Published OnlineFirst December 17, 2013; DOI: 10.1158/1940-6207.CAPR-13-0400
Cancer
Prevention
Research
Commentary
See related article by Singh et al., p. 12
Physical Activity and Gastric Cancer: So What? An
Epidemiologist's Confession
Tim Byers
Abstract
Epidemiologists, like many scientists, tend to become specialized and focused on a particular disease,
even though behavioral risk factors such as physical activity have effects across many diseases. This
commentary is a personal reflection by an epidemiologist on the shortcomings of this disease-oriented
approach to prevention. Cancer Prev Res; 7(1); 9–11. Ó2013 AACR.
When I first read the meta-analysis published in this
journal by Singh and colleagues concluding that risk for
gastric cancer is 21% lower among those who engage in
higher levels of physical activity, I must admit my reaction
was "so what" (1). My first thought, frankly, was of the
image of the iconic New Yorker cartoon that has frequented
many epidemiology talks—a sketch of a newscaster in front
of two spinning circles of chance. One of the circles lists risk
factors like smoking, obesity, and physical activity, and the
other lists various diseases. The newscaster is reporting
about a new study showing that some random risk factor
has now been linked to some random disease. What could
any link between physical activity and gastric cancer possibly matter, I thought. Surely, we would not be designing
prevention programs specifically promoting physical activity for a 21% reduction in gastric cancer risk, because
physical activity is already known to have a larger and more
certain impact on more common conditions like heart
disease, stroke, and diabetes. That was my initial reaction
to the finding linking physical activity to gastric cancer, but
since that initial cynical impression, I have since been
thinking a lot about physical activity and even more about
my attitude.
I am a cancer epidemiologist. I like to think I have broad
interests and an open mind, but in fact, my professional
behavior tends towards narrowness. The articles I read and
the meetings I attend are mostly about a few specific diseases. Although I study risk factors, I tend to regard risk
factors more as pieces of puzzles to be arranged to form
pictures of specific diseases than as matters of direct interest.
The reality, though, is that quite often the very same risk
factors impacting any single cancer site are also impacting
both other cancers as well as many other chronic diseases.
When it comes to public health impact, common risk
Author's Affiliation: Colorado School of Public Health, Aurora, Colorado
Corresponding Author: Tim Byers, Colorado School of Public Health,
13001 E. 17th Place, B119, Building 500, Room W3122, Aurora, CO 80045.
Phone: 303-724-1283; E-mail: [email protected]
doi: 10.1158/1940-6207.CAPR-13-0400
Ó2013 American Association for Cancer Research.
factors trump common diseases every day. Physical activity
is a good example. As I have reflected more in recent days on
the importance of the association between physical activity
and gastric cancer beyond my initial "so what" reaction, I
have come to see this relationship as important in two
ways–-as a finding of broad epidemiologic importance and
as a finding with more global implications for public health
that we epidemiologists should better appreciate.
I have been concerned for some time about residual
confounding between obesity and physical activity because
until now the only cancer sites with convincing evidence for
physical activity as a protective factor have been those that
also show convincing evidence for obesity as a risk factor
(2–7). Because obesity and physical activity are so closely
linked and because physical activity and life-long obesity
are both imperfectly estimated, there is a real risk for
residual confounding. However, because gastric cancer is
a site that is not particularly associated with obesity (apart
from the minority of cancers occurring at the junction of the
upper gastric cardia and esophagus) and because of the
observation by Singh and colleagues that physical activity
was more strongly associated with distal gastric cancers than
with cancers of the gastric cardia, the association between
physical activity and gastric cancer is unlikely to be confounded by obesity (1). There are other potential confounders to worry about, of course, such as various aspects of diet
or social class-related factors that can contribute to risk for
chronic infection with Helicobacter pylori, but my concern
about residual confounding tied to obesity is much less for
gastric cancer than for obesity-related diseases such as
cancers of the breast, colorectum, endometrium, or kidney
or even for other obesity-related diseases such as heart
disease and stroke (2, 7–11).
With concerns about the confounding minimized, the
broader epidemiologic importance of the physical activity–gastric cancer relationship is that it can support reasoned speculation about causal mechanisms apart from
obesity per se and obesity-connected conditions like insulin resistance. Singh and colleagues nicely discuss the
many mechanistic possibilities-–that physical activity
might act as a general antiinflammatory factor and/or
that it might induce specific beneficial effects on immune
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9
Published OnlineFirst December 17, 2013; DOI: 10.1158/1940-6207.CAPR-13-0400
Byers
function (1). There is now a considerable body of evidence from randomized, controlled trials that physical
activity does indeed induce antiinflammatory effects via
myokines leading to reductions in circulating levels of
C-reactive protein, a marker of general systemic inflammation (12–15). If the physical activity–gastric cancer
relationship is, indeed, explained by general antiinflammatory systemic effects, then that same mechanism could
also account for benefits to not only many other types of
cancer but also to many other noncommunicable diseases
that are driven in part by inflammation.
The World Cancer Research Fund (WCRF) concluded
that there was convincing evidence that physical activity was associated with lower risk for cancers of the
breast, colorectum, and endometrium (2). Subsequently,
updated reviews have concluded that physical activity
is also clearly a protective factor for cancer of the
kidney, and more equivocally, for cancer of the prostate
(7, 16, 17). Both for the gastric cancer relationship
reviewed by Singh and colleagues and for these other
cancer sites, the risk is about 20% lower for those who are
more physically active as compared with those who
are largely sedentary. Interestingly, this is about the same
risk reduction seen in meta-analyses for physical activity
as related to cardiovascular diseases and stroke (8–11).
Although a 20% risk reduction for gastric cancer will not
likely justify specific gastric cancer prevention education
programs based on physical activity promotion, the
potential for benefits from physical activity of this same
magnitude across many other cancers and across many
other chronic diseases as well as on general longevity
should not be taken lightly (18).
Epidemiologists like me who are disease-focused in our
professional orientation need to reassess how we regard
common risk factors such as physical activity. Physical
activity is clearly a major factor accounting for many deaths
and years of disability from a large set of noncommunicable
diseases worldwide. In fact, the Global Burden of Diseases
project recently estimated that more disability-adjusted life
years are lost worldwide from physical inactivity than from
obesity (19). Cancer is now the leading cause of death in the
world. There will be about 8 million deaths from cancer this
year, about 7 million from cardiovascular disease, about
6 million from strokes, and only about 5 million from the
sum of HIV, tuberculosis, malaria, and childhood diarrhea
(20). The current emphasis on infectious diseases in the
United Nations Millennium goals is now being rethought
for the future as noncommunicable diseases have emerged
as a much bigger threat to global health (21). If we are to
achieve the ambitious goal of reducing mortality from
noncommunicable diseases by 25% by the year 2025,
people like me who are focused more on specific diseases
will need to begin thinking about chronic diseases as a
general class of disorders sharing a common set of behavioral etiologies (22).
So what? Learning that those who are more physically
active are at lower risk for even gastric cancer has served to
remind me that public health is more likely to be
improved if we epidemiologists pay more attention to
risk factors and less to diseases in the way we conceptualize, analyze, and communicate the actual threats to
health. That’s what.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Received November 20, 2013; accepted November 22, 2013;
published OnlineFirst December 17, 2013.
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Published OnlineFirst December 17, 2013; DOI: 10.1158/1940-6207.CAPR-13-0400
Physical Activity and Gastric Cancer: So What? An
Epidemiologist's Confession
Tim Byers
Cancer Prev Res 2014;7:9-11. Published OnlineFirst December 17, 2013.
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