December 6 — 7, 2007 Advisory Committee Meeting
All-Cause Mortality Subcommittee Report
I-Min Lee, M.D., Sc.D. presented the report on All-Cause
Mortality. Dr. Lee acknowledged the participation of William Haskell, Ph.D. from
the Committee and Bill Kohl, Ph.D. from CDC on the All-Cause Mortality
Subcommittee. Additionally, Dr. Steven Blair served as a consultant. The
subcommittee formulated the following five questions to organize their work: 1)
What data do we have on the topic? 2) Can we confirm there is an association
between physical activity and all-cause mortality and what is the magnitude of
the association? 3) What is the minimum amount of physical activity associated
with lower rates of mortalities in the studies? 4) What is the shape of the
dose-response curve for all-cause mortality? 5) How is the relation between
physical activity and all-cause mortality influenced by adiposity levels?
Question 1. What data do we have on the topic?
In extracting data from the CDC database the group attempted to be as inclusive
as possible. The subcommittee selected all age groups, all the study designs
that were there, all activity types and obviously the health outcome all-cause
mortality. This search resulted in 83 studies of which 10 were excluded. The
studies were excluded based on the following rationale:
Three excluded because of a relationship to adverse events
which will be covered in another chapter by the Committee.
Three physical fitness studies were excluded because they did
not provide direct information on the amounts and kinds of activity, etc,
which were relevant to our report.
Two cancer studies were excluded as that subject is covered by
One study on the relationship between BMI and mortality
stratified by physical activity levels was excluded as there was no data
directly related to physical activity.
The group attempted to be as evidence-based as possible and
looked for particular information related to volume, intensity, frequency and
duration. Of particular interest were special populations such as persons with
disabilities. Almost all of the studies that were reviewed were perspective
cohort studies. The net result was a large database of more than 300,000
observations in men and nearly 700,000 observations in women with more than
140,000 deaths. The disparity in observations between men and women was
primarily due to several large cohort studies that included many women. The
majority of the participants in the studies were 40 years of age or older,
Caucasian and for the most part healthy.
Question 2. Can we confirm there is an association between
physical activity and all-cause mortality and what is the magnitude of the
The studies consistently pointed out lower rates of mortality. There was also a
significant inverse association with at least with one sex or with one domain of
physical activity in 67 of the 73 studies. As all of the studies were
observational studies the subcommittee looked for alternate explanations for the
associations. Sources of bias such as sick people participating in the studies,
follow-up rates and confounding were interpreted to not have a significant
impact, if any, on the studies.
Question 3. What is the minimum amount of physical activity
associated with lower rates of mortalities in the studies?
There are complications with interpreting data from the studies to find out
minimum amounts of physical activity associated with lower rates of mortality
because the studies assess physical activity in different ways. For example, if
a questionnaire is used the questions asked may differ. Even if the same
questions were asked there may still be differences across studies because of
the differences in types of activities such as leisure, occupational, household
and commuting activity.
Additionally, analyzing data in different ways leads to problems
interpreting the science. Typically, studies will have different groupings from
low to high with different categories such as inactive, moderately active,
highly active, etc. However, one may not know what each of these categories
A common way to organize categories are by energy expended,
duration of physical activity or frequency of physical activity. Within the
studies 12 were done by energy, 9 by duration and 4 by frequency. Studies
analyzed by energy indicated that about 1,000 kilocalories a week primarily in
leisure time activity was associated with significantly lower levels of
mortality. 1,000 kilocalories a week roughly corresponds to 2.5 hours of
moderate to vigorous intensity physical activity, which is consistent with
current recommendations. Studies classified by duration indicated about 2 hours
a week as the minimum amount of activity associated with lower mortality.
Studies by frequency, some of which may not have duration built in, showed that
activities as infrequently as once a month to one to three times a week of
duration, perhaps 30 minutes or longer, was associated with lower risk.
Walking was cited as a good example of physical activity in the
1995 CDC/ACSM recommendations on moderate physical activity. From the studies in
the CDC database it appears 2 hours a week corresponded with lower mortality. In
terms of distance, 1 – 2 miles a day was associated with lower risk. Pace of
walking was very consistently associated with lower risk as well (i.e., the
faster your pace the lower your risk). Three studies looked at walking/cycling
combined. Although some only looked at active commuting and/or leisure activity
combined, these studies generally showed 15 – 20 minutes of walking/cycling per
day showed benefit.
Question 4. What is the shape of the dose-response curve for
While the minimum amounts of physical activity found to be a benefit in the
studies correspond to current health recommendations the relationship is most
likely not an all or nothing proposition. Rather, the benefit is more of a
gradient response. As such the group wanted to look at the shape of the
dose-response curve. As before it can be problematic analyzing different studies
due to the different ways physical activity can be classified so each category
was reviewed. In plotting the studies a curvilinear relationship was found.
While there was a lot of benefit at early levels the benefit tapers off and much
more activity is required to gain similar benefits.
The group also questioned the impact of intensity of physical
activity, specifically, do all activities count equally? When looking at the
results of the studies on the surface the higher the intensity level of the
activity the more benefit one receives. In most cases though volume of total
energy expended is not accounted for. For example, if 2 individuals engage in
the same intensity activity but one has a larger body the person with the larger
body expends more energy.
Question 5. How is the relation between physical activity and
all-cause mortality influenced by adiposity levels?
The last question the subcommittee looked at is the physical activity
mortality association and whether body weight was an influence or does the
association exist merely because people who exercise are leaner? Sixty percent
of the studies adjusted their findings for body mass index or some measured
adiposity and all consistently observed significant inverse associations. A
handful of studies looked at individuals of different body mass indices and
their results suggested that no matter what your level of adiposity, if you are
physically active, at about the levels recommended, you will have significantly
lower risks of mortality.