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Recently, an editorial in the British Medical Journal explored
the relationship physical activity and cancer. Epidemiologists David
Batty from London and Inger Thune from Norway reviewed some of the
evidence regarding the wide range of potential cancer-protective
effects of exercise. Below are some excerpts and important points
from the editorial.
To start, they note that exercise has significant effects on several
functions of the human body that may influence cancer risk. These
effects include changes in the following:
| cardiovascular capacity |
energy balance |
| pulmonary capacity |
immune function |
| bowel motility |
antioxidant defence |
| hormone levels |
DNA repair
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They note that in the past decade alone, over 200 population
based studies have linked physical activities to cancer risk.
The most researched cancers are those of the:
Bowel Cancer
Cancer of the large bowel is the most commonly investigated cancer
in relation to physical activity (1-4) with physically active men
and women experiencing around half the risk of their sedentary counterparts.
Plausible mechanisms of protection include the beneficial effects
of exercise on levels of the following substances, which influence
the growth and proliferation of cells in the colon:
In addition, exercise reduces bowel transit time and thereby
the duration of contact between fecal carcinogens and the mucosal
lining of the colon.
Breast and Endometrial Cancer
Concerning these strongly hormone-dependent cancers the editorial
states:
Endogenous sex hormones are strongly implicated in the development
of breast and endometrial cancer. Physical activity may modulate
the production, metabolism, and excretion of these hormones, so
an association with these cancers is biologically possible.
Physical activity may also reduce the risk of cancer through
its normalising effect on body weight and composition. Evidence
from population based studies suggests that occupational, leisure,
and household activities are associated with about a 30% reduction
in breast cancer rates (5) ... Those studies that have explored
the link between physical exertion and the risk of endometrial
cancer suggest a negative association (1,3).
Prostate Cancer
The editorial notes that since athletes show lower levels of circulating
testosterone than non-athletes, and testosterone influences the
development of prostate cancer, this has led to the hypothesis that
physical activity may protect against this cancer. Most, but not
all, studies suggest a protective effect (3).
Lung Cancer
The editorial notes that:
Although physical activity improves pulmonary ventilation
and perfusion, which may reduce both the concentration of carcinogenic
agents in the airways and the duration of agent-airway interaction,
the association of activity with lung cancer has received relatively
little attention. Findings from most, but not all, studies suggest
a negative relation ... (1,3)
Exercise as a Cancer Treatment
The editorial notes that, in addition to the apparent role of exercise
in the prevention of some cancers, there is growing interest in
and evidence of its use in the treatment and rehabilitation of patients
with cancer (12, 13). Physical activity may reduce the likelihood
of recurrence and enhance survival through its capacity for doing
the following:
- Improving bodily
movement
- Enhancing immune function
Conclusion
The editorial notes that since exercise seems to have protective
effects against some types of cancers and does not increase the
risk of any cancer, it should be more actively encouraged. They
state that "in light of the decreasing population prevalence
of total physical activity, doctors should advocate moderate endurance-type
activity, such as walking and cycling. As well as reducing the risk
of chronic diseases such as coronary heart disease and non-insulin
dependent diabetes, such physical activity
does seem to protect against some cancers."
British
Medical Journal December 9, 2000;321:1424-1425
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