controversial.

Early studies on cardiorespiratory fitness and obesity indicated that moderate to high levels of cardiorespiratory fitness substantially attenuate, or perhaps even eliminate, the mortality risk of obesity.2 11 Stimulated by these early mortality reports, recent studies have evaluated the combined effects of physical activity or cardiorespiratory fitness and obesity for various disease outcomes. The results are somewhat inconsistent for different disease endpoints, study populations and exposure measurements.

The effect of physical activity on the health hazards of obesity has also been examined from different analytical perspectives. Early studies reported an independent protective effect of physical activity on morbidity and mortality after adjustment for obesity as a confounder or obesity-stratified subgroup analysis. However, recent studies have used physical activity or cardiorespiratory fitness and obesity joint stratification analysis to examine more completely the interrelationship of physical activity and obesity on various health outcomes. In this review, we focus only on the joint stratification analysis of physical activity or cardiorespiratory fitness and obesity on chronic disease and mortality.

JOINT ASSOCIATIONS OF PHYSICAL ACTIVITY OR CARDIORESPIRATORY FITNESS AND OBESITY WITH HEALTH OUTCOMES

Several investigators evaluated the relative influence and combined effects of physical activity or cardiorespiratory fitness and obesity on diverse health outcomes. Cardiorespiratory fitness is an objective and reproducible means of assessing physical activity, and may be a better method for evaluating the association of physical activity and obesity with health outcomes.12 We briefly reviewed 20 recent observational studies using joint analysis of physical activity or cardiorespiratory fitness and obesity on various health outcomes (10 with mortality and 10 with cardiovascular disease (CVD), type 2 diabetes, or hypertension).

Mortality

For all-cause, CVD, or cancer mortality, we reviewed three studies that used self-reported physical activity, and seven studies that used cardiorespiratory fitness as an exposure. In the three physical activity studies, physical activity attenuated, but did not eliminate, the negative effect of obesity on death rates.1 5 10 Two Aerobics Center Longitudinal Study (ACLS) reports used measured cardiorespiratory fitness and % body fat, and reported that moderate to high fitness eliminates the elevated risk of all-cause, CVD, and cancer mortality associated with obesity.11 13 Results were similar in a group of older adults14 and for men with diabetes,15 in which obese individuals who were fit had no increased risk of CVD or all-cause mortality. In all of these ACLS reports, obese and fit individuals had a lower risk of mortality than normal weight but unfit individuals. In the Lipid Research Clinics study, both cardiorespiratory fitness and obesity were significant mortality predictors in both women and men, but being fit did not completely reverse the elevated mortality risk associated with obesity.2 16 However, adjustment for cardiorespiratory fitness attenuated the association of obesity with mortality more than did the adjustment for body mass index (BMI) on the association of cardiorespiratory fitness with mortality. Another Lipid Research Clinics study showed that fitness eliminated the harmful effect of obesity in Russian men, but both fitness and fatness were associated with mortality in American men, even after each was adjusted for the other.17

Chronic disease morbidity

We reviewed three studies on the joint association of self-reported physical activity and obesity with CVD morbidity. All found that both physical activity and obesity are independent contributors to CVD, but the risk of obesity was not eliminated by physical activity, although physical activity attenuated the increased risk of obesity in relatively healthy populations.4 7 18 However, the Women’s Ischemia Syndrome Evaluation study indicated that physical activity was stronger than obesity as a predictor of incident CVD in women with CVD symptoms at baseline.19 Compared with the other three CVD studies, the different results in Women’s Ischemia Syndrome Evaluation studies could be due to CVD symptoms at baseline or to a more accurate physical activity assessment, which was previously validated against maximal oxygen uptake.

We reviewed four prospective studies of type 2 diabetes. Three used self-reported physical activity and one from the ACLS used measured cardiorespiratory fitness. The three physical activity studies showed similar results in that the magnitude of the association of obesity with type 2 diabetes was greater than that observed for physical activity in combined analyses.9 20 21 The ACLS, which used measured cardiorespiratory fitness and BMI, showed that both exposures made similar contributions to the risk of incident type 2 diabetes.3

Few studies have examined the joint relationship of physical activity or cardiorespiratory fitness and obesity with incident hypertension. We reviewed two observational studies, one prospective study with self-reported physical activity and one case–control study with measured cardiorespiratory fitness. In both studies, physical activity or cardiorespiratory fitness showed a consistently protective effect regardless of obesity levels, and the adverse contribution of BMI was attenuated substantially when cardiorespiratory fitness was controlled in the multivariate models.6 22

Summary

Based on reviews of the above papers, we conclude that physical activity or cardiorespiratory fitness ameliorates the health hazard of obesity in most studies, regardless of obesity measures, gender, or baseline health status after adjustment for possible confounders. In addition, regardless of exposure measurements, study populations, or study outcomes, the highest morbidity and mortality risk was observed in individuals who are both obese and inactive or unfit.

Findings from ACLS reports, which used objectively measured cardiorespiratory fitness, indicate that cardiorespiratory fitness appears to eliminate the increased mortality risk associated with obesity. However, some studies suggest that obesity is more strongly associated with the risk of type 2 diabetes than is physical activity, but cardiorespiratory fitness appears to be as important as obesity in the development of type 2 diabetes.3

Different study populations, outcomes and physical activity measures lead to the variation of results on the interrelationship of physical activity and obesity in joint analyses. For example, the protective effect of cardiorespiratory fitness on the increased risk of mortality by obesity was greater in Russian men than in American men.17 The magnitude of association of BMI with incident type 2 diabetes was likely to be greater than physical activity,9 20 but when incident coronary heart disease was the outcome, both physical activity and obesity appeared equally important.4 7 Also, in the study with cardiorespiratory fitness instead of self-reported physical activity as an exposure, obesity was unlikely to be more strongly associated with type 2 diabetes than cardiorespiratory fitness.3

PHYSICAL ACTIVITY, CARDIORESPIRATORY FITNESS AND OBESITY ASSESSMENT ISSUES

People may be likely to overestimate their physical activity levels and this may be even more true for overweight individuals.23 This could lead to results suggesting that obesity is more important than physical activity as a cause of chronic disease due to the misclassification of physical activity. Using the objective measure of cardiorespiratory fitness probably leads to less misclassification and results in finding stronger associations with health outcomes. We found that measured cardiorespiratory fitness predicted mortality better than self-reported physical activity.24

Figure 1 shows interrelationships among physical activity, cardiorespiratory fitness, obesity, clinical factors and health outcomes. In this diagram we postulate that physical activity is associated with both cardiorespiratory fitness and obesity, and that inactivity may be the starting point for a chain of events that lead to morbidity and mortality.

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Figure 1 Associations among physical activity, cardiorespiratory fitness (CRF) and obesity on health outcomes.

CONCLUSION

Regardless of the differences in study populations, accuracy of physical activity or obesity measurements and the specific health outcomes, some findings were consistent across studies. First, physical activity or cardiorespiratory fitness appears to ameliorate the health hazard of obesity, although it is still unclear if physical activity or cardiorespiratory fitness can eliminate the risk of obesity for a variety of health outcomes. Second, the highest risk of morbidity or mortality was observed in individuals who were both obese and inactive or unfit. Finally, the discussion about whether inactivity or obesity is more important is somewhat academic. The recommendation for both conditions is to increase physical activity.25 Unfortunately, the rate of physician counselling about healthful lifestyles is low,26 and it is critical for physicians to encourage regular physical activity and healthful diets to assist with weight control and to enhance health. It is important for us to develop effective and affordable systems and methods to assist physicians and other clinicians in helping their patients to follow healthful lifestyles.

An enjoyable individual sport or physical activity in daily life may help patients maintain an active lifestyle for weight management and disease prevention. Riding a bicycle to work or school may be more pleasant than bicycle exercise in a gym, at least for some people. Parking a car further away in the parking lot, using the restroom one floor up or down in the workplace or walking a dog can be recommended as convenient ways to increase physical activity in everyday life. We are confident that if physicians help patients meet the consensus public health recommendation of 150 minutes of moderate intensity activity each week,27 28 which can be obtained by three 10-minute walks per day on at least 5 days of the week, we will dramatically improve individuals’ and the public’s health. It is critical that physicians “put physical activity on each patient’s agenda” and strongly encourage them to seek ways to reduce sedentary behaviour and become more active.