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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.
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