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Chapter 8
Cancer
Chapter overview
• Introduction
• Carcinogenesis
• Physical activity and colorectal cancer
• Physical activity and breast cancer
• Physical activity and other cancers
• Mechanisms
• Population-attributable risk
• Physical activity in cancer survivors
• Summary
Definition and risk
Cancer is not one disease, but a set of diseases
characterized by unregulated cell growth leading
to invasion of surrounding tissues and spread to
other parts of the body.
In developed countries, the lifetime risk of
developing cancer is around 1 in 3.
Role of environmental factors in
cancer causation: migrant studies
Normal cells
• Reproduce themselves exactly;
• stop reproducing at the right time;
• stick together in the right place (cell adhesion);
• self-destruct if they are damaged (apoptosis);
• become specialized or mature.
Cancer cells
• Carry on reproducing;
• do not obey signals from neighbouring cells;
• can become detached from the primary tumour and
travel to other parts of the body;
• do not become specialized, but stay immature;
• do not die if they move to another part of the body.
Carcinogenesis
• Damage to a cell’s DNA is called mutation.
• Cancer derives from an accumulation of errors.
• Few cancers are familial.
• Most cancers are initiated by exposure to some
environmental agent – chemicals, radiation, viruses.
• There is no single cause – cancer is a multifactorial
disease.
Risk factors
• Age – the most important factor;
• genes that increase susceptibility, e.g. two genes are
known to increase risk for breast cancer;
• impaired immune function, e.g. in AIDs;
• diets high in fat, meat, salt or alcohol increase risk for
some cancers;
• agents in the environment, e.g. tobacco smoke, sun,
radiation, workplace hazards, asbestos;
• viruses can cause genetic changes in cells that make
them more likely to become cancerous, e.g. hepatitis B
increases risk for liver cancer.
US Nurses’ Health Study:
colon cancer risk
Colon cancer risk in people with high
energy intake and high BMI
Activity status
Odds ratio
95% CI
Low physical activity
3.35
2.09–5.35
High physical activity
1.28
0.81–2.03
Evidence for the relation between
physical activity and colon cancer
• Consistent across:
– occupational and leisure activity
– different study designs;
• comparing most active with least active, median
relative risk is 0.7;
• independent of confounding factors;
• some evidence of dose–response;
• overall, evidence classed as ‘convincing’.
Canadian case-control study:
breast cancer risk
Breast cancer risk in Nurses’ Health Study:
dose–response evident
Evidence for the relation between
physical activity and breast cancer
• Around 60 observational studies;
• consistent when total activity was measured;
• independent of confounding factors;
• comparing most active with least active, median
relative risk is 0.8 (20% risk reduction);
• overall, evidence ‘convincing’, but interaction with
menopausal status still to be clarified.
Potential mechanisms
• Systemic:
– low levels of metabolic hormones, e.g. insulin,
IGFs;
– improved weight regulation, low body fatness;
– enhanced immune function in moderately active
people.
• Site-specific
– sex steroid hormones, e.g. oestrogen and breast
cancer risk;
– for colon cancer, bowel transit time.
Changes in bowel transit time with
six weeks of running training
Preventive potential
Population-attributable risk estimates from
International Agency for Research on Cancer
(2002):
• 13–14% for colon cancer
• 11% for postmenopausal breast cancer
Evidence linking physical activity with
cancer risk
Cancer site
Plausibility
Evidence level
Colon
Several hypotheses
Convincing
Breast
Several hypotheses
Postmenopausal –
convincing/probable:
premenopausal – limited
Endometrium
A few hypotheses
Probable
Prostate
Some hypotheses
Possible
Pancreas
At least two hypotheses
Possible, limited
evidence
Lung
None known
Possible, limited
evidence
Summary I
• Cancer is major source of morbidity and mortality.
• Risk factors vary by tumour site.
• Carcinogenesis disrupts the orderly fashion in
which normal body cells grow, divide and die.
• Physical activity is associated with reductions in
the risk of several site-specific cancers. Evidence
is strongest for cancers of the colon and breast.
Summary II
• Dose–response relationships are not well described.
• At least 30–60 minutes per day of moderate-tovigorous activity may be needed for optimal
reduction in risk of colon and breast cancer.
• Potential mechanisms can be advanced, some
systemic, some site-specific.