February 28 - 29, 2008 Advisory Committee Meeting Minutes
Subcommittee Report: All-Cause Mortality
I-Min Lee, M.D., Sc.D. presented the subcommittee report on all-cause mortality. The subcommittee reviewed 5 research questions:
Is there an association between physical activity and mortality; what is the magnitude of the association?
What is the minimum amount of physical activity associated with significantly lower mortality rates?
Is there a dose-response relation between physical activity and mortality?
What is the shape of the dose-response between physical activity and mortality?
Is the physical activity-mortality relation independent of adiposity?
The subcommittee reviewed 83 potential studies from the CDC database and ultimately included 73 studies in their research. All were observational studies and almost all were prospective cohort studies. The studies were conducted in North America, Europe, the Middle East, Asia and Australia. The samples consisted of 254,514 men and 576,574 women of which there were 113,358 deaths. 60% of the subjects were 40 or older and 20% 65 or older. Most were Caucasian and most were identified as healthy.
Dr. Lee summarized the subcommittee’s conclusions for question 1. There is strong evidence of an inverse association between physical activity and all-cause mortality. Active people have approximately a 30% lower mortality risk compared with inactive people. The association is seen in men, women, young people and old people in populations in both the U.S. and internationally among different races as well as in people with high risk factors. One study of disabled people also appeared to hold this relation.
For question 2, there is a challenge in quantifying the minimum amount of physical activity associated with lower mortality rates are the different assessments and categorizations of physical activity. Most studies measured leisure-time physical activity but it would be helpful to also measure the impact of routine physical activity. Ultimately, there is a challenge to synthesize the data across all studies and express the findings in a fashion that can be readily translated for public health purposes. The data is clear however I showing that the equivalent of at least 2 to 2.5 hours per week of moderate-intensity physical activity is associated with significantly lower risk of mortality. It is important to note that this level does not represent an "all-or-none" threshold.
Regarding question 3 on the dose-response relationship between physical activity and mortality Dr. Lee summarized as follows. Most of the data in the studies measured total volume of energy expended. The data consistently demonstrates an inverse dose-response relationship. While the data shows that at least 2 to 2.5 hours of moderate-intensity activity per week is needed to significantly reduce all-cause mortality rates, this amount does not represent a threshold level. Rather, the dose-response relation supports a "some is good; more is better" message. There is limited data showing that vigorous-intensity activity is associated with additional risk reduction. There is no data on dose-response relations for duration and frequency of physical activity, independent of their contributions to total volume of energy expended.
Summarizing the conclusions for question 4 the dose-response curve appears curvilinear. On average, across studies, compared to less than half an hour of moderate-to-vigorous activity per week, 1.5 hours per week is associated with a 20% reduction in risk. Additional amounts of activity is associated with additional risk reductions but at smaller magnitudes.
Finally, Dr. Lee, addressed the final research question looking into the potential relation of adiposity to physical activity and mortality. The inverse relation between physical activity and mortality appears independent of adiposity. Additionally, a few studies have stratified findings by Body Mass Index (BMI) and these studies show the inverse association between physical activity and mortality is similar across strata of BMI.
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