Physical
Activity and Cancer
by Lisa Saffron
When weighing
up the risk of cancer from food, it's not only what you put into your
body that matters, although the type and quantity of food certainly
contribute to cancer risk. However there's another side to the cancer
and food equation and that is physical activity. It may not be intuitively
obvious but there is evidence that physical activity itself protects
against cancer.
For colon cancer, the evidence is strong and convincing. It's less
convincing, but nonetheless possible that physical activity protects
against breast cancer, lung cancer and reduces overall cancer rates.
In September 1997, the World Cancer Research Fund presented a comprehensive
review of the evidence linking food, nutrition and the prevention
of cancer. From the review, I discovered the current state of knowledge
about the link between physical activity and cancer
Energy Balance
For optimal health, adults need to achieve a balance between the energy
taken into the body via food and the energy spent by the body via
metabolism and physical activity. An energy-dense diet full of fatty
foods combined with a sedentary lifestyle tips the energy balance
out of equilibrium. When energy intake exceeds energy output over
a long period of time, the result is an increase in many chronic diseases,
including cancer.
Colon Cancer
Very active people have a risk of colon cancer that is only about
60% of that of sedentary people. The risk rises as the amount of activity
drops. It's not certain from the evidence whether lifelong activity
is necessary to achieve this protection although it seems likely.
The evidence is convincing because the results are so consistent.
Of the 20 studies which look at the link between colon cancer and
physical activity, all but 3 show that activity protects and they
all show roughly the same level of protection.
Each study has limitations which should be recognised before applying
the results to other populations. Most of the studies are about men
and most are done in the USA. In any epidemiological study, it is
hard to pinpoint one factor such as physical activity and claim that
it is the factor responsible for the drop in cancer risk. In affluent
societies such as the USA, people who are most physically active tend
to be more health conscious, smoking less and eating a healthier diet.
Separating out these other important factors is not an easy task.
Most studies have not investigated the effect on cancer risk of both
energy intake and physical activity. The few studies that did look
at both energy intake and activity found that they interact. The people
with the highest cancer risk had the greater body mass and were also
the least physically active.
Various theories have been proposed to explain how physical activity
might affect colon cancer. One suggestion is that physical activity
speeds up gut activity so that carcinogens in food or carcinogens
produced during digestion spend less time damaging cells in the colon.
Another theory is that physical activity has a positive effect on
the immune system and on the levels of growth factors that inhibit
the growth of cancer cells.
Breast Cancer
Physical activity possibly reduces the risk of breast cancer, according
to evidence from epidemiological studies. In one American study, the
breast cancer rate of university graduates was observed years after
graduation. Those who had not been athletic at university had an 86%
greater risk of breast cancer than the women who had been athletes
at university. Another study estimated physical activity levels from
the occupation listed on the death certificate. Those women who had
been in physically demanding jobs had a lower rate of breast cancer.
There is also corroborative evidence from animal studies where exercise
protects against the growth of mammary tumours in experimental animals.
The evidence is not as convincing as for colon cancer. There are only
5 epidemiological studies. Nevertheless, all of these, except one,
found evidence of a protective effect of physical activity.
The way that physical activity affects breast cancer is probably through
its effect on oestrogen metabolism. It is well-established that natural
oestrogens, the oestrogens produced in the body during a woman's menstrual
cycle, increase the risk of breast cancer. The risk is greater the
more one is exposed to high levels of oestrogen, which is why early
age at menarche, late menopause, late age at first pregnancy, and
no pregnancies are associated with a higher risk of breast cancer.
Physical activity decreases the body's production of oestrogen and
lowers both the age of menarche and the age of menopause.
Ways Forward
The lifestyle of most people in the developed world and increasingly
of those in urban areas of developing countries creates an energy
imbalance with energy intake often greater than energy output. One
way to attain energy balance is to change to food with a lower energy
content, eating a diet based on bulky plant-foods, mainly starchy
staples, fruit and vegetables. Another way is to eat less food. A
third way is to step up the energy output by increasing the amount
of physical activity. All of these will reduce the risk of cancer.
Source
Food, Nutrition
and the Prevention of Cancer: a global perspective, World Cancer Research
Fund and the American Institute for Cancer Research, 1997.
Articles
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