Time and again, research correctly finds that people in developed countries don’t exercise enough. But what information are policy efforts to promote physical exercise based on? This column presents new – and more sophisticated – research that disentangles recreational exercise and cumulative exercise – such as physical activity in (or on the way to) the workplace. An individual’s total physical activity, and not just their recreational exercise, is the salient input into health production because exercise is typically a very small portion of an individual’s total daily physical activity. The sole focus on recreational exercise may well be misleading.
Physical activity is a problem for many high-income nations:
- Half of all US adults do not engage in sufficient leisure-time physical activity outside of their work (based on data from the 2011 Behavioural Risk Factor Surveillance System-BRFSS) and about 25% of US citizens are not physically active at all;
This high prevalence of physical inactivity is not just confined to the US:
- A survey of 15 countries in the EU found that about two-thirds of the adult population did not participate in recommended levels of physical activity (World Health Organization 2006).
In reaction, the public health sector has mobilised resources to propose policies to promote physical activity. The US Surgeon General, for instance, released its first report on physical activity in 1996 (US Department of Health and Human Services 1996), noting recommendations for people of all ages to include at least 30 minutes of moderately intensive physical activity on most, if not all, days of the week. Similar guidelines were also adopted by the World Health Organization in 2002. The US Surgeon General’s Report further acknowledged that, for most individuals, activity levels which are of more vigorous intensity or of longer duration would impart even greater health benefits. Healthy People 2020, which provides science-based national objectives for improving population health in the US, includes key objectives relating to reducing the proportion of adults with no leisure-time physical activity and increasing the proportion of adults who meet current federal physical activity guidelines by 10%.
In the EU, 41 guidelines have been put forward to ensure consistency and integration of individual member policies. These suggest priorities for policies and call for cooperation among the relevant public and private actors to promote increased physical activity.
The research basis for public policy
All of these calls to action and guidelines are predicated on the potential health-promoting effects of physical activity. The US Surgeon General’s report, for instance, had concluded that physical activity is protective of many of the risk factors for heart disease, such as obesity, high blood pressure, and a rapid pulse, as well as reducing the incidence of associated morbidities such as Type 2 diabetes, angina, heart attack, and stroke. Warburton, Nicol, and Bredin (2006) also provide an excellent review of some of the literature linking physical activity with morbidity and mortality.
However, the studies upon which these conclusions and reviews rest are limited:
- While the effects found in randomised controlled studies are plausibly causal, they may lack external validity since such studies are often based on small, convenience samples of individuals whose behaviour may not necessarily generalise to the overall population (Arroll and Beaglehole 1992, Kelley and McClellan 1994).
- Studies that have utilised population-based samples, while are able to capture generalisable and representative effects, often lack internal validity.
They generally do not control for the many confounding factors that affect both physical activity as well as health, and it is not clear whether results can be considered causal (Sherman et al. 1994, Hu et al. 2007).
For instance, a person may have inherited a tendency to worry about his health or the individual may be relatively more forward-looking or risk-averse, which would cause him to watch his diet as well as to exercise, or persons who have poorer health for other reasons may find it more difficult to be active.
If these factors remain unobserved, then the estimated association between physical activity and health may be overstated. In addition, the population-based samples used in these studies mainly contain self-reported rather than objectively measured characteristics, such as height, weight, and the presence of high blood pressure, which may be subject to systematic misreporting or under-reporting and thus lead to biased estimates. Oftentimes, these studies also conflate or fail to distinguish between recreational exercise and other physical activity; presumably, it is total physical activity, not just exercise per se, that is ‘health-promoting’.
In recent research, we address some of these limitations to assess credible causal effects of physical activity on the risk factors for heart disease (Colman and Dave 2013). The analyses are based on the first National Health and Nutrition Examination Survey, conducted between 1971-74, and its follow-up study, the NHANES I Epidemiologic Follow-up Study, conducted between 1982-84. The nature of the National Health and Nutrition Examination Survey enables us to make a number of contributions:
- First, unlike virtually all of the prior studies, we estimate the effect of both recreational exercise as well as of other physical activity.
The salience of disentangling the effects of exercise from other physical activity stems from the likelihood that both types of activity may have heterogeneous effects on health. Given that recreational exercise typically constitutes only about 3-4% of total daily physical exertion for the average individual (Colman and Dave 2011), an exclusive focus on exercise can also lead to biased or incomplete conclusions.
- Second, the measure of physical activity in the National Health and Nutrition Examination Survey also includes activity at work, which is often excluded in prior studies.
Work-related physical activity constitutes the largest component of total physical exertion for the average individual: approximately 32% for working-aged males, and 20% for working-aged females (American Time Use Surveys 2010). Hence, it is important to consider the totality of an individual’s physical activity including their work-related activity, while separating the effects of exercise from that of other activities.
- Third, the availability of repeated observations on the same persons makes it possible to control for many of the unobserved confounding factors that affect both physical activity and health.
- Fourth, the objectivity of the measurements minimises measurement error that may have plagued prior research.
Person-specific fixed effects specifications are estimated that account for a host of unobservable confounding factors (for instance, stable factors such as time- and risk-preference, family background, parental investments, and any other relevant baseline determinants of health prior to the sample period), and compared with specifications with lagged outcome measures. This allows us to bound plausibly causal effects under reasonable assumptions.
Four key patterns of results are evident:
- The lagged effect of physical activity is almost always larger than the contemporaneous effect.
This suggests that current risk factors, not only obesity but also high blood pressure and heart rate, take years to develop, which underscores the importance of consistent physical activity to ward off heart disease.
- In general physical activity reduces risk factors for heart disease even after controlling for all unobservable confounding influences that are constant over time, such as genetic factors and family background.
- Not only recreational but other physical activity, which includes work-related activity, appears to protect against heart disease.
- There is evidence of a dose-response relationship such that higher levels of recreational exercise and other physical activity have a greater protective effect.
The National Health and Nutrition Examination Survey I and its follow-up study predate the sharp rise in obesity in the US (and in most other countries as well) and thus bypass the difficulty of disentangling the causal effect of physical activity from the strong nationwide trends in obesity that began in the early 1980s, which in turn has probably affected physical activity. Hence, these analyses establish baseline effects of physical activity on the risk factors for heart disease over a time period when obesity was stable, which can then be used to inform whether and to what extent physical activity can account for subsequent observed trends in these risk factors, conclusions we draw in our study.
Studies have documented a decrease in overall physical activity in the US, which may have played a role in the increase in certain morbidities. Brownson, Boehmer, and Luke (2005), for instance, report longer-term trends (up to 50 years for certain measures) in physical activity in the US. They conclude that there is:
- An overall trend of declining total physical activity due to a decrease in work-related physical activity.
- Declining transportation activity.
- Declining home-based activity.
- Increased sedentary activity.
Church et al. (2011) document a decrease in daily occupation-related energy expenditure by more than 100 calories between 1960-62 and 2003-06, based on the National Health and Nutrition Examination Survey, confirming a declining trend in work-related physical activity levels. Our estimates suggest that such reduced levels of physical activity will lead to adverse consequences on weight-based health and heart-related morbidity by increasing the incidence of obesity, high blood pressure, and the resting heart rate, ceteris paribus. Specifically, the estimated contemporaneous and strong cumulative effects from our study indicate that the observed declines in high levels of recreational exercise and other physical activity, observed over the sample period, can account for between 12-30% of the increase in obesity, hypertension, diabetes, and heart disease, ceteris paribus.
Our research (Colman and Dave 2011, 2013) also underscores that it is an individual’s total physical activity, not just their recreational exercise, that is the salient input into health production, particularly since exercise is typically a very small portion of an individual’s total daily physical activity. This distinction is particularly relevant when factors potentially cause different activity components to move in opposite directions, and thus the sole focus on recreational exercise may be misleading.
For instance, there is considerable substitution between work-related physical activity, exercise, and other forms of recreational activity. Differences in exercise across gender and racial/ethnic groups are somewhat counteracted by differences in other forms of work-related and non-work-related activities to some extent. These differences in total levels of physical activity can therefore play a role in explaining some of the health disparities across population groups (Saffer et al. 2011).
Furthermore, during an economic downturn, job loss induces individuals to increase their recreational exercise, presumably due to relaxed time constraints, though their total physical activity level declines due to a large decrease in work-related physical activity. This decline in overall physical exertion may have adverse effects on long-term health. Due to the concentration of low-educated workers in boom-and-bust industries such as manufacturing and construction, the drop in total physical activity during recessions is especially problematic for vulnerable populations and may play a role in exacerbating the socioeconomic status (SES)-health gradient during recessions.
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